Unit 9 Compression, Absorption, Refrigeration Flashcards
Refrigeration is the artificial production of
Cold, using mechanical means
First Fundamental principle of refrigeration
A substance must absorb or reject latent heat in order to change state
When latent heat is added to a liquid
It evaporates
When latent heat is removed from a gas
It condenses
What happens when an ice cube tray is placed into a freezer
First, the refrigerant removes sensible heat from the water, and then it removes latent heat of fusion
The evaporation of a liquid can lower a body’s temperature by the extraction of sensible heat, and the evaporation process can cause a change in state if;
The liquid evaporation occurs at a low enough temperature
The second fundamental principle of refrigeration
The pressure exerted on a surface of a boiling liquid affects the temperature at which the liquid boils
Saturation temperature
The temperature at which a substance is heat saturated, and additional heat causes a change in state, but no temperature change.
saturation temperature depends on
The pressure applied
Saturation pressure
The pressure applied to the boiling liquid surface when saturation temperature is reached
Third principle of refrigeration
For every saturation pressure, there is one corresponding saturation temperature
ASHRAE
American Society of heating, refrigeration and air conditioning engineers
The fourth principle of refrigeration
One of the physical properties of every pure liquid is a characteristic set of saturation pressures and temperatures
A physical property of every pure liquid is a set of
Saturation pressures and temperatures
Refrigerant
A liquid that is capable of boiling at low temperature
The temperature of a refrigerant can be controlled by
Varying the pressure in the vessel
The refrigerated space
Inside an insulated room
Refrigerated medium
The substance being cooled
The evaporator in a refrigeration system is in physical contact with
The refrigerated medium, so that heat May transfer to the refrigerant
A back pressure regulator, in the form of a throttling valve installed at the evaporator Outlet
By adjusting the valve, refrigerant Vapor flow can be adjusted, which changes the saturation pressure and saturation temperature in the vessel.
Vacuum pump at the evaporator Outlet
The vacuum pump can withdraw refrigerant Vapor faster than the rate at which it boils. So, the vacuum pump can lower the evaporator pressure to below atmospheric pressure
Liquid receiver
Reservoir of refrigerant to supply liquid to the evaporator. Continuously fed to replace the refrigerant boiled in the evaporator. Used in larger refrigeration systems
Metering device
Installed between the liquid receiver and the evaporator, to control refrigerant flow.
If too much refrigerant enters the evaporator
Both vapour and unboiled liquid will leave the evaporator. The liquid will damage the compressor
If the evaporator is fed to little refrigerant
The evaporator will starve or run dry. Heat flow to the evaporator will be reduced due to insufficient boiling liquid
Refrigerant Vapor must be
Recovered and reused
The refrigerant Vapor that leaves the evaporator is;
Compressed, and then cooled in a heat exchanger (the condenser). In the condenser, The Vapor gives off latent heat to a coolant or cooling medium and returns to its liquid state. The liquefied refrigerant then flows from the condenser to the liquid receiver until it’s reused in the evaporator
To get refrigerant Vapor to release its latent heat
The Vapor must be hotter than the cooling medium
Most condensers use a relatively warm cooling medium
Between 0 and 40 degrees Celsius
A compressor draws refrigerant vapour at a low pressure and discharges it at
A higher pressure and also raises the refrigerant Vapor temperature.
After the pressurized, high temperature vapour leaves the compressor
It enters the condenser, transfers heat to the surrounding coolant, and condenses. The liquid refrigerant drains to the receiver, and eventually returns to the evaporator, where the cycle repeats.
5 mandatory components in a refrigeration system
Refrigerant
evaporator
refrigerant metering device (control valve, orifice or capillary tube)
Condenser
Compressor
Optional components for a refrigeration system
liquid receiver
Condenser cooling fan
Evaporator fan
Metering device Inlet
Condenser Outlet
Evaporator Inlet
Metering device Outlet
Compressor Inlet
Evaporator Outlet
Condenser Inlet
Compressor Outlet
Energy is consumed in order to compress refrigerant. The energy consumed by the compressor results in
An increase in enthalpy, pressure and temperature
In the condenser, the refrigerant first gives off
The energy added by the compressor. Then, the saturated refrigerant Vapor gives off the latent heat it absorbed in the evaporator, condensing back to saturated liquid.
Batch process
A discrete mass of refrigerant circulates through the system, and repeatedly undergoes thermodynamic processes
Constant flow cycle
When the heat transfer is continuous in a refrigeration cycle
Subcooled
Cooled to below its saturation temperature
Thermostatic expansion valve
Small bulb located at the evaporator Outlet, monitors evaporator superheat and controls the opening of the metering device.
Due to length of the pipe, internal roughness of the pipe and bends in the evaporator and condenser, the pressure of the refrigerant;
Drops from the metering device Outlet to the compressor Inlet, and from the condenser Inlet to the metering device Inlet
In both the high and low sides, pressures drop in;
The direction of refrigerant flow
The evaporator pressure drop reduces;
The compressor suction pressure, increasing the pressure ratio and making the compressor work harder
Internal combustion engines are compared according to
Boilers are compared according to
Refrigeration plants are compared according to
Kilowatt output
Horsepower
Tonnes of refrigeration
Evaporator capacity (evaporator tonnage)
Cooling capacity of the evaporator. Expressed in tonnes of refrigeration
The evaporator transfers less Heat than;
The condenser, because it does not need to handle the energy added to the refrigerant by the compressor. Therefore, evaporators often have smaller capacity than condensers
Condenser capacity
The heat rejection capability of the condenser. Must have greater heat transfer capability then the evaporator
Net refrigerating effect (NRE)
Heat absorbed per kilogram of refrigerant circulated through the evaporator (kj/kg)
Flash gas
The portion of refrigerant that evaporates.
Flashing occurs because
The liquid has higher enthalpy in the high side then it can have on the low side. This excess enthalpy is converted to latent heat in the evaporator, producing flash gas.
_____ reduces the amount of flash gas produced
Subcooling
The refrigerant that flashes into vapour
Will not take part in the actual Refrigeration process. Only the remaining liquid water absorbs heat from the surrounding medium for evaporation.
NRE is considerably less when
The liquid refrigerant entering the evaporator is at a temperature higher than the boiling point in the evaporator.
