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127 Cards in this Set
- Front
- Back
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Can scroll compressor be liquid charged? |
Be careful of sweeping the oil |
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What is the purpose of the evaporator externalized equalizing line for? Is it an opening force or a closing force? |
An externally equalized line on a thermostatic expansion valve (TXV) allows for pressure equalization between the valve's diaphragm chamber and the evaporator outlet. This ensures accurate control of the refrigerant flow rate based on the temperature and pressure conditions at the evaporator outlet. By equalizing pressures, the TXV can maintain proper superheat levels, improving system efficiency and performance. The force given from the equalization line is a closing force because (equalized suction pressure) + (spring) = (closing force) |
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In to increase what on a txv? |
What are you controlling? Increase superheat |
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Multiplex: How do they control condensing pressure?
Where is the number 1 fan? |
Fan controlled speed by outlet temperature, or temperature in the middle of the coil. fan staging with pressure switches turning on more and more fans as needed. (-) lots of leaks Closest to the condenser header is the number 1 fan, this fan will have speed control. |
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What is a condensate line max velocity? |
120-150 ft/min |
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What do you size liquid lines to? |
Max 300 ft/min If the liquid line is too fast it will hammer the liquid line solenoid |
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What does 90 at 90 mean? |
It means the reciever must be no more than 90% full at 90F but that 90% IS 100% of the charge. |
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If a system has two evaporators that have the same back pressure what can be said of the operating temperatures? |
They are the same. They would need an EPR (ORI) to have different temperature evaporators. Higher temperature evaporator will have an EPR and the lower temperature evaporator will have a check valve |
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If a system has two evaporators that have the same back pressure what can be said of the operating temperatures? |
They are the same. They would need an EPR (ORI) to have different temperature evaporators. |
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Do EPRs (ORI) enforce a minimum or a maximum pressure in the evaporator? |
They enforce a minimum pressure. The coldest evaporator will not have an EPR. The medium temp will so that the EPR will hold back gas till the higher suction pressure is reached. |
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What is temperature difference? |
The difference between inlet air and saturated suction temperature. Temperature drop is (supply temperature) - (return temperature) |
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What is the hinge point on a evaporator? |
It is the point where box temp = SST. (0 capacity @ 35F SST) The evaporator capacity is linear. So (20,000 btu/hr @ 25 SST) Or (2,000 btu/hr @ 34F SST) ———————— How do you plot an evaporator capacity line?
(1) find your system capacity and evaporator temperature difference. 12,000 BTU/hr 10F difference (2) plot a point at 34F and 0 capacity and another point at the ideal balance point. (12,000 btu/hr and 24F SST) Connect the line (3) plot all the evap and compressor lines. (4) choose the system that has the same or lower SST. choose the system with the same or higher capacity |
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What is temperature difference? |
The difference between inlet air and saturated suction temperature. Temperature drop is (supply temperature) - (return temperature) |
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What is the hinge point on a evaporator? |
It is the point where box temp = SST. (0 capacity @ 35F SST) The evaporator capacity is linear. So (20,000 btu/hr @ 25 SST) Or (2,000 btu/hr @ 34F SST) |
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For a compressor, if the SSP is high then the volumetric efficiency is high because the piston does not have to travel down far till the piston pressure is lower than the suction pressure. So….on the capacity vs SST chart as the suction pressure drops what happens to the compressor capacity? (Bonus: What happens to the capacity of the evaporator as the temperature difference increases?) |
The capacity of the compressor decreases as the sst decreases. (Bonus: The capacity of evaporator will be increasing with higher temperature difference and at some point the two capacity line will meet and this is the system balance point.) |
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Why is getting a 2.5 ton coil acceptable in a location with no humidity? (Fixed orific system this works better) |
If the temperature difference is smaller then you wont remove as much humidity as quickly. |
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What does pressure drop on a system do to your evaporator performance curve? The temperature penalty will shift your SST... |
To the left. It will decrease your balance point with your chosen compressor. |
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How does head pressure control utilize the ORI? Aka Hold back valve aka flood valve |
When the condenser pressure drops it hold back the refrigerant because it is enforcing a minimum condensing pressure. This will drop the receiver pressure, dropping the pressure across the TXV, dropping the capacity. (TV capacity dependent on pressure across the valve and subcooling) So we add an ORD in parallel with the condenser to pass hot gas past the condenser and push on the liquid and drive up the capacity of the txv. |
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To drive up the head pressure what would you do? |
Block the condensing inlet, not the outlet because your motor will die... supposedly. |
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What is the equation for total capacity of the system? |
Capacity = NRE (net refrigeration effect) x MFR (mass flow rate) |
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What is an OROA? |
ORD and an ORI together A head master uses ambient temperatures |
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What do you need when you have 2 compressors in parallel? (Bonus: What does a multiplex system have above the normal components?) |
You need an oil equalization line (or oil management system). A common discharge header, a common suction header, and a common liquid line, 2 compressors. |
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How does oil management work |
Oil is separated out of the discharge line, suction pressure is piped to the top of the oil reservoir but the OCV allows for a slightly higher pressure (+15 psi) inside of the oil reservoir… this allows for the flow of refrigerant to the oil level control, which is a float that will allow oil in if the compressor oil level drops.
