140301aB - Load Calculations Flashcards
What are the 4 temp range categories and their respective ranges?
- Med temp 28F to 40F
- Low temp 0F to -20F
- Ultra Low Temp (ULT) -40F to -120F
- Cryogenic Temp -58F to -148F
What is medium temp used for?
Dairy, meats, deli, etc
What is low temp used for?
Frozen meat, vegetables, dairy, and ice cream
What is ULT used for?
Laboratories, pharmaceuticals, biological samples, vaccines, and the production of certain metals and for temperature testing
What are some applications of cryogenics? How do they reach these temperatures?
Used by engineers and scientists to study the effects of very low temps on materials. MRI, NMR spectroscopy, FLIR cameras, etc. Liquid nitrogen and liquid helium are used to reach these very low temps
What is capacity and how is it measured? What is a ton of cooling equal to?
Capacity is a measure of a system’s ability to reject heat, measured by the NRE in tons of cooling, nominal BTUh or kWatts. 1 ton of cooling is 12,000 BTUh or 3.517kW
What is the purpose of any refrigeration system?
Remove heat from where it is not wanted and send it to where it is not objectionable
What does the configuration of a refrigeration system refer to?
The type of system, eg: cascade, air cooled, water cooled, ammonia absorption, centrifugal chiller, etc
What 3 terms might a system’s category include?
- Residential (Domestic)
- Commercial
- Industrial
Define the 2 main categories of HVAC and what they include
- Packaged Units: RTUs, Computer rooms, heat pumps
2. Built Up systems: split ACs and ductless splits
Define: Design Balance Point
The temperature at which the heating or cooling capacity of a unit matches the rate of heat or cooling loss in that building
Define: Direct Expansion System (DX system)
A refrigerant vapour compression system utilizing an evaporator in direct contact with the medium it cools (air, water or glycol etc) also known as dry expansion system
Define: Enthalpy
The amount of heat in 1lb of a substance starting from a zero base point and at a universally accepted temperature. -40C/F is the base point for refrigerants
Define: CPR Valve
Crankcase Pressure Regulator Valve. Operates on basis of CRO, close on rise of outlet. As crankcase pressure rises, valve closes to prevent compressor motor from overloading on pull down or while coming off defrost. Begins to open again once outlet pressure starts to drop
Define: EPR Valve
Evaporator pressure regulator valve. Operates on ORI basis, opens on a rise of inlet pressure, closes on a drop in inlet pressure. Keeps evaporator at a set pressure/temp, above the SST/pressure of a rack system
Define ORD Valve in relation to head pressure control
A Valve piped in between comp discharge and condenser outlet (receiver inlet) but after ORI. Part of head pressure control system with ORI Valve
Define ORI Valve for head pressure control
valve piped downstream of condenser and before ORD connection and receiver. Opens as condenser pressure rises, otherwise allows ORD to bypass hot gas to reciver
Define OROA Valve
Combination ORI and ORD valve used in head pressure control, has 3 ports and controls condenser flow to receiver based on condenser outlet pressure
Define packaged unit
a single unit containing all components, high and low side
Define Refrigerant
A substance able to change from a gas to liquid and back again and that has the ability to carry heat
Define RTU
rooftop packaged unit that can heat, cool, and provide fresh filtered air to a space
Define temp difference (TD)
the diff in temp between two substances such as temp of air in space and refrigerant in the evap
Define VRF unit
a unit that uses one condenser with multiple evap units. Compressor speed control changes the refrig flow to the evaps
Define Volumetric Capacity
performance rating of a certain compressor based on capacity
How can you categorize refrigeration applications? 6 points
capacity category configuration remote install purpose temp range
What functions can a computer room AC perform?
Cooling, humidification and dehumidification and filtration
What extra components do heat pumps have compared to an AC system
they have a reversing valve/three way valve and some models have an accumulator
What are the different types of heat pumps? 3 total
air to air
water to air
ground to air
What is a built up system? 4 points
- a system built on site
- customized to the requirements of the job
- Allows the amount of cooling to be altered
- Gives designer flexibility to mix and match components based on requirement
What is evap selection based on? 5 points
Capacity Setpoint SST Space limitations temp diff TD
What TD would an evap need to maintain humidity?
+/- 1Fdegree
What if equipment is sized too large?
