When I started the Facebook group last fall, I posted a series of short articles derived from my blog about refrigeration cycle diagnostics daletrossi/. There were very few members then. I am going to re-post the series now for the benefit of most members that didnt see it. Number 25 Evaporating temperature (ET) The evaporating temperature is the low side saturation temperature. The suction pressure converts to the saturation temperature on the T-P chart. Increase the suction pressure by increasing the refrigerant flow through the metering device, by increasing the heat transfer into the refrigerant in the evaporator or by decreasing the pumping capacity of the compressor with unloading. When is evaporating temperature low? Many technicians have seen equipment running with low suction pressure. How do we know that it’s low? What are our criteria for judging if suction pressure is low? Remember, suction pressure converts to evaporating temperature on the T-P chart. Water freezes at 32°F, so when the evaporating temperature remains below that temperature for an extended time, ice forms on the evaporator coil. Ice is an obstruction to airflow as well as insulation against heat transfer. One problem we are trying to avoid in air conditioners by keeping the evaporating temperature above a minimum temperature is icing of the evaporator coil. Low evaporating temperatures and especially low superheat might mean that the refrigerant isn’t absorbing enough heat from the air passing over the evaporator coil. When this problem occurs, the diagnosis is a low-side heat transfer problem. We care about this because the heat absorbed by the refrigerant in the evaporator is needed to evaporate the liquid refrigerant. When there is either too much refrigerant in the evaporator or too little heat available to evaporate the liquid refrigerant, it remains liquid. Liquid refrigerant entering the compressor leads directly to pre-mature compressor failures. A low-side heat transfer problem may have one or more of several root causes. Some of the causes of low-side heat transfer problems are dirty evaporator coils, dirty filters, or a slow blower fan. A slow blower fan may be caused by a worn belt, a worn or improperly adjusted motor sheave or a multi-speed fan motor set at the wrong speed. Other times obstructed or under-sized duct work will cause low air flow. Whatever the problem is, it should be corrected, or the technician will be forced to reduce the capacity of the unit to compensate when possible or the unit will operate with a low side heat transfer problem. Operating with a low side heat transfer problem increases the system’s energy use and it increases the risk of “slugging” the compressor, meaning allowing liquid refrigerant to enter the compressor. Slugging is a primary cause of premature compressor failure. Another possible cause of low evaporating temperatures is an insufficient refrigerant charge. High superheat or low subcooling along with low suction pressure will characterize this problem, as we will see in the diagnostics section. Low evaporating temperatures cause low capacities and low efficiencies. There are a wide range of evaporating temperature goals depending on the driving conditions, the design efficiency of the unit, and the metering device type. A 35° evaporating temperature can be high and a 45° evaporating temperature can be low depending on the design efficiency (EER) or the unit, the metering device used and the driving conditions meaning the ambient temperature and the return are wet bulb temperature. Next: When is evaporating temperature high? I will post another installment each weekday.
Posted on: Mon, 24 Mar 2014 12:56:10 +0000
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