The amount of liquid that may flash into vapour can be as high as
30%
Coefficient of performance (COP)
The ratio of the amount of heat absorbed from the refrigerated medium by the evaporator, to the amount of energy used to drive the compressor.
A higher COP means
A more effective refrigeration system
COP formula
NRE/Compressor power in Kw
Pressure ratio
Absolute compressor discharge pressure/Absolute suction pressure
Refrigerants are categorized according to
Flammability and toxicity, then placed in a safety group.
Refrigerants are also classified according to
Their impact on the environment, chemical Origins, and operating temperature suitability
ASHRAE denotes refrigerants with the capital
“R” followed by a dash and a number
There are over ____ ASHRAE designated refrigerants and refrigerant blends
300
Leaks are inevitable. Leaking refrigerant may have considerable environmental impact on
The Earth’s ozone layer and on global warming
Refrigerants that contain chlorine
May have ozone-depleting potential (ODP).
Unlike combustion equipment, refrigeration systems do not produce
CO2 (carbon dioxide)
Global warming potential (GWP)
Refrigerants rated according to their global warming potential.
Natural refrigerants
Refrigerants that occur naturally in the environment.
EX; ammonia, water, carbon dioxide
Ammonia natural refrigerant
Desirable since it is an energy efficient refrigerant with zero ODP and zero GWP
Many refrigerants begin as hydrocarbon compounds, but are
Chemically modified to achieve certain physical properties
CFC’s
Chlorinated fluorocarbons. Because they are chlorinated, they have high ODP.
R-11 and R-12 refrigerants
Phased out chlorinated fluorocarbons, they have high ODP.
Halocarbons
Includes; CFC, HFC, HCFC
Thermodynamic properties are
The physical properties that directly affect the movement of heat
Examples of thermodynamic properties
6
Pressure, temperature, volume, density, enthalpy and entropy
Refrigerants with low evaporator pressure
Air May leak into the refrigeration circuit, therefore are purgers must be installed to continuously remove air from the system
Refrigerants operating pressures vary with temperature
Important, since it determines the strength of the equipment required, cost of construction, and operator Staffing requirements
When choosing a refrigerant it should have a
Low condensing pressure. As well, evaporators should preferably operate above atmospheric pressure to prevent air and moisture infiltration, which causes operational problems
Refrigerants with lower specific volume
The refrigeration compressor system can have a smaller displacement compressor, as well suction lines can be made smaller in diameter.
Refrigerant liquid density informs designers
About how heavy liquid lines will be. This helps determine the size of control valves and piping.
The NRE is the heat absorbed
In the evaporator, under standard operating conditions
Latent heat of evaporation formula
HFG=
HG-HF
Enthalpy of saturated vapour - Enthalpy of saturated liquid
6 important physical properties of refrigerants
Miscibility, leakage tendency, odour, toxicity, formability/explosiveness, moisture reaction
Miscibility
Refers to when two or more liquids are soluble in all proportions
Refrigerants come in contact with Lube oil in the
Compressor crankcase, cylinder walls and screws. Refrigerants carry some of this oil into other parts of the system
Miscibility is dependant on
The type of refrigerant and the type of lube oil
Lube oils are categorized as
Conventional mineral-based oils and synthetic polyester oils
Ammonia is not
Miscible with Lube oil
Oil miscible refrigerants dilute
Compressor crankcase oil, lowering its viscosity and lubricating ability
When non-miscible refrigerants are used
Oil tends to build up in the evaporator and condenser, reducing system capacity.
May need oil separators and oil return systems
Ammonia systems require oil
Separators and oil return systems. These remove oil from the piping and other low points in the system, and return the oil to the compressor
Refrigerants that have greater molecular mass, have larger molecules and are less likely
To escape through tiny openings (leaks)
4 Leakage tendency factors
Operating pressure, viscosity, density, and chemical effects on seals and gaskets
At high temperatures, refrigerants are capable of absorbing greater amounts of
Moisture than at low temperatures. When a warm moisture saturated refrigerant is cooled to a lower temperature, it will produce free water
Moisture in the refrigerant should be avoided for 3 reasons
May cause ice to form between valve and valve seat of metering device
May cause acid formation, resulting in corrosion
Water combining with ammonia
Ammonia combines with water to form
Ammonium hydroxide, which is highly corrosive to copper and alloys. Ammonia systems must never use components made of copper or copper alloys
By decreasing the pressure exerted on a refrigerant
It’s boiling point decreases, and it will absorb latent heat of evaporation from its surroundings. After evaporating, the refrigerant is pressurized and returns to its liquid state by discarding latent heat
Closed cycle refrigeration system
Vapour from the evaporator is collected continuously. The vapour is compressed, condensed, and returned to the evaporator so the same refrigerant is used over and over
Two classes of closed-cycle refrigerating systems
Compression system and absorption system
Closed cycle compression refrigeration system principal parts
5
Evaporator
Compressor
Condenser
Metering device (liquid refrigerant control/regulating valve)
Liquid receiver
Closed cycle compression system two sections;
High-side and low-side, divided by the centre line
Closed cycle Compression High side
Contains refrigerant at high pressure and temperature. Refrigerant leaving the compressor is high pressure superheated gas. Refrigerant in the condenser is both liquid and gas. In the condenser, refrigerant is at saturation temperature and pressure. Refrigerant leaving the condenser is high pressure, high temperature liquid. Refrigerant entering metering device is high pressure and temperature, but subcooled by a few degrees.
Closed cycle compression low side
Contains refrigerant at low pressure and temperature. Refrigerant leaving metering device is low pressure saturated liquid and Flash gas. Refrigerant in the evaporator is both liquid and gas, at saturation pressure and temperature. Refrigerant leaving the evaporator is low pressure, low temperature gas. Refrigerant entering the compressor is low pressure and temperature, but superheated to an extent
Closed cycle compression low side system
Includes all equipment Downstream of the metering device including;
The evaporator, suction side of compressor, all interconnected tubing and piping
Closed cycle compression
Design pressure of low side is determined by the;
Temperature requirements for the cooled medium
Closed cycle compression
The high side consist of;
Compressor discharge, condenser, liquid receiver, piping Upstream side of the metering device, connected tubing and piping
Closed cycle compression
High side design pressure is determined by;
Required condensing temperature of the refrigerant vapour, which depends on the temperature of the available condensing medium
Direct system- direct expansion systems or DX
The evaporator surface is in direct contact with the material or space being refrigerated.
