I Dave s Statement If the thermostat calls for cooling, and the furnace fan is running properly, and the coil airflow is adequate, and the condenser fan is running properly, and the condenser airflow is adequate, and the compressor is running, and the subcooling is not excessive, and the superheat is not too high, and the superheat is not too low, then The system is operating properly. I ll break you knee caps
If the thermostat calls for cooling, and II the furnace fan is running properly, and the coil airflow is adequate, and the condenser fan is running properly, and the condenser airflow is adequate, and the compressor is running, and the subcooling is not excessive, and the superheat is not too high, and the superheat is not too low, then These questions must be asked in the proper sequence. Excessive subcooling Excessive subcooling is caused by noncondensables (usually air, nitrogen), over-charge, or restriction. After determining that the systevm does not contain noncondensables, we simply remove refrigerant until the subcooling is no longer excessive. If the system was overcharged, we just took care of it. If the system is restricted, this will show up in the superheat measurements. High superheat High superheat is caused by undercharge (leak), or restriction. If we add refrigerant until the subcooling is normal and superheat is still high, the system is restricted. Note: For diagnostic purposes we may consider an underfeeding metering device to be a form of restriction (expansion valve). If the superheat is no longer high, the system was undercharged. Locate and repair the leak. Low superheat L ow superheat (floodback) is caused by overcharge, overfeeding metering device, or inefficient compressor. If we remove refrigerant until the superheat is normal, and the system still has measurable subcooling, the system was overcharged. If the system has no subcooling, check the compressor efficiency. If the compressor is OK, the metering device is overfeeding.
III Condensers When you remove heat from a vapor, it condenses into a liquid. This is the purpose of the condenser. As the vapor circulates through the condenser it loses some of its heat, and in doing so condenses into a liquid. If the airflow through the condenser is adequate, the efficiency of the condenser (at design ambient) will be determined by how much area it has to work in. If there is too much refrigerant in the high side, excess liquid will back up into the condensing area, limiting the ability of the condenser to reject its heat. The condensing temperature quickly rises and, at the same time, the liquid has more time to cool before reaching the receiver outlet, thus the difference between the condensing temperature and the receiver outlet temperature increases if there is too much refrigerant in the high side of the system. The difference between the condensing temperature and the receiver outlet temperature is the subcooling temperature. The higher the subcooling is, the more refrigerant is contained in the high side of the system. Some subcooling is needed to assure a steady supply of liquid refrigerant to the evaporator (coil), but too much will reduce the capacity of the condenser. Cold Ambient Temperatures Cold air flowing through the condenser coil increases its ability to reject heat and therefore increases its capacity. A system which has sufficient subcooling during cold weather may be grossly overcharged during warm weather. It is necessary, under cold ambient conditions, to simulate design ambient temperature by blocking off part of the condenser until the condensing temperature is approximately 100 to 110 degrees Fahrenheit before measuring the subcooling.
IV Measuring Subcooling Most refrigeration gauges have both pressure and temperature scales. Get in the habit of reading the temperature scales rather than the pressure scales. Start thinking in terms of condensing temperature instead of head pressure. This will make diagnosing systems a lot easier. Subcooling is measured by subtracting the liquid line temperature (at the condenser outlet) from the condensing temperature. Keep in mind that we are comparing two temperatures. Either temperature by itself will not tell us what we need to know. Excessive Subcooling In most systems anything over 15 degrees Fahrenheit subcooling should be considered excessive. In those rare systems which specify more than 15 degrees Fahrenheit (some as high as 20), 15 degrees will work just fine, but will not be quite as energy efficient. Consult the manufactures specifications if possible (name plate). If in doubt 15 degrees is reasonable limit for subcooling. Excessive subcooling is caused by non-condensables (usually air, nitrogen), refrigerant overcharge, or restriction. Noncondensables Under Normal conditions, the head pressure will tell us the condensing temperature. Air or other noncondensable gases, however, add pressure to the system without raising system temperatures, thereby distorting the pressure/ temperature relationship. If possible, pump down the system, then compare condenser coil temperature to head pressure. If the pressure indicates a higher temperature than the coil temperature, there are noncondensables in the system.