Bonus: beads float on top of the oil and you can see them in the sight glass. Bonus Bonus: the 10-15 psi is higher than the highest crank case pressure pressure in the system. So if there are low temp cases and medium temp cases the oil differential will be 10-15 psi higher than the medium temp |
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What is the benefit of a pilot operated EPR vs a EPR?
SORIT (suction throttling valve) De-energized then they are functioning as a suction stop valve |
EPR is not serviceable You can change out the sORIt guts without removing the body unlike a standard EPR. The sORIt is bigger than the EPR. THE sORIt has a smaller pressure drop than the ORI |
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The solenoid on the sORIt can be used to change the box temp from medium to low temp on demand. However what is it normally used for? |
Suction stop in hot gas defrost |
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What does PID stand for? |
Proportional Intergel Derivative? Proportional control will swing with a +/- 5F swing PI will measure how far away from the set point. PID will look at how far away it is from setpoint. +/- 0.5F |
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What is the benefit of a stepper motor EPR? |
It can find and maintain a box temperature by modulating the evap suction pressure ….gradually. PID will limit hunting. A standard EPR can have overshoot, therefore, the suction pressure increases and decreases rapidly….so the unloading system will turn on/off compressors, and then the condenser will turn on/off fans, and the TXV will also hunt…. The system will loose efficiency |
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What is the benefit of a stepper motor EPR? |
It can find and maintain a box temperature by modulating the evap suction pressure ….gradually. PID will limit hunting. A standard EPR can have overshoot, therefore, the suction pressure increases and decreases rapidly….so the unloading system will turn on/off compressors, and then the condenser will turn on/off fans, and the TXV will also hunt…. The system will loose efficiency |
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What is the most efficient defrost? |
Reverse hot gas defrost |
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What are the two most common initiation types for defrost? |
Temp-time Time-time |
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How does a reverse hot gas defrost work? |
Remove all refrigerant from evaporator by closing liquid line solenoid, then energize hot gas solenoid, de-energize sORIt, the gas will move through the check valve past the txv, move through the check valve past the Suction line Solenoid. The refrigerant will then flow into the suction line header. This is possible because De-energizing the sORIt will put the valve into Suction stop. This will make sure the sORIt does not short cycle the refrigerant once the hot gas enters the evaporator through the hot gas solenoid. The hotgas that is now liquid and will enter the suction header. This can be done because of the defrost differential valve. Defrost differential valve maintains 10-15 psi below discharge pressure to ensure refrigerant will flow backwards in a defrost. This valve will fail to the defrost pressure. One the thermistor beside the txv senses hot temperature it will stop defrost. |
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What is drip time? |
This will delay evaporator fans and is included in the recovery time. |
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What is the other names for 2 pipe and 3 pipe systems? |
Reverse flow hot gas defrost is = 2 pipe Conventional flow hot gas defrost = 3 pipe |
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What is the other names for 2 pipe and 3 pipe systems? |
Reverse flow hot gas defrost is = 2 pipe Conventional flow hot gas defrost = 3 pipe |
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Conventional flow first strike hits the drain pan first? T/F? |
T |
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For conventional flow, a suction solenoid is used to make sure liquid is not returned to the compressor. Where is the refrigerant diverted to? |
A re-evaporator or an accumulator. You will need a crank case pressure regulator. You will set this up at at highest load and use the service factor and allow it to go to 1.25 FLA. |
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Is suction solenoid and suction stop the same thing? |
Suction stop is in the sORIt Suction solenoid is for conventional flow |
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What is the discharge velocity? |
750 ft/min -1500 ft/min |
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Split condenser has 2 separate coils within it. Once you isolate one what happens to the condensing pressure? |
It will increase the condensing pressure.
The condenser enters into split at 55F. It exits out of split if the pressure gets too high even after all condenser fans running it will hit the high pressure set point. It can also exit split if the ambient goes above 55F |
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How do you cycle the reversing valve on a heat pump if the slide valve is stuck? |
You block the condenser and drive up the condensing pressure then cycle the valve back and forth.