Household refrigerators and air-conditioners are examples of
Direct systems
Evaporators used in DX systems
Are called; DX coils
Indirect system
Liquid, such as brine, glycol or water is cooled by the refrigerant and then circulated by means of a pump to the material or space being refrigerated.
Advantages of indirect systems
Hazardous refrigerants can be used to cool brine. They are also cost-saving, most of the system is filled with chilled water or brine, Which is far less costly. Chilled water or brine transfers heat from occupied space, reducing the amount of refrigerant required
Chillers
Evaporators used to cool water or brine
Packaged refrigeration unit advantages
Components designed to match each other for greatest operating efficiency. Compact since its on a single skid, less space requirements needed. Installed easily and quickly. Factory tested and manufacturer takes full responsibility for design and performance.
Economizer installed between a condenser and chiller
Serves 3 purposes;
Produces flash gas to cool the compressor motor
Increases the net refrigerating effect (NRE) of the evaporator
Reduces power consumption of the compressor
When evaporator flash gas is reduced
Net refrigerating effect increases, which increases the cooling capacity of the system
Compression system with economizer installed in between the condenser and chiller
Only the refrigerant Vapor produced by the evaporator goes through
2 compression stages
Economizer provides an _________ effect for the compressor
Intercooling.
The cool vapour from the intermediate chamber cools the first stage compressor discharge gas, reducing power required for compression in the second stage
Refrigeration compressors are cooled without
Fins
Refrigeration compressors are designed specially to prevent or inhibit
Refrigerant leakage
Refrigeration compressor three main functions
Draws refrigerant gas from the evaporator as it’s produced.
Raises the refrigerant gas pressure so refrigerant can flow from the high side to the low side
Raises the saturation temperature of the gas to above the temperature of the condensing medium
Three types of refrigeration compressors
Reciprocating, rotary, centrifugal
Reciprocating compressors
A piston travels back and forth in a cylinder, drawing in and compressing the vapour
Rotary compressors
Use helical rotors or an eccentric rotor with vanes to compress the vapour
Centrifugal compressors
Rapidly revolving impellers to draw in the vapour and discharge at high velocity by centrifugal force. High velocity, low pressure vapour is converted to low velocity, high pressure vapour before it leaves the compressor
Compressor safety head
Entire head lifts when the pressure in the cylinder becomes too high, due to liquid refrigerant in the cylinder. Lifting action allows liquid to pass into the discharge line without doing serious damage
Rotary compressors in compound refrigeration systems
The compressor is called a booster. Produces very low evaporator pressure for deep freeze applications
Helical rotor compressor design is primarily in
Medium and high capacity Refrigeration
New compressors must be able to efficiently operate over
A pressure range, rather than a single evaporator or condenser pressure
Heat exchange results in one process fluid
Gaining Heat and the other losing Heat
Two main heat exchangers used in a refrigeration system
Evaporator and condenser
Evaporator is the part of the refrigeration system in which
Liquid refrigerant is vaporized by the absorption of heat from the medium to be cooled
Three types of evaporators
Direct expansion “or dry” evaporator
Flooded evaporator
Liquid recirculating (or liquid overfeed) evaporator
They differ in the method of refrigerant circulation
Dry evaporator (direct expansion)
Expansion valve admits only enough liquid refrigerant to maintain desired temperature. Amount of liquid refrigerant entering, balances the amount of refrigerant Vapor leaving, with no recirculation. Evaporator only contains a small amount of liquid refrigerant at any time
Flooded evaporator
Kept almost completely filled with liquid refrigerant regardless of load demand. Respond rapidly to changes in load as refrigerant has already gone through a pressure reduction and is awaiting exposure to heat.
________ has a higher heat transfer rate than the dry evaporator
Flooded evaporator
Flooded evaporator is applicable for
Pasteurization processes where on Startup, fluid must not pass through without being cooled.
Flooded evaporator disadvantages
Requires a relatively large refrigerant charge and is bulky
Liquid recirculating evaporator (liquid overfeed)
Used in overfeed refrigeration systems. Has a constant flow of liquid refrigerant regardless of load demand. Has a pump that feeds around three times the amount of liquid then the evaporator requires.
Critical element of the liquid recirculating evaporator
The low pressure receiver. Separates liquid and Vapor returning from the evaporators, so that the vapour can be re compressed or condensed. Without it, liquid would enter and damage the compressor
Low pressure receiver is also called
A surge drum or accumulator
Recirculating evaporator advantages
Higher heat transfer since it is completely wetted with liquid. Respond rapidly to changes in load as refrigerant has already gone through a pressure reduction and is awaiting exposure to heat.
Liquid recirculating evaporator is applicable for
Quick freeze and loads that swing from almost no load to full load quickly.
Liquid recirculating evaporator disadvantages
Requires a relatively large and costly refrigerant charge
Needs additional equipment such as surge drum, pumps, level controller. Adds to initial cost
Classes of evaporators according to their Construction
4
Bare tube, plate surface, finned tube, Shell & tube
Bare tube evaporator
Single coil bent in various shapes, or various coils placed in parallel and connected to Common headers. Steel or copper tubing. Used in any type of evaporator
Bare tube evaporators can be used where
Temperature is maintained below 1°C, such as coolers and freezers. Can be submerged in liquid
Plate surface evaporator
Uses a coil and two plates. Can be used individually or in Banks. Usually used as a direct expansion evaporator
Plate surface evaporator is widely used in
Refrigerators, freezers, display cases and Locker plants because it can be easily cleaned with manual scraping.
Finned tube evaporator
Bare tube evaporator coil with fins attached. Used as a direct expansion evaporator.
Finned tube evaporator is the most widely used in
Air conditioning applications. Also used in walk-in coolers and display cases
Shell & tube evaporator
Cylindrical steel Shelf with multiple straight bare tubes in parallel. Used for almost any type of liquid cooling application.
