RACT Unit 21 Flashcards
_________ is the process of removing heat from a place it is not wanted and transferring it to a place where it makes little or no difference
refrigeration
Typically, the fresh food compartment temperature is about _____F
35F
Heat in refrigerator is transferred to a cold refrigerant coil and pumped by the system compressor to the condenser where it is released into the room much like using a ______ to move water
sponge
High temp refrigeration applications will normally involve box temperatures of ____F to ____F. High temp is commercial refrigeration
47 to 60F
Medium temp applications range from ___F to ___F. The household refrigerator fresh food compartment is medium temp
28 to 40F
Low temperature applications produces temperature below the freezing point of water, 32F. Low temp food storage applications generally start at ____ and go as low as ____F
0F and as low as -20F
The ________ in a refrigeration system is responsible for absorbing heat into the system from whatever medium is to be cooled.
evaporator
The heat absorbing process is accomplished by maintaining the evaporator coil at a ______ temperature than the medium to be cooled
lower
The evaporator operating at these low temperatures removes ______ and ______ heat
latent and sensible heat
Two important factors in understanding refrigeration are the refrigerating _______ temperature and _______ temperature
boiling temperature and condensing temperature
The _________ temp of the liquid refrigerant determines the coil operating temperature
boiling temperature
When the evaporator removes heat from the air and lowers the temperature of the air, _______ heat is removed
sensible
When moisture is removed from the air, _______ heat is removed
latent
The difference in temperature between the air being cooled and evaporator is directly related to the amount of ________that will be removed
moisture (humidity)
The greater the difference in temperature the more ______ is removed
moisture
The rate of heat exchange is governed by 3 things;
- The evaporator ______ through which heat has to be exchanged
- The _____ to which the heat is transferred.
- The ____ factor. Relationship between the medium giving up heat and the heat exchange surface.
- The _______ _________ between two mediums in which the heat exchange is taking place
- material
- medium
- film factor
- temperature difference
What type of evaporators were the first for cooling air?
natural-convection type
The stamped evaporator was one of the first designs to create a ______ pipe surface that improved efficiency
larger
Finned-tube evaporators are used more today than any other type of heat exchanger. It is efficient because the fins are in good contact with the _____ carrying the refrigerant
pipe
The longer the evaporator the more ______ drop associated with refrigerant flowing through them
pressure drop
The ______ (tonnage) of the heat exchanger can vary with its height and length
capacity
An efficient evap design incorporates an aluminum parallel-flow, flat-plate-and-fin configuration with small parallel channels inside the flat plate. The plates are _______, streamlined _____ , each one of which is split into smaller parallel ports
flattened , tubes
Name two ways to increase internal surface area;
- Increasing the number of ______ channels inside the flat plates
- increasing the number of ____ plates (decreasing space between them)
- parallel
2. flat
The vertical orientation of flat plates allows condensate removal to occur naturally, relieving any water drainage issues from the evaporator. This technology is being used with ______ as well as _________
condensers, as well as evaporators
Some of the benefits of this parallel-flow, plate-and-fin heat exchanger technology are as follows;
- reduces ______ pressure through the coil
- _____ system costs
- The all-aluminum coil, header, fins enhance _________ resistance
- Reduced internal _______ reduces ref charge
- Reduces ____ depth for the evap and cond which leads to easier cleaning
- reduced _____ area of cond and evap
- smaller _______ for the condensing unit
- static
- lower
- corrosion resistance
- volume
- coil
- face
- footprint
Field repair of leaking heat exchangers can be accomplished by;
- Recovering the ________
- Cleaning the leaking area with a ______
- Brushing the area with a ____ brush
- Pulling a vacuum with a ______ pump
- refrigerant
- solution
- wire brush
- vacuum pump