The reversing valves always face up. |
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If there is a water heater (reclaim device) desuperheats the discharge gas before going the the condenser? What are they piped in? |
Series |
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What is a satellite compressor? |
It is a dedicated independent compressor. It usually runs the low temp. Usually not that many loads on a grocery store need to run such a such low pressure. |
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Compressor ratio is rated as? |
Hp/ton 1 hp/ton for high temp 1.25 for a mid temp cooler 1.5 for low temp freezer |
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If the satellite fails you can run the freezers of the main rack? |
True. Usually the satellite will run the low temp because it is sized for the evaporator sst. |
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Why do they have mechanical subcooling? |
To ensure you have subcooling. This is important in Kelowna where it can be so hot that you will lose capacity. You need to remember that the subcooler is a load on the compressor. It is no different than any other evaporator on the system. Too much subcooling will be paid in discharge pressures if you dont strike a balance. |
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What is the temperature split? |
Condenser Temperature in vs SDT |
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What is the temperature split? |
Condenser Temperature in vs SDT |
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What is the subcool-o-matic? |
It has an EEV that monitors the superheat leaving the “evaporator” (mechanical subcooler) while ensuring proper subcooling is given to the liquid refrigerant headed for the liquid header. It is fed by the medium temperature header. |
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What are the three different modes an epr can operate in? |
(1) Hold back (2)modulating (3) suction stop |
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A stepper motor EPR valve works off of a PI loop and not a pid? |
True. Just PI
Proportional integral differential |
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If increase spring pressure on an EPR what are you increasing? |
You are holding back the evap more gas. In-to-increase evaporator pressure. |
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If you show up to a multiplex system the service call is usually one case. So the issue is usually the EPR, the TXV, or something between the two like a plugged TXV strainer screen. What else usually effects one unit? |
Product over-loading, air fowling, dirty honey comb… door open, fans |
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If you show up to a multiplex system the service call is usually one case. So the issue is usually the EPR, the TXV, or something between the two like a plugged TXV strainer screen. What else usually effects one unit? |
Product over-loading, air fowling, dirty honey comb… door open, fans |
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Refractonomer can tell you the ratio between…. |
Mineral oil and POE.
Spread some on the lense and look up. |
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Wax in the system might accumulate where in the system that would make one case not work? |
At the strainer in the txv |
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Wax in the system might accumulate where in the system that would make one case not work? |
At the strainer in the txv |
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A loose bulb will cause what symptoms? |
It will flood and then starve. Hunting
It waits to get cold to shut off… it throws refrigerant at the evaporator. The compressor floods, then foams, then the oil leaves the compressor. |
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Wax in the system might accumulate where in the system that would make one case not work? |
At the strainer in the txv |
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A loose bulb will cause what symptoms? |
It will flood and then starve. Hunting
It waits to get cold to shut off… it throws refrigerant at the evaporator. The compressor floods, then foams, then the oil leaves the compressor. |
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Where do you put the txv on a pipe? |
Mickey mouse ears (10 and 2)
If the pipe is small you can stick it on the top. |
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Wax in the system might accumulate where in the system that would make one case not work? |
At the strainer in the txv |
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A loose bulb will cause what symptoms? |
It will flood and then starve. Hunting
It waits to get cold to shut off… it throws refrigerant at the evaporator. The compressor floods, then foams, then the oil leaves the compressor. |
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Where do you put the txv on a pipe? |
Mickey mouse ears (10 and 2)
If the pipe is small you can stick it on the top. |
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Check evaporator pressure to leaving temperature for superheat? |
If you have an EPR you wont see a pressure problem but superheat will be off. EPR enforces a minimum so blocked strainer wont show up as low back pressure BUT you will get a small temp drop across the evaporator and high super heat. |
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Wax in the system might accumulate where in the system that would make one case not work? |
At the strainer in the txv |
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A loose bulb will cause what symptoms? |
It will flood and then starve. Hunting
It waits to get cold to shut off… it throws refrigerant at the evaporator. The compressor floods, then foams, then the oil leaves the compressor. |
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Where do you put the txv on a pipe? |
Mickey mouse ears (10 and 2)
If the pipe is small you can stick it on the top. |
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Check evaporator pressure to leaving temperature for superheat? |
If you have an EPR you wont see a pressure problem but superheat will be off. EPR enforces a minimum so blocked strainer wont show up as low back pressure BUT you will get a small temp drop across the evaporator and high super heat. |
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If you find 3 out of the 8 compressors are off what caused it? |
Cause: Oil level, oil feed level sensor, oil pressure
Single phasing would impact all High pressure is a condenser problem effects all compressors Low pressure effects all |
