RA Pressure Temperature Chart. PRESSURE-TEMPERATURE CHART .. Refrigerants 22, a, A, and values are based on °F liquid temperature and the stated evaporator. PRESSURE-TEMPERATURE CHART .. Refrigerants 22, a, A, and values are based on °F liquid temperature and the stated.

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By cycling the compressor in response to the suction low side pressure, box temperature can be controlled.

Refrigerant Pressure – Temperature Chart

This type of control is most popular in small-refrigerated boxes such as beer coolers. To control box temperature with a low-pressure control, the system must use a thermostatic expansion valve, non-bleed type. As box temperature decreases, the evaporator temperature decreases, and a lower suction pressure results.

There are some advantages to this type of system. Short cycling, fhart to door openings, etc. 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 Char As an example for a beer cooler: This will be the cut-in setting.

The differential is the difference between the cut-in and the cut-out pressures. Usually it is advantageous to keep the compressor on as long as possible for maximum efficiency. To set the differential, 4004a consequently the cut-out setting, four factors need to be determined: The TD chrt the evaporator coil. What all this means is that one picks cut-in and cut-out settings that should result in good temperature control, monitors the system, chsrt then fine-tunes each specific job to achieve the desired results.

Figure 1 shows the usual starting set points to chatt up a LPC for various applications. Our example of a beer cooler could be set up as follows: Set the cut-in at 31 psi. Allow 2 – 3 psi for suction line D P. Set the differential at 11 psi. The compressor will now go on at 33 psi and go off at psi suction pressure. 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.

Fine-tuning the differential setting should produce the desired results.

Refrigerant Pressure – Temperature Chart

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.


In a pump down system, a thermostat controls a solenoid valve in the liquid 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.

p t chart a –

These brief occasional cycles are not objectionable, but if they occur too often, are charh indication of a leaky solenoid valve or leaky compressor valves. The LPC cut-in setting should be selected first. For units located indoors, determine the lowest operating temperature of the unit. The cut-out 440a should be a reasonable amount of PSI lower than the cut-in, but 044a 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 or RA 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, RA refrigerant.

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 cgart temperature.

Figure 3 is chxrt 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.


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. Too small a differential will cause short cycling of the condenser fan and shorten the fan motor life.

P T Chart 404a

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. December 13, – 1: The TD of the evaporator coil 3. Since we know the cut-in we want, this is easily determined. 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. 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 practicalthis D P can be estimated.

Since these systems are small and usually closely coupled, the D P in the suction line should be somewhere around 2 to 4 psig.

This D P between the evaporator and the LPC connection will result in an increase in the differential setting. Testing Wireless Solutions Testing wireless transmitters link.

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