Evaporator evaluation can be considered as one of the ______ ____ in organized trouble shooting
starting point
The evaporator _____ heat, the compressor _____ it and the condenser _____ it
absorbs, pumps, rejects
How an Evaporator functions correctly;
- Warm ____ passes through a small
opening in the expansion device into the low pressure side - Due to pressure drop across the expansion device, ____% liquid and ____% vapour occurs
- As sat. mix moves through the evaporator more liquid changes to a ______ . This is called _____ and is a result of heat absorbed into the coil from whatever medium the evaporator is cooling
- Finally, near the end all the liquid is boiled to a vapour and the refrigerant is known as _______ vapour
- liquid
- 25% vapour 75% liquid
- vapour, boiling
- saturated vapour
The latent heat absorbed during the change of state is much more ________ than the sensible heat that is added to the vapour leaving the coil
concentrated
To get max eff from the evap heat exchange, some evaporators are operated full of liquid or flooded and are equipped with a device to keep the liquid refrigeration from passing to the _______
compressor
The _______ evaporators are specially made and normally use a float type metering device to keep the liquid level as high as possible
flooded
If the evaporator is not flooded when the refrigerant starts out as a partial liquid and boils away to a vapour in the heat exchange pipes it is known as a ____\ type or direct expansion evaporator
dry
To check performance of a dry type evaporator the technician would first make sure that the refrigerant coil is operating with enough _______ inside the coil
liquid
To determine if enough liquid is inside the coil to check evaporator performance , the technician must calculate the evaporator ________ which is done by comparing the ______ temp of the refrigerant inside the coil with the line temp leaving the coil. The difference should be __ to ___F
superheat
boiling temp
8 to 12F
The difference in temperature between the boiling refrigerant and the evaporator outlet temp is known as ________ ________
evaporator superheat
Super heat is the ________ heat added to the vapour refrigerant after the change of state has ocuxted
sensible
Superheat is the best method of checking to see when a refrigerant coil has a ______ level of refrigerant
proper
When a metering device is not feeding enough refrigerant to the coil, the coil is said to be ______ and the superheat is greater
starved
A hot pull down is essentially an excessively ________ evaporator
loaded
When the refrigerated space has been allowed to warm up considerably, the system must got through a ____ ________
hot pulldown
When a system is under a hot pull down, the technician should allow the system to _________ before trying to calculate an evaporator superheat reading
stabilize
When a dry type coil is fed too much refrigerant, not all refrigerant changes to a vapour. This is thought of as a _______ _____
flooded coil
Multicircuit evaporators are used when the coil would become too _____ for a single circuit
long
A dry type evaporator has to be as full as possible with refrigerant to be ______
efficient
Each circuit in a multi circuit evap should be feeding the same amount of refrigerant. If this needs to be checked the service technician can check the _____ ______ tap for the boiling pressure which can then be converted to to temperature. The temp will then have to be checked at the ______ of each circuit to see if any circuits are being _______ or ________
common pressure tap
outlet
overfed or starved
Some reasons for uneven feeding on a multi-circuit evaporator are;
- blocked ________ system
- dirty _____
- uneven ____ distribution
- coil circuits of _______ lengths
- distribution
- coil
- air
- different
With larger evaporators and longer suction lines it is best to measure the refrigerants pressure at the _______ outlet not at the compressor ______ valves -when measuring evaporator superheat
evaporator outlet
compressor service valves
It is best to measure refrigerant pressure at the same location that the evaporator ______ temp is taken when measuring superheat
outlet
_____ taps can be used to gain access to evaporator outlet pressure for more accurate readings
line
A different type of evaporator is required for liquid cooling. It functions much like the one for cooling air and is normally a ___ type expansion evaporator in smaller systems.