Function of the refrigeration condenser
Removes heat from compressed refrigerant Vapor until it changes to liquid. The condenser must remove both the heat absorbed in the evaporator by The Vapor, and the heat of compression added by the compressor
Three types of condensers
Air-cooled, water cooled, evaporative
Air-cooled condensers
Uses air to remove heat from refrigerant vapour. Cooling air is circulated by fans or blowers. Commonly mounted Outdoors on a roof location
Air-cooled condenser disadvantages
Cooling load increases in hot weather, condenser capacity is reduced as temperature difference between cooling medium and refrigerant decreases.
Air-cooled condenser advantages
Do not require cooling water or water treatment chemicals, and do not breed harmful bacteria. Preferred when environmental considerations are of importance
Water cooled condensers
Use cooling water to remove heat from refrigerant vapour
Three types of water cooled condensers
Double tube, shell and coil, Shell and tube
Double tube condenser
Water flows through inner tube, while refrigerant flows in the opposite direction. Lowest efficiency. Sometimes used as a booster condenser
Shell and coil condenser
Very efficient and compact, easily adaptable to packaged Refrigeration units. Difficult to clean. Repair costs may increase when leaks develop
Shell & tube condenser
Number of straight tubes fastened in the tube sheets. Has water boxes to direct the cooling water through two or more passes.
Shell in tube condenser is used in
Ammonia installations, as well as medium and large-sized air conditioning installations using other types of refrigerants.
Evaporative condensers
Uses air and water to provide Cooling in order to condense the refrigerant vapour.
Evaporative condensers are favoured in locations where
It is necessary to conserve water. 80 to 90% in water consumption savings
Evaporative condensers may be located either
Indoors or Outdoors.
Indoors requires Ducts Which are connected to the outside.
In Outdoors, care should be taken to prevent freezing of water during cold weather. Can be done by using a heating coil in the sump of the condenser
Cooling towers used with water cooled condensers to
Limit water consumption
Three types of refrigeration system controls
Operating controls (system capacity)
Actuated or secondary controls
Limiting and safety controls
Operating controls
Start or stop the compressor, or regulate its capacity when the process conditions approach or deviate from set points
Actuated or secondary controls
Either indirectly control the changes in operation called for from the primary controls, or regulate the cycle during operation
Limiting and safety controls
Protects the system against operation Beyond the limits it was designed for
Human machine interfaces (HMI’s)
Allows operator to receive operational data and alarms on a screens, and ability to make changes to set point. Allows a unit to be stand alone
Refrigeration plant cooling capacity must always equal
The cooling load
If a refrigeration plant operates at full capacity when the load is low ______
If a refrigeration system operates at low capacity when the cooling load is high ______
The temperature of the refrigerated medium will drop below set point
The temperature of the refrigerated medium will rise above set point
Compresser capacity can be adjusted by
Cycling it on and off, regulated with multi-stage control and fully modulated
Compressor Control Systems respond to
3
Temperature, pressure and humidity
A suction pressure actuated control cannot be used with
An automatic expansion valve, or a capillary tube
Chilled water flow switch
Installed in chilled water Outlet line, protects the chiller from freezing due to lack of water flow. Opens compressor motor circuit when water flow drops below the safe minimum flow. Prevents compressor from starting if flow is not established
Motor demand limiter
Most centrifugal Chillers have this, limits the maximum current flow to the compressor Drive motor. Overrides the water temperature sensor if the motor load reaches the maximum amperage setting.
Motor demand limiter
One of the most common and useful purposes it serves
Limit the current flow when the machine is first started. Prevents low temperature trips on Startup
Instead of demand limiters, some chillers use
Soft loading strategies on Startup. PLC control strategy that brings the chilled water loop temperature from its start value to its set point in a controlled manner. Prevents Chiller from going to full capacity during the pull down period
Restart inhibit
Keeps compressors from short cycling or restarting in a short period of time. Allows motor to cool off for 10 or more minutes
In the condenser, non condensable gases
Occupy space, impede heat transfer, and raise condenser pressure
Chiller low limit thermostat
Immersed in the water at the coldest point of the chiller, to prevent freezing. Opens the control Circuit of the compressor several degrees above the freezing point
Refrigeration compressor crankcase and oil sumps are both under
Low side pressure
many compressors pump down
The low side before they cycle off. Reduces low side pressure to just above atmospheric
Net Lube oil pressure
The difference between the oil pump supply pressure and the crankcase pressure
Flow switches are operated by the force exerted on
A flexible vane immersed in a flowing fluid
Flow switches are used in;
Chilled water lines, cooling water lines and air ducts
Flow switches are often used as
Safety lockout switches if the fluid flow becomes insufficient or ceases. Can also be used to close flow indicator circuits
When compressor Lube oil is cool
It dissolves more refrigerant than when it is hot. Dissolved refrigerant dilutes Lube oil, which causes excessive bearing wear. When the compressor starts, refrigerant in the oil vaporizes, forming bubbles. When lube pump draws bubbles, it may lose its prime and starve bearings of oil
Refrigeration system pressures and temperatures must remain constant while the system is in operation. For this to occur
The amount of refrigerant fed to the evaporator must always equal the amount of refrigerant circulated by the compressor
As cooling load increases, refrigerant flow must
Increase
Flow of refrigerant through the compressor is controlled by;
Flow of refrigerant through the evaporator is regulated with;
Various capacity control methods, including on off cycling and unloading
A metering device
If too much refrigerant enters the evaporator
The cooling load will not be able to vaporize all of the refrigerant. Remaining liquid will enter the compressor suction line and destroy the compressor
Metering devices include;
3
Capillary tubes, fixed orifices, expansion valves and float valves
Manual expansion valves are
Seldom used in modern refrigeration systems. Occasionally used in bypass lines around automatic control valves or defrosting system
Automatic expansion valve (AEV)
Pressure regulating, maintains constant pressure in the evaporator whenever the compressor is running regardless of load. Automatically shuts off liquid flow when compressor stops
When temperature of the refrigerated space or medium drops below set point, a thermostat stops the compressor and
The expansion valve stays open and allows liquid to enter the evaporator. Liquid continues to vaporize, causing the evaporator pressure to increase.
AEV’s are used on;
Small refrigerating units, such as refrigerators and freezers.
Thermostatic expansion valve (TEV)
Most widely used metering device. Similar to automatic expansion valve, but features thermal power element.
