Uses of Refrigeration Low Pressure Controls
The temperature of a refrigerated box can be controlled by a low-pressure control (LPC), instead of a thermostat, because of the pressure/temperature relationship in the refrigeration system. By cycling the compressor in response to the suction (low side) pressure, box temperature can be controlled. This type of control is most popular in small-refrigerated boxes such as beer coolers.
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To control box temperature with a low-pressure control, the system must use a thermostatic expansion valve, non-bleed type. The condensing unit must be located in an ambient, which is warmer than the box’s highest operating temperature.
As box temperature decreases, the evaporator temperature decreases, and a lower suction pressure results. When the suction pressure reaches the low-pressure control’s cut-out setting, the LPC’s contacts open and stop the compressor. As the box temperature rises, the evaporator’s temperature also rises, the evaporator pressure increases, and when the cut-in setting of the LPC is reached, its contacts close and the compressor is started.
There are some advantages to this type of system. The low-pressure control will act as a “loss of charge” control. Short cycling, due to door openings, etc., is prevented. The same is not true for a standard temperature control. Wiring is simplified and installed cost is reduced.
Two settings need to be made on the LPC:
1. THE CUT-IN
The cut-in is the pressure that closes the LPC’s contacts and starts the compressor. This pressure relates to the refrigerated box’s highest temperature. As an example for a beer cooler: The desired box temperature is 36°F; the system is R-134A refrigerant. From a pressure/temperature chart, we see that 31.3 psig is 36°F for R-134A. This will be the cut-in setting.
2. THE DIFFERENTIAL SETTING
The differential is the difference between the cut-in and the cut-out pressures. The differential will determine the compressor “on” time: the time it takes to pull the suction pressure down to the cut-out setting. Usually it is advantageous to keep the compressor on as long as possible for maximum efficiency.
To set the differential, and consequently the cut-out setting, four factors need to be determined:
1. The box’s lowest temperature
2. The TD of the evaporator coil
3. Desired compressor “on” time
4. The DP in the suction line between the evaporator and the LPC connection at the compressor.
In the “real world” we’ll have to make some educated guesses for answers to the four factors.
- Usually, a 2°F to 3°F temperature differential is maintained in refrigerated boxes. Since we know the cut-in we want, this is easily determined. In our example, it would be 34 or 33°F.
- Unless you were the designer of the box, you may not know the design TD picked for the system. TD is the temperature difference between the box temperature and the refrigerant temperature in the evaporator. Refrigerant temperature is the design suction temperature when the compressor is on. In most cases, this will be 10°F or 15°F. All freezers are 10°F, as are applications requiring high humidity in the box. Meat cases are generally 15°F splits. As we’ll see below, using 10°F or 15°F isn’t critical, because of other unknown variables that will enter into our setting. To simplify this factor, assume a 10°F TD if the actual TD is unknown. It is a good starting point.
- The differential chosen will determine the compressor on time. It should be long enough to prevent short cycling, but not so long as to cause wide temperature variations or excessively low suction pressure, which may cause motor overheating or inadequate lubrication.
- Unless gauges are installed at the outlet of the evaporator and at the LPC connection to the system, (not practical), this DP can be estimated. Since these systems are small and usually closely coupled, the DP in the suction line should be somewhere around 2 to 4 psig. This DP between the evaporator and the LPC connection will result in an increase in the differential setting.
What all this means is that one picks cut-in and cut-out settings that should result in good temperature control, monitors the system, and then fine-tunes each specific job to achieve the desired results.
Figure 1 shows the usual starting set points to set up a LPC for various applications. (R-134A can be substituted for R-12 and R-404A for R-502).
Our example of a beer cooler could be set up as follows:
Refrigerant R-134A
Highest box temperature desired 36°F
36°F R-134A is 31 psig
Set the cut-in at 31 psi
Coil TD selected 10°F
36°F minus 10°F is 26°F
R-134A at 26°F is 23 psig
Allow 2 - 3 psi for suction line DP
23 psig minus 3 psig = 20 psig
31 psia minus 20 psi = 11 psi
Set the differential at 11 psi. The compressor will now go on at 33 psi and go off at 20-psi suction pressure. This should result in a box temperature of 34°F to 36°F. See Figure 2.
Depending on how well the system balance was made, that is, matching the evaporator to the compressor capacity at the selected suction temperature, the compressor run time will be long enough to give good efficiency and not short cycle. If box temperature varies more than 2°or 3°F, or if the compressor short cycles, adjust the differential only, not the cut-in setting (raising the cut-in setting raises the box temperature)! Fine-tuning the differential setting should produce the desired results.