It will not run long enough to maintain a constant temperature
What if equipment is sized too small?
it will run longer to try and maintain setpoint, sometimes not shutting off
What is a balancing graph useful for?
Balancing condenser and evaporator capacity
What are some variables in refrigeration compressor selection? 3 categories
-overall capacity
Evap:
- Setpoint
- SST/low pressure
- suction line press drop
- suction gas temp
Compressor:
- comp ratio
- displacement
- volumetric efficiency
- voltage
Condenser:
- condenser size
- temp of condenser medium
- quantity of condensing medium
How many evaporators and compressors should you plot on a balancing graph when seeking the design balance point?
at least 3
What are the selection criteria for refrigeration condensers? 5 points
- BTU/h capacity
- quantity of cond medium
- temp of cond medium
- low side pressure/SST
- compressor size
Why is condenser correction factor (CCF) vital to know?
So we can calculate how much higher the cond capacity must be in relation to the evap
How is CCF applied in relation to evap tonnage? Formula to calculate what?
Cond Capacity = Evap tonnage x CCF
How is a receiver selected/sized?
It is sized to hold the entire refrigerant charge of a system
How is an accumulator sized?
According to the type of metering device and the refrig charge. Keep pressure drop to a minimum, and return oil and refrig without affecting flow
How is accumulator sized with Txv?
Sized based on 50% of the charge
How is accumulator sized based on fixed orifice?
Sized to 70% of charge
How are metering Devices selected? 5 criteria
- BTUH capacity
- Refrigerant type
- SST
- pressure drop across valve (higher drop = higher capacity)
- liquid refrig temp entering valve (lower temp = higher capacity)
How are flow regulating components selected? eg EPR valve or solenoid
Based on size of port through which refrig passes and sometimes refrig used
How are filters, separators, and piping selected?
They must be sized to match the capacity of the evap directly or according to correction factor
What are the 3 most important components in a system?
- compressor
- evaporator
- condenser
3 criteria for comp selection?
capacity
suitability
design factors
What is the maximum accepted design pressure drop of a suction line and why does this matter
2degF, because SST directly relates to compressor capacity, so higher suction line pressure drop = loss of capacity
Why does suction pressure affect capacity?
As suction pressure drops, the refrig specific volume increases and the density decreases. (lower pressure = less particles of refrigerant in a given space) A comp pumps the same volume of refrig at all times so lower density = comp moves less refrig = system is less efficient
What are 3 key factors in compressor selection?
- capacity is key
- voltage
- application
What is the most important component in a system?
The compressor, but all components must be balanced for it to properly perform
2 methods of capacity control?
hot gas bypass and VRF
How does hot gas bypass work?
Hot gas from discharge line is introduced to evap inlet via solenoid and modulating control valve. The hot gas keeps the evap pressure and temp at a certain point even when the space is satisfied
5 benefits of hot gas bypass?
- better oil return
- lower compression ratio
- keeps evap ice free
- cooler compressor windings
- reduce short cycling of comp
Downside of hot gas bypass? Explain answer
less energy efficient than other capacity control methods because unit operates when there is no load
What is a VRF system suitable for?
zoning applications since each room can have its own evap and temp control connected to one central CU
Benefits to VRF system?
zoning, units can heat or cool while using one CU, rotary compressors allow speed modulation, more energy efficient due to less stopping and starting (runs longer) so saves $$
How do refrigerants affect compressor suitability?
Different refrigerants have different compression ratios
How to convert psig to psia?
Add 14.7 to psig
Why is it important to calculate compression ratio?
To ensure the compressor is operating in the conditions it was designed for
What would happen if system compression ratio doubled from 10:1 to 20:1? 3 points and why
- The compressor would struggle to handle the pressure increase
- Lower its volumetric efficiency
- shortening its life.
Why? You are shoving twice as much in and out per stroke of a piston
How does compression ratio affect compressor construction?
Greater ratio requires a stronger built compressor to handle the larger differential pressure (low temp refrig)
How does compressor speed affect construction?
higher motor RPMs require stronger built components to handle increased velocity and kinetic energy. Stronger motor shaft, bearings, crank, pistons and other components may need modification
How does refrigerant type affect comp construct?