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Wax in the system might accumulate where in the system that would make one case not work? |
At the strainer in the txv |
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A loose bulb will cause what symptoms? |
It will flood and then starve. Hunting
It waits to get cold to shut off… it throws refrigerant at the evaporator. The compressor floods, then foams, then the oil leaves the compressor. |
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Where do you put the txv on a pipe? |
Mickey mouse ears (10 and 2)
If the pipe is small you can stick it on the top. |
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Check evaporator pressure to leaving temperature for superheat? |
If you have an EPR you wont see a pressure problem but superheat will be off. EPR enforces a minimum so blocked strainer wont show up as low back pressure BUT you will get a small temp drop across the evaporator and high super heat. |
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If you find 3 out of the 8 compressors are off what caused it? |
Cause: Oil level, oil feed level sensor, oil pressure
Single phasing would impact all High pressure is a condenser problem effects all compressors Low pressure effects all |
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How does a oil feed sensor work? |
It times the oil float and if it runs too long it will shut off the compressor |
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Wax in the system might accumulate where in the system that would make one case not work? |
At the strainer in the txv |
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A loose bulb will cause what symptoms? |
It will flood and then starve. Hunting
It waits to get cold to shut off… it throws refrigerant at the evaporator. The compressor floods, then foams, then the oil leaves the compressor. |
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Where do you put the txv on a pipe? |
Mickey mouse ears (10 and 2)
If the pipe is small you can stick it on the top. |
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Check evaporator pressure to leaving temperature for superheat? |
If you have an EPR you wont see a pressure problem but superheat will be off. EPR enforces a minimum so blocked strainer wont show up as low back pressure BUT you will get a small temp drop across the evaporator and high super heat. |
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If you find 3 out of the 8 compressors are off what caused it? |
Cause: Oil level, oil feed level sensor, oil pressure
Single phasing would impact all High pressure is a condenser problem effects all compressors Low pressure effects all |
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How does a oil feed sensor work? |
It times the oil float and if it runs too long it will shut off the compressor |
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Control issues can be mitigated by leaving what in the control section? |
An accurate diagram |
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Suction filters are infront of each compressor, if one compressor dies you can add filters in front of all compressors to filter out acid. |
Filter dryers are installed for 72 hours. Suction driers are installed on a 45 degree into the side and 45 degrees out of the bottom. |
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Suction filters are infront of each compressor, if one compressor dies you can add filters in front of all compressors to filter out acid. |
Filter dryers are installed for 72 hours. Suction driers are installed on a 45 degree into the side and 45 degrees out of the bottom. In through the outside and then it gets pulled through the center.
BONUS: put a high flow filter on the outlet to stop rocks from heading to the compressor |
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Suction manifold piping? |
The inlet to the suction receivers are staggered so they dont line up with the outlets headed to the compressors. The outlet to the compressor are piped so the opening is in the middle of the reciever. This is to allow whatever refrigerant in the suction reciever to not enter the compressor. The oil floats on the refrigerant so there is a hole on the side of the outlet pipe that will allow the oil to enter and head towards the compressor. |
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Hot gas reverse flow uses a 15-20 psi differential valve to make a differential between the defrost header and the liquid header. So the hot gas goes from the defrost header to the liquid header.
What is the name of this valve? |
Liquid differential valve |
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Hot gas reverse flow uses a 15-20 psi differential valve to make a differential between the defrost header and the liquid header. So the hot gas goes from the defrost header to the liquid header.
What is the name of this valve? |
Liquid differential valve |
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The heat reclaim coil is piped in ________ with the condenser. |
Series |
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After the reverse p what is the access fitting for? |
Remove non-condensibles High condensing pressure OR turn the system off and the standing pressure will be higher than the ambient temperature. |
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How big should the drop be before you tie the summer and winter condenser back together? |
7-8 ft
Winter condenser will have a check valve to allow refrigerant in 1 direction |
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What is the benefit of a split condenser? |
You only have to flood one condenser. |
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Why is a winter charge a common solution? |
The receiver is now undersized for the system after an addition to the system. So you add refrigerant in the winter to maintain head pressure in low ambient. You are not allowed to tie a tank to the receiver to increase the size of the receiver. |
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When the system is experience higher load the suction will go up. When we stage compressors on a multiplex system what do they use to monitor the suction pressure? |
Transducer on the suction header.
They used to use suction pressure switches that would stage the compressors on. |
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When the system is experience higher load the suction will go up. When we stage compressors on a multiplex system what do they use to monitor the suction pressure? |
Transducer on the suction header.
