There are several types of refrigerant with different qualities and boiling points. These different boiling points affect pressures in the evaporator and the condenser coils.
By increasing or reducing the pressure in the evaporator, an increase or decrease in temperature can be achieved. By looking at the chart on page 124, you’ll see how each type of refrigerant produces different temperatures under various pressures.
It is possible to achieve a desired evaporator temperature by manipulating the low-side pressure. The lower the pressure under a saturated condition (holding as much vapor as it can), the lower the temperature of the evaporator coil. Under a lowered pressure, liquid refrigerant in the evaporator coil vaporizes more rapidly and absorbs more heat from the walls of the evaporator coil, reducing the temperature of the freezer or the refrigerated area even further.
In residential refrigeration units, this pressure-temperature relationship is very often precalculated by the manufacturer. When servicing these units, check the nameplate to find the type of refrigerant and the required amount of charge. That amount of charge automatically produces the correct pressure in the evaporator to bring the coil to its predetermined temperature.
Charging commercial units is a simple operation. By using the temperature-pressure chart on page 133, across from the desired evaporator temperature, move horizontally to determine the corresponding pressure in the column of the refrigerant type used. Hook up the gauges and charge the unit until the compound gauge registers the desired pressures.
As the unit is charged, both the head and back pressures go up. As soon as the desired back pressure is reached, shut off the valve and disconnect the refrigerant tank from the unit.
EXAMPLE: To determine the corresponding low-side pressure to produce the desired evaporator temperature of 33°F in a walk-in cooler using R-12 refrigerant, refer to the temperature-pressure chart.
ANSWER: Across the line on which 33°F is shown, in the R-12 column find 30.9 as the pressure needed to produce 33°F. Charge the system until the compound gauge (low-pressure side) reaches 30.9 lbs/in 2; turn off the valve and stop charging.
The cabinet temperatures in different types of refrigeration units are fairly standard.
Figure 86 shows recommended cabinet temperatures for different units.
As a rule of thumb, evaporator temperature should be set 20°F lower than the desired cabinet temperature.
EXAMPLE: To determine the required low-side pressure in a unit (using R-12) to maintain a general cabinet temperature of 45°F, subtract 20°F from the cabinet temperature to get 25°F (45-20=25).
ANSWER: Referring to the chart, across the line from 25°F in the R-12 column, 24.6 psi is the required low-side pressure to maintain the general cabinet temperature at 45°F.
The evaporator size should match the rest of the system. If an oversized evaporator is used, it will starve; and if an undersized evaporator is selected, it will become flooded. In both cases, the temperatures produced in the evaporators will never drop to a desired point.