Refrigerants at diff temps and pressures react differently to certain metals and gaskets. eg ammonia attacks copper so steel is used instead. R-123 destroys plastic so other materials are used
How does voltage affect comp construct?
higher voltage means smaller wire required for motor construction. Motor starters also affect internal motor wirings eg across the line start vs VFD
How does environment affect comp constuct?
Dirty dry atmospheres not great for open drive compressors since motor shaft and bearings will be exposed or shaft seal can be damaged = leak. water jackets for hot rooms or insulation jackets for condensation and rust prevention
Name some features of compressors
oil stub sight glass service valve flange oil heater well sensor and thermistor wells
What are oil stubs for?
joining oil return lines for multi compressor systems that are piped together
What is a comp sight glass used for?
for mechanics to monitor oil return to comp. May be replaced with oil float control
What is a service valve flange for?
for larger bodied compressors if they must be isolated for repair or service work. can separate high and low side of system
Why is an oil heater well useful
to install external oil heaters or CCHs to avoid liquid refrigerant accumulating in crankcase. also warmer oil lubricates better
What are some evap selection factors?
materials
fin spacing
evap TD
refrig feed methods eg DX, flooded, liquid overfeed
Evap materials matter why?
cant use copper with ammonia. Fins may need a protective coating to resist corrosion
why is fin spacing important
application determines fin spacing. Colder the temp larger the fin spacing. Spacing also affects dehumidification
What is TD a function of?
Evap and compressor capacity
How does coil depth affect TD?
Deeper coil = higher TD
How does refrigerated space humidity relate to TD?
higher TD = lower RH%. Greater TD removes more moisture
What problem can an oversized evap cause?
low refrig velocities and poor oil return, oil logging in evap
What can an undersized evap do to TD?
same as a thicker coil, cause low SST and high TD
3 methods of refrigerant feed?
Direct expansion
flooded
liquid overfeed
Other name for direct expansion and why?
Dry expansion because all the liquid should be completely boiled off and superheated prior to exiting evap
How efficient are DX evaps compared to other systems?
the least since 80% of the evap is used for latent heat transfer and 20% is used to superheat
Which evap type can use EPR valves and what do they do?
DX systems, EPR valves are ORI and open when the evap pressure rises, they close when the pressure drops
Benefits and drawbacks of a flooded evap?
benefits are most efficient due to most interior wetted tube surface for heat transfer
drawback is requires the most liquid refrigerant holding charge.
How do flooded evap systems meter refrig?
High or low side float
How does a liquid overfeed system work?
also know as the multiple evap system, more refrig than can be evaporated is fed to multiple evaps. the system recirculates the refrig constantly via an accumulator system. all evaps have solenoids, and a DP relief valve is used with a pump when some evaps are at SP. often they have hand expansion valves or fixed orifice
What is a condenser?
a heat transfer device responsible for rejecting all the heat that the system absorbs plus HoC. heat goes from high pressure high temp gas to the ambient medium
Why is a head pressure control important?
It allows the condenser to maintain a set head pressure, keeping the comp windings from overheating. keeps comp operating more efficiently both mechanically and electrically. low head pressure starves the evap
How does a head pressure controller work?
different valves work to maintain discharge pressure by bypassing hot gas around the condenser. only difference is whether the valve controls condenser inlet pressure or receiver inlet pressure
3 types of head masters?
modulating valves
ORD and ORI valves
3 way OROA valve
How does a modulating valve headmaster work?
it is an ORI placed between hot gas and condenser. on startup discharge line cannot flow to condenser due to ORI, so via bypass valve opening on drop of outlet flows to receiver. Once discharge rises the ORI opens allowing flow to condenser and eventually closing ODO bypass valve.
How does ORD and ORI combo work?
they work to control the receiver pressure. ORI is placed on outlet of cond and ORD between receiver inlet and discharge but after ORI. as cond pressure falls ORI closes causing ORD to open to send hot gas to receiver
How does OROA control head pressure and what does it stand for?
Open on rise of outlet and differential is a combo ORI and ORD valve with 3 pipes. bottom of valve T goes to condenser outlet, flow goes in from discharge line and out to receiver. bypasses hot gas when cond pressure falls
Two ways to control head pressure in winter?
condenser fan cycling and condenser flooding to reduce surface area (headmaster)