They used to use suction pressure switches that would stage the compressors on. |
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What operating variable would be monitored for condenser fan cycling? |
Discharge pressure or SDT in the center of the condenser |
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When the system is experience higher load the suction will go up. When we stage compressors on a multiplex system what do they use to monitor the suction pressure? |
Transducer on the suction header.
They used to use suction pressure switches that would stage the compressors on. |
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What operating variable would be monitored for condenser fan cycling? |
Discharge pressure or SDT in the center of the condenser |
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Advantages of a multiplex? |
(1)Efficiency (2)Better control (lead/lag, balanced hours and starts for compressors and condensing fans) (3)Ability to dial out (4)remote monitoring (5)view alarm history |
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How many cases can be on defrost at once with reverse flow defrost? |
20% |
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Why do they delay on the evap fans after a defrost? |
Bleed time is used to ensure the water on the coil refreezes Prevents ice on floor. |
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How many cases can be on defrost at once with reverse flow defrost? |
20% |
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Why do they delay on the evap fans after a defrost? |
Bleed time is used to ensure the water on the coil refreezes Prevents ice on floor. |
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How does cool gas defrost work? Less damaging? |
Reverse flow from high side reciever is saturated. This will allow a quick phase change because the gas does not need to be desuperheated. (+) less thermal shock (-) not as fast |
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What velocity is the air curtain on a open freezer? |
600 cfm |
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Why do they have a load limit on a 5 deck open freezer? |
If product obstructs the curtain then warm air will enter this will cause ice formation |
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Why do they have a load limit on a 5 deck open freezer? |
If product obstructs the curtain then warm air will enter this will cause ice formation |
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What are the three air curtains used for in the 5 deck open freezer? |
(1) ambient or comfort zone curtain is not recirculated. Prefiltered to prevent plugging homey comb (2) guard jet is recycled and is somewhat refrigerated because of contact with the case (3) the air curtain or freeze jet is refrigerated |
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The evaporator fan sometimes are left on and sometimes turn off during defrost depending on the case manufacturer. T/F? |
True. The manufacturer might decide to shut them off and wait or just leave them on. Hussman leaves them on Hill phoenix one turns them off |
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What is the cut in and cut out for a demand defrost? |
Cut out is 34F and cut in is 24F The box temp is set to 34F and the evaporator has a 10F drop. So the cut in turns the compressor off when the box is satisfied 34F return minus 10F = 24F It turns the compressor back on when it knows there is no ice. Aka coil temp is above freezing. |
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What happens if the defrost is not long enough? |
The ice on the evaporator will be hard ice that is halfway down the evap. On the bottom. Not light and fluffy ice. Lihht and fluffy ice is caused by humidity, air leakage, bad air curtain. |
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Time clock runs the the full time and there is no ice on the evaporator.
What might be the cause? |
Bad defrost termination coil or defrost termination switch |
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If you were a modulating defrost differential regulator what would you fail to? DDR |
It will fail to differential. This will allow the cases to continue to defrost |
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If the overcurrent devices are located on the control string then they are not line side. They are… |
Pilot duty |
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Where do you feed of the liquid header? |
The bottom. |
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Where do you feed of the liquid header? |
The bottom. |
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Hot gas header feeds off…. |
The top. |
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Where do you feed of the liquid header? |
The bottom. |
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Hot gas header feeds off…. |
The top. |
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The pilot header feeds off… |
The top or side. It just is a container for static pressure. |
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How do copeland digital discus compressors unload? |
They throttle the suction so no gas is entering the cylinder.
The heads will get hot. But they will pump down to 10% for duty cycle so 10% of the duty cycle the suction solenoid will open and allow refrigerant through. |
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EPR enforces a minimum? CPR enforces a maximum? |
Yep, yep
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What is a CRO valve enforcing? |
-Maximum crank case pressure. CPR is a CRO Closing force is Crank case pressure Opening force is spring Suction inlet gas does not effect the valve |
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For 3 pipe defrost, does the suction solenoid fail open or closed? |
Closed, it fails closed. |
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PIDs can limit humting to +/- 0.5 F?
True |
Proportional Integral Differential |
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Things to know for the test |
Opening force and closing force: ORI,ORD,CRO Oil pressure control piping What is the defrost sequence of operations How does the three way valve on the front of the condenser work? |
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What are turbulators? |
They are welded onto the tubes to increase heat transfer. |
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De-energize to create suction stop on an EPR? |
Yup,
DDR valve de-energize to create a differential pressure (fails to differential so coils can defrost) |
