It’s a documented (and unfortunate) fact that a significant percentage of HVAC refrigeration systems that have been serviced, or checked for seasonal operation on a regular basis, are overcharged. The underlying cause behind this problem is the misinterpretation of a suction side pressure reading that appears lower than normal, leading the technician to the conclusion that “adding a little gas” will bring the operation of the equipment to a higher level of performance. However, when refrigerant is added to a system without considering other factors that could affect the low-side pressure reading, the result is a negative effect on the performance and efficiency of the equipment. And, the factor at the top of the list regarding the proper operation of a refrigeration system is its relationship to the volume and velocity of air flow through the indoor coil.
Looking at this idea from a simple perspective, we’ll first consider the general approach of actual coil temperature and how it relates to what your gauges should show when you check a low side pressure. Generally, when considering the fundamental design of a tube and fin coil, it is common to find that the average temperature of the coil is about 5-degrees warmer than the refrigerant in the coil. What this comes down to is this: If a technician performed a simple temperature test of a comfort cooling system indoor coil at an approximate mid-way location with an accurate digital device, and the result was a 50-degree coil, the actual refrigerant temperature should then be 45-degrees. In Figure One, we’re showing the pressures that would result in both an R-410A and R-22 system with the 45-degree temperature we calculated.
From a theoretical point of view, this simple example explains the process of temperature affecting pressure. If the heat load in the building was found to be minimal once an accurate temperature was recorded, a lower suction pressure would be expected. And, a higher-than-normal heat load would result in a higher indoor coil (and subsequently, refrigerant) temperature, which, in the end would result in an increase in suction pressure. The point to keep in mind is that a properly charged refrigeration system in conjunction with correct air flow will allows the equipment to operate at the evaporating and condensing temperatures necessary for the efficient transfer of heat out of the building. The partial temperature-pressure chart in Figure Two explains this point further.
When we apply the 5-degree rule, and consider a 45-degree coil in a situation in which the air flow is correct, we arrive at the conclusion that a suction pressure of 118.1 could be expected for an R-410A system. And, considering R-22 equipment, the suction pressure could be as low as 68.6 PSIG.
What this comes down to is that, when accomplishing PM on a comfort cooling system, we can consider the temperature-pressure information above in conjunction with performing a simplified evaluation of air flow performance of the equipment. (See Figure Three).
With outdoor ambient temperature recorded, liquid line temperature measured, and superheat considered relative to manufacturer’s charging charts, four simple air flow temperature checks can provide valuable information about air flow through the duct system.
With a dry bulb temperature measurement accomplished first at the return air grille, and then finding a significant temperature difference with a second measurement at the point where the air enters the evaporator coil, the indication is return duct system leakage and/or insufficient insulation. A difference in wet bulb readings that ultimately shows a change in specific humidity indicates duct leakage that needs to be corrected. The same principles apply to differences in dry bulb and wet bulb temperature readings found between the leaving point of the evaporator coil and the supply register.
With these fundamental tests accomplished, and conditions corrected when necessary, the groundwork has been laid for further necessary testing to ensure the efficient operation of the equipment.
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