With the variety of refrigerants a technician can encounter, remembering all the different acceptable system pressures can be difficult or even overwhelming. To make this task more manageable, think in terms of temperature and then convert to a pressure. Focus on remembering and understanding the various design temperatures of a system. Then, with the use of a basic pressure/temperature card or app, determining the required pressure will become much easier.
For example, instead of remembering that a typical reach-in cooler using R-134a should have an operating evaporator pressure of around 15-18 psig, remember the relationship between the coil temperature and the entering air temperature, referred to as the evaporator’s temperature difference (TD). Once you know the evaporator’s operating coil temperature, you can easily convert it to a pressure. This enables you to determine the correct operating pressure no matter what refrigerant is in the system.
The same is true for the condensing pressure. For example, on an air-cooled condenser, if you know the design temperature difference (commonly referred to as the condenser split) between the air entering the condenser and the refrigerant’s condensing temperature, you can easily determine the refrigerant pressure in the condenser.
Medium-temperature systems generally will use one of two design TDs. Many walk-in coolers will be designed based on an evaporator TD of 10°F. A typical reach-in cooler will have a design temperature of 20°. So, if you’re working on an older reach-in cooler that is using R-22 instead of R-134a and you measure a 40º air temperature entering the evaporator, the evaporator pressure should be around 43 psig (based on a 20° coil temperature), instead of 18 psig if it were using R-134a.
Air-cooled condensers are generally selected with a design TD from 10°-30°. The actual condenser TD will depend on the size of condenser selected for the system but should fall within the 10°-30° range. If the system incorporates any head pressure controls, and you are working on the system during low-ambient conditions, the TD observed will be much higher. As the ambient temperature drops, the low-ambient controls will prevent the condensing pressure from dropping below a predetermined minimum value. Generally, these controls will prevent the condensing temperature from dropping below 90°. Systems with water-cooled condensers generally are designed for a 105° condensing temperature based on a 75° entering water temperature when using a municipal (city) water source, and 85° when used in conjunction with a cooling tower.
Please note, the values stated above are the generic design values used by many refrigeration equipment manufacturers. You may encounter a specialized system or equipment manufacturer that has designed a system using different temperature values. In that case, you should obtain the design values from the manufacturer in order to determine the acceptable system pressure for that system.
Thinking in temperature in-stead of pressure can help make the crossover to a new refrigerant much easier. The next time you’re faced with determining the correct operating evaporator or condensing pressure, think in terms of temperature and then convert it to pressure.
Publication date: 6/29/2015
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