The thermostatic expansion valve adjusts the amount;
Of liquid admitted to the evaporator so that, under all load conditions, nearly the entire evaporator surface is used to transfer heat to the evaporating liquid refrigerant. Ensures no liquid leaves the evaporator with the vapour. Vapor leaving the evaporator must be superheated
To compensate for a large evaporator pressure drop, a thermal expansion valve may be equipped with
An equalizing line
In a flooded evaporator, the low pressure float valve is used to
Maintain a constant level of liquid refrigerant. Installed in the low side of the system
High pressure float valve
Liquid level operated control valve, located on the high side. The amount of refrigerant charge in a system with a high pressure float valve is critical
Capillary tube
Simplest of all metering devices. Fixed length of tubing with a small inside diameter. Create considerable pressure drop along its length, restricting the flow of liquid from the condenser to the evaporator and maintains pressure difference between the high-side and low-side.
Capillary tubes are limited to
Small refrigeration units, since it is small and not very sensitive to load changes. Used in refrigerators, freezers and air conditioners
Evaporator capacity can be controlled by using
Sectional evaporators or evaporator dampers
Sectional evaporators
Divided into two or more sections, each with a flow control valve. Sections of the evaporator can be shut off as cooling load decreases.
Evaporator dampers
Uses a face damper to vary the quantity of air passing over the evaporator coils and controlled by a positioner.
Evaporator face damper disadvantage
Air flow varies with cooling load. In HVAC, reduction in air flow caused when the dampers closed reduces ventilation air to unacceptable levels
Evaporator face and bypass damper
Dampers connected to same damper Drive. Regardless of damper position, quantity of air passing through the ducts remains constant
Methods used to control compressor capacity
2
Intermittent operation and continuous operation with reduced output
Intermittent operation
Compressor is stopped when the desired low temperature of the substance is reached, and started up when the temperature rises to a certain level.
Intermittent operation is only used
On small systems with fairly constant loads, as the most power is consumed on Startup
In larger systems, the compressor operates continuously but
At reduced capacity
Capacity of reciprocating compressors is reduced by
4
Cylinder unloading, cylinder bypass, Hot Gas bypass, variable speed Drive
Cylinder unloading
Method of capacity reduction that deactivates one or more cylinders in sequence, based on the cooling load
Cylinder unloaders work by
Keeping the intake valves of one or more cylinders in the open position, preventing the compression of vapour drawn in during the suction stroke
Cylinder bypass
Bypasses the discharge from one or more cylinders back to the suction side of the compressor
Hot Gas bypass
Capacity control. Has a normally closed solenoid valve located in the bypass line. When energized, some hot high pressure gas is allowed to go directly into the suction line
Hot Gas bypass disadvantages
Little or no reduction in power consumption when bypass line is open. Compressor overheating can occur.
Hot Gas bypass is used only on
Small compressors. Often used in conjunction with other types of capacity control
Centrifugal compressor capacity control
3
Speed control, suction throttling, variable Inlet guide vanes
Suction throttling
Uses a butterfly damper installed at the inlet to a centrifugal compressor.
Suction throttling is not economical because
The input power to the compressor does not decrease by the same amount as the capacity. Has been replaced by variable Inlet guide vanes
Variable Inlet guide vanes
Vanes are linked so they operate together. Operated by a Rack & Gear Arrangement attached to a piston.
Variable Inlet guide vanes efficiency
Very efficient, compared to suction throttling. Reduces compressor power consumption
Pressure gauges are rarely installed on
Low capacity commercial refrigeration systems
Accumulator
Simple liquid trap located in the compressor suction line. Collects and holds liquid so that it does not enter the compressor
Accumulators must not be insulated
So heat may be absorbed through the accumulator wall.the heat vaporizes the trapped liquid and returns it to the compressor suction. If insulated, liquid refrigerant could accumulate and eventually become entrained in the compressor suction
Oil separator
Separates oil that leaves the compressor with the high pressure vapour, and returns it to the compressor crankcase. Maintains oil level in the crankcase and reduces the need for oil addition
Oil separator should be installed in
The discharge line between the compressor and the condenser
Refrigerating systems using oil miscible refrigerants use
Oil separating devices in the compressor discharge line
Suction strainer
Installed in compressor suction lines to keep impurities from entering compressors, which could damage them.
Filter drier
Removes moisture, particulate and scale from liquid refrigerant. Installed in the liquid refrigerant line ahead of the expansion valve
Moisture in a refrigeration system can cause the following problems
2
Ice buildup in metering device.
Acid formation resulting in, corrosion, sludge, deterioration of motor insulation
Sightglass and moisture indicators
Small housing, with one or two lenses installed in liquid refrigerant line to observe liquid flow.
Vapor appears in the liquid line when the system contains insufficient refrigerant, causing the pressure drop in the liquid line to the metering device to increase. Vapor bubbles also show if the flow in the liquid line is restricted
Economizers are used for three reasons
Reduce the temperature of liquid refrigerant
Prevent flashing of the liquid due to the pressure drop that occurs as refrigerant flows through the liquid line
Increase the temperature of the vapour passing through the economizer
If liquid passes through the evaporator, the economizer will
Evaporated before it reaches the compressor suction. Prevents liquid slugging in the compressor
Distributor
Placed in the liquid line directly Downstream from the expansion valve, and ensures each evaporator coil is fed refrigerant equally. Used when a large direct expansion evaporator has more than one refrigerant circuit
Vibration absorber
transmission of vibration and noise is prevented, by connecting the piping to the main components of the refrigeration system using vibration absorbers
Vibration absorber should be located
As close as possible to the compressor for maximum effect
Ammonia steel pipe should be
Type S (seamless)
or
Type E (Electric resistance welded)
Horizontal refrigeration lines should slope
Downward in the direction of refrigerant flow, minimum slope of 4.2mm per metre.