The settings in Figure 1 are only a beginning reference. Variations in systems will probably require small corrections of the settings.
Remember—too close a differential may maintain close temperature control, but cause short cycling, greatly shortening equipment life. A wide differential will give longer running time, but may cause wide temperature swings. The final chosen differential has to be a compromise.
Probably the most common use of an LPC pump is in “pump down”. In a pump down system, a thermostat controls a solenoid valve in the liquid line. On a rise in temperature, the thermostat energizes the solenoid valve, allowing refrigerant to the TXV (pump-down systems must use TXV’s) into the evaporator and suction line. The refrigerant pressure increases, causing the LPC to cut-in, starting the compressor. When the thermostat opens the circuit to the solenoid valve, the valve closes and the compressor pumps the refrigerant from the evaporator and suction line into the receiver and condenser, reducing the refrigerant pressure to the cut-out setting of the LPC and stops the compressor.
If, during the off cycle, refrigerant leaks into the low side to raise the pressure to the LPC cut-in setting, the LPC will start the compressor for a short period until the pressure is lowered to the cut-out point and the compressor is once again stopped. These brief occasional cycles are not objectionable, but if they occur too often, are an indication of a leaky solenoid valve or leaky compressor valves.
While pump-down is a low-cost, easy to install refrigeration control system, it is not necessary to wire from the refrigerated box to the compressor, the main benefit of a pump down system is the fact that the refrigerant is isolated in the condenser and receiver when the compressor is not running, preventing migration of refrigerant to the compressor’s crankcase. The last place we want liquid refrigerant is in a compressor’s crankcase!
The LPC cut-in setting should be selected first. For units located indoors, determine the lowest operating temperature of the unit. Subtract 3°to 5°F from this temperature. Using a T/P chart set the cut-in at that value. The cut-out setting should be a reasonable amount of PSI lower than the cut-in, but not so low that the compressor will have difficulty reaching the cut-out setting. Avoid cut-out settings that result in a vacuum. Even low temperature freezers using R-502 or R-404A should not be set lower than 0 psi cut-out.
If the beer cooler, used as previous example, were to use a pump-down system, the LPC cut-in setting would be determined as follows:
Indoor unit, R-134A refrigerant
Lowest operating temperature of the unit 24°F
(Lowest box temperature 34°F, 10°F coil TD equals 24°F)
Subtract 3 to 5°F from 24°F. Let’s use 4°F. This results in 20°F. From a T/P chart, we see that R-134A at 20°F is about 18 psi. Set the cut-in at 18 psi. A differential setting of 5 psi to 10 psi will result in a reasonable cut-out setting of 8 to 13 psi.
For condensing units outdoors, either the coldest unit operating temperature or the coldest ambient temperature selects the cut-in setting, whichever is the lowest temperature.
Figure 3 is a guide for setting the LPC for outdoor units.
Figure 4 is typical of the piping for a pump-down system. Note that when the solenoid valve is closed (off) the refrigerant is essentially trapped between the solenoid valve and the discharge valves of the compressor.
Figure 5 and Figure 6 show two of the more common wiring diagrams for pump-down systems. Another common use of LPC’s is to cycle condenser fans to maintain head pressure during cool weather conditions (note the word “cool”, not cold). Air-cooled condensers outdoors need head pressure control when required to operate in ambient temperatures below 60°F, for air conditioning, and below 50°F for refrigeration systems.
Fan cycling is only good to about 20°F above zero. Below that, flooded condenser valve systems should be used.
An LP fan cycling control senses discharge (head) pressure and closes on rise of pressure. The control opens on a fall in head pressure and shuts off the condenser fan, or fans.
Condensing temperatures range from 95°F to 105°F. The correct adjustment of the on/off control differential is important. Too small a differential will cause short cycling of the condenser fan and shorten the fan motor life. Too wide a differential will cause large fluctuations in head pressure and cause TXV hunting. A 35 to 50 psi differential is suggested, depending on usage.
Figure 7 shows suggested pressure settings for a single fan condenser.
Figure 8 is a chart for condensers with multiple fans. The pressure control for each fan should be set to cut-in about 10 psi apart.
Very large condensers having four or more fans, usually have the fan cycling controls control two or more fans at a time. For instance, a condenser with nine fans would have each fan cycling control turn three fans on and off at the same time.