dry type
Liquid cooling evaporators have saturated liquid/vapour refrigerant in the shell and the water to be chilled flows in the _____ bundles
tube
Liquid cooling evaporators use a ____ side float to meter the refrigerant into the shell of the evaporator to maintain the proper refrigerant level
low side float
Liquid cooling evaporators have more then ____ refrigerant circuit to prevent pressure drop
more than one
Liquid evaporators have a normal superheat range similar to air-type (__ to ___F)
8 to 12F
List two problems that are typical on the liquid side of the evaporator;
- mineral _______ build up on the liquid side, acting as an insulator and causing a poor heat exchange
- poor _______ of the liquid to be cooled where a circulating pump is installed
- mineral deposits
2. circulation
A pull down can not be _______
rushed
Low temperature evaporators uses for cooling space or product to below freezing are designed differently because they require the coil to ______ below ________
operate below freezing
Low temp coils have wider spacing than medium temp coils to prevent ____ flow blockage when freezing occurs
air flow blockage
Low temp evaporators are normally ____ type. They have one or more ____ to circulate air across the coil
dry type
fans
The defrosting of a low temp evaporator coil has to be done by _______ the temp of the coil above freezing to melt the ice
raising
defrosting of low temperature evaporators can be accomplished with heat from inside the system using hot gas from the discharge line of the compressor or using __________ resistance heaters
electric
Hot gas defrost is accomplished by routing a ___ _____ line from the comp discharge line to the outlet of the expansion valve and installing a solenoid valve to control the flow
hot gas
When hot gas enters the evaporator to melt the ice it will soon turn to a liquid. This liquid will either go to an _________ or flood the compressor crankcase if there is no _________
accumulator
flooding compressor can cause a lower oil level in the crankcase and score the bearings in the compressor which is known as ___________ washout
bearing
Electric defrost heaters are often embedded in the actual _______ _____ and cannot be removed if they burn out
evaporator fins
If the evaporator fan is not turned off during the defrost, tow things will happen;
- The heat from defrost will be transferred directly to the _________ space
- The ______, conditioned air will slow down the defrost process
- conditioned
2. cold
Defrost occurs when the thermostat is ________ and the refrigeration system cycles _____
satisfied
off
Medium temperature refrigeration systems operate with evap temperatures below ___F and box temperatures above 32F
32F
The evaporators function is to efficiently transfer heat heat from the surrounding area to the phase-changing ________ flowing through the ________
refrigerant
evaporator
As the evaporators heat load changes, an _______ type _________ device at the entrance of the evaporator will control the flow of refrigerant through the evaporator
expansion type metering device
The expansion type metering device will ensure that all of the liquid in the evap has fully _________ prior to exiting
vaporized
Many low temp commercial and industrial evaporator assemblies consist of:
- The evaporator _____
- F___
- An ______ device
- _______ heaters
- A liquid line ________ valve
- The evaporator coil
- fans
- an expansion device
- defrost heaters
- Liquid like solenoid valve
Although frost is unavoidable on an evaporator it is the most common cause of ___________ in the evaporator coils
inefficiency
The dew point temperature is the temperature where ________ starts to form
moisture
When the evaporators coil temp drops below the dew point temperature, moisture begins to collect on the evaporators coil surface. Eventually this moisture turns to _____ if temp continues dropping
ice
The control system is based around a refrigerant __________ cycle
refrigerant pump down cycle
In a pump down cycle, once the refrigerated space temperature is satisfied, the controller deenergizes (closes) the liquid line _______ valve and turns the evap fans off. The comp continues to run until any liquid refrigerant in the evap and suction line is fully vapourized and pumped to the _______ of the system
solenoid valve
high side
During the ref pump down, the temperature of the evaporator coil will _____ because of the low pressure created in the coil from the pump down process.
This residual/flywheel effect will drop the _________ below the refrigerated space temp
drop
coil temp
Once the coil temperature is below the space temperature in a pump down cycle, the controller will then cycle the evaporator fans back on without starting the _______
The space temp is now controlled by only the _____ cycling on and off blowing air over a very cold evaporator coil
compressor
fans
Frost removal by Sublimation:
During punk down, the evaporator fans are blowing air through a very cold compressor coil with ______ accumulating on its surfaces. The frost will change its phase to vapour through the process of ________.
frost , sublimation,
_________ is a process where a solid changes directly to a vapour, skipping the liquid phase
sublimation
The sublimation process reduces the frost on the coil _______
naturally
As frost changes to a vapour during sublimation, it also returns valuable _______ to the refrigerated space to maintain higher humidity and reduce product shrinkage
moisture
Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):
The evaporator efficiency controller uses a proprietary defrost control algorithm that eliminates the dependency on _____ for defrost _______
time
initiation
Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):
Instead of depending on time, evaporator efficiency controllers monitor evaporator coil _______ to initiate defrost and also minimize effects of defrost on space temp
efficiency
Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):
Maintaining a lower TD between the evap coil and the refrigerated case throughout the run time will reduce rate of frost formation on the ____. This will lead to extended time between defrost and save _____.
coil
energy
Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):
Important functions of the evaporator efficiency controller;
- Monitors evaporator efficiency and ______
- Initiates defrost once the evap coil efficiency is less than ___%
- creates an initial evap profile from series of __________
- identifies the temp relationship between the evaporator coil and refrigerated space temp through two _______ locations
- Monitors evaporator efficiency and performance
- Initiates defrost once the evap coil efficiency is less than 90%
- creates an initial evap profile from series of measurements
- identifies the temp relationship between the evaporator coil and refrigerated space temp through two sensor locations