As an added precaution against leakage along valve stem
Packed valves may be equipped with a valve cap that covers and seals the valve stem. Cap must be removed before valve can be operated
Liquid receiver gauge glass
Installed in Vapor space and the liquid space. Usually a reflex glass, since it makes clear liquids appear as black
Purge valve
Used to vent non-condensable gases from the system
Charging valve
Used to charge the system with refrigerant
Two most common pressure relief devices in refrigeration systems
Fusible plug and spring loaded relief valve
Fusible plugs
Device containing an alloy that melts at a specified temperature to relieve pressure. When plug melts entire refrigerant charge escapes. New plug and new refrigerant charge are required to resume operation
Fusible plugs are used
In smaller systems, and only to protect smaller volume components. Located above or below the liquid refrigerant level, except on the low side. Primarily used to protect from explosion in case of fire.
Refrigeration safety valves do not have
Manual try leavers or exposed Springs. Refrigerant must be contained within the system. Valves must be replaced every 5 years
Emergency discharge system consists of
3
Pipe in connection to the top of a receiver or vessel where refrigerant is stored
Emergency discharge valve, outside of the building
Diffuser, located at high elevation to spread the refrigerant Vapor over a large area
After a refrigeration system is installed, repaired, or modified, the entire system must be
Thoroughly inspected for leaks
__________ must not be used for system pressure testing
Oxygen and flammable gases. An explosion could result. Nitrogen gas is commonly used for leak testing
Positive pressure pneumatic testing
Inert gases, such as dry nitrogen or carbon dioxide, are used as the testing medium. Gas supplied from high-pressure cylinders, connected through a pressure-reducing valve to the high or low side pressure of the system
Before a pressure test
Piping joints must be uninsulated, welded joints unpainted and free of rust, dirt, oil. Fields test should be witnessed by jurisdictional inspector
For pressure testing, valve lineup must permit the
High and low pressure sides of the system to be initially tested together, at the same pressure
Fluids that are not to be used for leak testing an ammonia refrigeration system
4
Oxygen or combustible gas mixtures
Carbon dioxide
Halogen refrigerants
Water or Water Solutions
Refrigeration system pressure test
System must be held under pressure until;
Proven tight with no more than 1% loss in pressure, after accounting temperature change.
For safety, and to prevent over pressurization of the system, the pressure testing cylinder must have;
5
Shut off valve, bleed valve, pressure regulator, adjustable pressure relief valve, calibrated cylinder and line pressure gauges
Pneumatic pressure applied to a refrigeration piping system under test shall not exceed
130% of the design pressure of any system component
If no further leaks are discovered during the pressure test, the system may be left pressurized for
About 24hrs
Detecting leaks in a system that operates below atmospheric is
More difficult. Leakage of air in the system will cause The Purge unit to cycle more than usual, resulting in a loss of refrigerant because it’s impossible to totally separate air from the refrigerant during purging
To test a sub atmospheric pressure refrigeration system for leaks, it is necessary to
Shut down the compressor and Break Free the vacuum by pressurizing system with dry nitrogen.
Leak Detectors used to find leaks
5
Electronic Leak Detectors, litmus paper, phenolphthalein paper, sulfur candles, soap and water
Two most common Leak Detection methods
Soap and water test and electronic leak detector
Electronic leak detector
Draws vapour through a tube with a sniffer at the end. Sniffer is placed in areas where a leak is suspected. Measures electrical resistance of vapour sample. When a sample contains refrigerant, the change in resistance causes the detector to react
Sniffers are available to detect
Ammonia, HFC’s, HCFS’s
Litmus paper detector
Change in colour in the presence of an acid or base.
Red litmus paper turns ______ in the presence of ammonia
Blue
Phenolphthalein paper
Used when a more sensitive test for ammonia is required. White papers which change to red when exposed to a solution with a pH greater than 8.3
Sulphur candle test
Can detect ammonia leaks. When the flame from The sulphur candle comes in contact with leaking ammonia, thick white smoke is created
Sulphur dioxide formed when a Sulphur candle is burned is an
Irritating and toxic gas. Only done and well-ventilated areas
Soap and water leak test
Brush the solution on the area where leak is suspected. Bubbles will appear if there is a leak
Copper plating
Acids remove Copper from heat exchanger surfaces and redeposit it at points of high temperature, such as bearings and compressor exhaust valves
To prevent copper plating, large temporary dryers are installed
As liquid refrigerant passes through the drying agent, any existing moisture is absorbed
Before charging an empty refrigerating system, the entire system is
Put under a very high vacuum (5000 microns or 0.67kpa absolute) with a special vacuum pump. System compressor must not be used since it is not designed for this and could be damaged.
After the vacuum is obtained,
The vacuum is broken by admitting dry nitrogen gas. The gas is then evacuated from the system buy the pump to once again produce a high vacuum. Second vacuum removes the last traces of moisture from the system. If pressure does not increase, it is free of leaks and moisture
Before system charging is done
System drying and evacuating is done prior
Only ____________ should attempt to charge refrigeration systems
Technically qualified persons, trained and certified in the handling of refrigerant
Initial refrigerant charge is added to
The high pressure side of the system. A dehydrator should be installed in the line between the refrigerant drum and the liquid charging valve.
King valve is also called
Also called the liquid line shut off valve.
In preparation for charging the system, the liquid line shut off valve is
Closed at the receiver Outlet
If the refrigeration system uses ammonia, the vacuum is broken
Gas and not liquid
In small refrigeration systems, Frost formation on the compressor suction line indicates
Overcharging. This causes high suction and discharge pressures, and hi compressor power consumption.
Refrigeration compressors are internally lubricated by
Hand pumps and pressure rated hoses with quick connectors
Refrigeration compressors have _______ to show crankcase oil level
Bullseye gauge glasses. Normal oil level is about 1/2 the glass. If oil is low, first try to return Lube oil from the oil separators back to the crankcase
Oil separators that do not automatically drain to the crankcase, may drain to
oil pots.
Oil pots
Small pressure vessels left open to the system to gather oil.
When a refrigeration system is shut down for a prolonged of time, it should be
Pump down and all refrigerant should be stored in the liquid receiver. This prevents unnecessary strain on the low pressure side of equipment and loss of refrigerant while system is shut down
Direct expansion evaporator, it is essential that the evaporator be pumped down
Every time the compressor is shut down to make sure all refrigerant is removed from the evaporator. Prevents liquid slugging and compressor damage on Startup
When shutting down compressor
Never pump down below 7 kPa to 14 kPa since a slight positive pressure is needed to prevent air from being drawn in through minor leakes on compressor shaft seal.
Non condensable gases must be removed from the system either by
Manual or automatic purging.
Purge connections are made at the
Highest point of the condenser and receiver, since this is where gases tend to collect
Purgers
Refrigerate the mixture of refrigerant and non-condensable gases. The resulting liquid returns to the receiver. The non-condensable gases are vented to the atmosphere.
The condenser and receiver should be purged
Separately
Purger chilling coil
Causes vapour to condense and separate from the gas mixture, since non-condensable gases that enters the purger may carry a considerable amount of refrigerant vapour
Whenever non condensable gases are released through the relief valve,
A small amount of refrigerant is always discharged with the gases. Refrigerant losses increase with size of installation
Air-cooled condensers should not be cleaned with
Pressure washers, the high pressure water will bend the cooling fins and restrict airflow. Bent cooling fins can be straightened with a special comb
Absorption systems require
4
A condenser, liquid receiver, expansion valve, evaporator
A compressor is not required
Absorption system function 1
Absorber
A low pressure absorber containing a liquid (the absorbent), that withdraws the refrigerant vapour from the evaporator
Absorption system function 2
Generator
A high-pressure generator that contains and heats up a concentrated absorbent / refrigerant solution. This drives the refrigerant Vapor out of solution and raises its vapour pressure
Absorption system function 3
Heat source
A heat source that adds energy to the system
Absorption system function 4
Pump
Pump that moves the concentrated solution from the low pressure absorber to the high pressure generator
Many absorption systems are designed to use waste
Heat from boiler flue gas or gas turbine exhaust.
Absorption system generator may be used for
Steam or hot water boiler needs to be kept in operation
Electrical energy used by an absorption system is
Less than a similar capacity compression refrigeration system. Because pumps in an absorption system require less power than a refrigeration compressor. Also, smaller emergency generators can be installed
Absorption system refrigerants
Use only natural refrigerants, like ammonia and water. Reduces contribution to global warming if a leak occurs, and by reducing electricity consumption and production of greenhouse gases
In an absorption system, carry over from the evaporator
Will not cause damage. Permits continuous operation of the absorption system without changing set points or continuously monitoring of loading. Slugging and evaporator freezing are not a problem
Absorption systems require more _________ then compression systems
Cooling water. Since the absorption condenser must reject the heat added in the generator plus the heat absorbed in the evaporator, which is way more than the heat of compression added by compressor
More equipment and piping are needed for
An absorption system.
Absorption systems are very ________ in operation
Quiet. Well-suited for concert Halls, theatres and other applications where noise of a compression system would affect human comfort.
Water is used as the liquid absorbent in the ammonia absorption systems because
It has the ability to absorb ammonia Vapors in large quantities, and at a rate equal to the rate of vapour production in the evaporator
Ammonia absorption system;
When the low temperature, low pressure vapour is discharged from the evaporator to the absorber
It dissolves into a spray of water. Rapid absorption of ammonia occurs by the absorber which maintains the low pressure in the evaporator. Forms a solution of refrigerant vapour and water which forms a concentrated aqueous solution, called strong aqua.
Ammonia absorption system;
A pump transfers the strong aqua from the low-pressure absorber to the high-pressure generator.
In the generator, a heat source, raises the temperature of the strong aqua. This boils off the ammonia absorbed by the water in the absorber, producing high pressure, high temperature ammonia vapour. Water left in the generator is called week Aqua, which flows back to the absorber
Absorption system high side
Consists of the generator and condenser
Absorption system low side
Consists of the evaporator and the absorber
Ammonia absorption systems are used in
Cold Storage, food processing, or other applications where extremely low temperature cooling is needed. Can be cooled in the -30°C range, which lithium bromide systems cannot.
Ammonia absorption systems are seldom used in
Air conditioning purposes because of the toxic properties.
Lithium bromide absorption systems are more suitable for
HVAC use
Lithium bromide absorption systems
Large-capacity packaged water chillers, used in HVAC service. Use water as a refrigerant, and lithium bromide solution as the absorbent.
Lithium bromide absorption system;
When water is the refrigerant, to lower the saturation temperature to a temperature useful for cooling
The system must operate under an extremely high vacuum
Since very low temperatures are not required for air conditioning purposes, lithium bromide absorption systems maintain an evaporator temperature of
4°C to 7°C
Two factors lithium bromide absorption units are based on
Lithium bromide has the ability to readily absorb water vapour
Water boils at low temperatures when under a high vacuum
Lithium bromide absorption system
Generator is also called
A concentrator
Absorption units are all built on the Hermetic principle, because
Air infiltration must be prevented. Parts of the units which are under vacuum or a completely seal welded.
Crystallization
When a saturated solution is cooled, it will not be able to hold the same amount of salt in solution. Some salt will precipitate out of the solution
Crystallization occurs when the temperature of a solution is
Lowered so far that it’s salt concentration exceeds the maximum concentration the solution can hold at that lower temperature.
Crystallization of a solution is a problem in
Lithium bromide systems. When it occurs, the precipitated salt can restrict or block circulation
Lithium bromide absorption system
The Vapor pressures, depends on the temperature and concentration of
The solution in each section
The solution in the absorber is not in equilibrium since
More molecules of water vapour are absorbed than leave the solution. The solution becomes increasingly dilute
Two reasons for mixing dilute and concentrated Solutions together to form an intermediate solution
If the concentrated solution is sprayed directly on absorber cooling coils, it’s temperature would drop below that required to keep the salt in solution, and salt crystals would precipitate. Dilution prevents this
A higher solution flow rate is required in the absorber than in the concentrator. Some recirculation of the dilute solution is required to obtain this higher rate
Before a lithium bromide absorption system is shut down, it must go through
A dilution cycle. During the cycle to dilute and concentrated Solutions are mixed so that crystallization is prevented when the temperature drops during the shutdown.
Crystallization is of particular concern when
A power failure occurs and the unit cannot operate through its normal dilution cycle
Heat exchanger bypass
Pipe that leads from the concentrator to the absorber
The purpose of the heat exchanger bypass is to;
2
Limits the level of the solution in the concentrator by bypassing it’s solution directly back to the absorber
The bypass conducts the full flow of hot concentrated solution directly back to the absorber, in case the regular return through the heat exchanger is blocked by crystallization
The lower loop of the bypass tube is filled with solution at all times. This forms
A loop seal between the high and low pressure side of the system
The concentrator, absorber, and evaporator circulating pumps used on an absorption unit are of the ______
Hermetic type, to prevent infiltration of air in this High vacuum unit.
Most common cause of non condensable gases in absorption systems
An air leak somewhere in the unit. There are always some non condensable gases in an absorption unit
Non condensable gases has the following effects on absorption units
3
Reduced capacity and higher chilled water Outlet temperature
Crystallization
Corrosion
Purge chamber
Basically a small absorber.
Octyl alcohol charge
Wetting agent that helps the lithium bromide solution absorb water vapour, increasing the efficiency of the system. Should be added to the system
Log sheets
Best way to spot gradual changes in performance of the absorption unit.
The degree of Hazard in a refrigeration system depends on
5
Toxicity or flammability of refrigerant
Amount of refrigerant in system
Building occupancy class
Leakage probability of the system
Internal pressure of refrigeration system
Refrigeration safety limit controls set at to not more than
90% of the system high side design pressure
Safety valves installed on compressors usually discharge to
The low side of the system. So that when pressure is relieved, refrigerant is contained within the system
3-way safety valve
Places only one valve in service at a time. Allows one valve to be isolated so it can be tested, recertified, serviced or replaced.
CSA B-52 requires overpressure protection for lengths of pipe that contain liquid refrigerant
And can be isolated. Because, if a section of pipe is isolated and the temperature increases, hydrostatic liquid expansion May over pressurized pipe and fittings
Size of water tank needed to absorb ammonia charge
Tank must be large enough to contain the water and ammonia without overflowing. Tank must be kept warm so water does not freeze.
The owner of a refrigeration system must provide
Directions for operating system, precautions in case of breakdown or leakage. Instructions in a conspicuous location near as practical to the compressor
Anything moist attracts and dissolves ammonia, forming
Highly corrosive ammonium hydroxide. Exposure to ammonia, will damaged eye tissue, mucous membranes, and skin. If inhaled, damages the lungs
When ammonia is released to the atmosphere,
It dissolves readily in atmospheric moisture, forming a dense, caustic White Cloud. Never enter a cloud of ammonia.
At atmospheric pressure, ammonia boils at _____
It also has a very high ________
-33°C
Latent heat of evaporation. Even a small amount sprayed on skin or eyes can cause frostbite and chemical burns
Those exposed to ammonia regularly may have a reduced ability to detect it by smell. Therefore,
The nose should not be used to assess ammonia concentrations.
Personal protective equipment should be used when there’s a possibility of ____ exposure to ammonia
50ppm
Before hot work is performed on refrigerant lines
Must be purged using inert gas to reduce combustion and toxicity hazards. Must be leak detected after work is completed
Refrigerant cylinders must be stored
A safe distance from an open flame or hot surface. Handled with care, because of potential of frostbite due to escaping liquid refrigerant
Any routine maintenance activity that may create a discharge of refrigerant,
Should be performed with the aid of a buddy. But he should have water source available and means of signalling for emergency assistance.
If the operator draining oil is overcome by fumes or must abandon the work station due to emergency,
The drain could be left open causing a deadly release of refrigerant. A self-closing emergency stop valve, Deadman valve, could be used.
According to the International Institute of ammonia Refrigeration (IIAR), ammonia piping labels must have;
The word “ammonia” in Black letters on an orange background. An arrow indicates direction of refrigerant flow. Label also has several colour bands.
One band shows the refrigerant _____
Another band indicates weather the piping is ___
State:
Yellow=liquid
Blue=vapour
Both bands=liquid and vapour
High side or low side:
Red=high pressure
Green=low pressure
Valves should be identified with;
A permanent tag that will not fade or fall off.
Include; valve number, fluid, and State
Employers must provide, and ensure that
Workers wear, appropriate personal protective equipment. Impervious clothing, gloves, Splash proof goggles, face shield
If a refrigerant leak occurs and there’s no way to identify potential for workers or facility occupants exposed to respiratory hazard, the leak must be considered
LDLH and appropriate emergency response taken
According to OSHA, the PEL for ammonia is a Time weighted average of
50 PPM
Persons trapped in an ammonia leak should
Breathe as little as possible and open their eyes only when necessary. Some protection is possible with a wet cloth covering the nose and mouth. Remain close to the door, since ammonia Vapor Rises. Then proceeded to source of ventilation air by traveling against the airflow
Persons exposed to ammonia should be
Removed to a warm, fume free location, placed in a reclining position with Head and Shoulders elevated. Keep warm with blankets
If liquid ammonia contacts a person’s skin
Brought to a safety shower and the affected area flooded with water for 15 minutes. If no shower available, body parts immersed in relatively warm water, victim’s clothing may be frozen to the skin.
If ammonia enter the nose or throat but victim can still swallow
Drink large quantities of water
If eyes made contact with ammonia
Wash with clean water for at least 30 minutes. Eyelids must remain open
Workers in refrigeration plant should not wear
Contact lenses
Ammonia Refrigeration plants require an E2 plan if the total amount of refrigerant on-site equals
4.5 tonnes
When using a Deadman valve, never
Prop open the spring loaded handle. Propping it open renders it ineffective
The secondary refrigerant is often called the
Brine system. Describes the secondary refrigerant, which includes salts and glycols.
The use of a brine system prevents the main refrigerant, which may be hazardous, from
Circulating in public areas. Also allows refrigeration system to use multiple evaporators providing brine at different temperatures
Salinity test
Common brine System test. Uses a hydrometer to test the concentration of the brine
Ammonia refrigeration systems require regular testing of
The brine for any levels of ammonia. Presence of ammonia in the brine indicates a leak in the primary refrigerant piping and must be dealt with.
At least once per year, a sample of the secondary refrigerant should be
Drawn and sent out to a qualified lab facility for a complete analysis
Testing the specific gravity and freezing point of brine indicates
The strength, concentration, of the brine.
The secondary Refrigeration side of the system is the
Pump and piping that will move the brine to the area being cooled
When liquid evaporates it _________
When liquid condenses it ________
Absorbs heat
Releases heat
Ammonia is explosive in concentrations of
15 to 28% volume in air