V Excess Refrigerant If the system does not contain noncondensables and the subcooling is more than 15 degrees, this tells us that the condenser contains excess refrigerant. There may be too much refrigerant in the system (overcharge), or the excess refrigerant has been borrowed from the low side (restriction). Overcharge At this point in the procedure it is not necessary to know whether the system is overcharge or restricted. We simply remove refrigerant until the subcooling is less than 15 degrees. If the system was overcharged, we have just taken care of it. If the system is restricted, we will find this when we check the superheat. Condenser Airflow Since we Are following the method procedure, we have checked the condenser fan and coil long before we measured the subcooling., therefore we know the airflow is adequate. Is it, really? What if a replacement fan motor or blade was improperly sized? What if the coil appeared to be clean, but really wasn t? Let s try one more test to confirm condenser airflow. Measure the temperature of the air entering the condenser and the air leaving the condenser. If the air flow is inadequate, the condensing temperature is higher and the slower moving air has more time to heat up, so the temperature difference across the coil is higher. If the difference between entering air and leaving air is more then 30 degrees, the airflow is inadequate.
VI Evaporator Coils When you add heat to a liquid it boils or, in other words, evaporates. This is the purpose of the coil. The liquid refrigerant circulating through the evaporator absorbs heat from the refrigerated space and in doing so, becomes a vapor. For the evaporator to do its job, it must have a steady supply of liquid refrigerant from the condenser. This is why we check the subcooling before measuring the superheat After the liquid evaporates, the vapor temperature will rise above the evaporating temperature. If there is less refrigerant in the coil, the evaporating temperature will drop and at the same time, the vapor temperature will have more time to rise before it reaches the compressor inlet. If there is more refrigerant in the coil, the evaporating temperature will rise, while at the same time, the vapor temperature will have less time to rise before it reaches the compressor inlet. Thus the difference between evaporating temperature and compressor inlet temperature (super heat temperature) will tell us if there is not enough refrigerant in the low side of the system (high superheat) or too much refrigerant (low superheat ). The superheat must be low enough to flood the evaporator and cool the compressor, but not low enough to flood the compressor. Cold Ambient Temperatures Low head pressure (low condensing temperature) will reduce the rate of refrigerant flow through the metering device, causing he superheat to be higher than normal. In cold ambients it is necessary to simulate condensing temperature at design ambient conditions by blocking off part of the condenser until the condensing temperature is 100 to 110 degrees before taking superheat (or subcooling) measurements.
VII Refrigerated Space Temperatures Warm air flowing through the evaporator coil increases its ability to absorb heat and there for increases its capacity. When the refrigerated (or cooled) space temperature is above its design temperature range we must expect the superheat temperature to be higher than normal. (This is particularly true on cap tube systems). For this reason we must recheck superheat measurements after the system has reached its design temperature range in order to obtain true readings. Measuring Superheat Most refrigeration gauges have both pressure and temperature scales. Get in the habit of reading the temperature scales. Superheat is measured by subtracting the evaporating temperature from the suction line temperature as close to the suction port as possible. keep in mind that we are comparing two temperatures either temperature by itself will not tell us what we need to know. High Superheat In most systems anything over 30 degrees superheat should be considered too high. In general, lower superheat is better for the compressor, however some manufactures specify higher than 30 degrees superheat for their systems (usually low temp). Consult the manufactures specifications if possible. If in doubt, 30 degrees (at design space temperature) is a reasonable limit for superheat. High superheat is caused by undercharge or restriction.
VIII Undercharge Undercharge of refrigerant is identified by a combination of high superheat and low or no subcooling. If we add refrigerant until the superheat is 30 degrees (at design space temperature or manufacturers nameplate rating) and the subcooling does not exceed the manufacturers recommended rating normally no more then 15 degrees the system was undercharge. Unless you have reason to believe that the system was not charged properly, it is reasonable to assume that there is a leak. Locate and repair the leak before continuing the procedure. Restriction By holding back the flow of refrigerant, a restriction causes refrigerant to accumulate in the high side while limiting the amount of refrigerant in the low side, therefore a restriction identified by combination of high superheat (at design space temperature) with subcooling (at design ambient). If we add refrigerant to the system until the superheat is 30 degrees or manufacturers recommend rating and the subcooling rises above 15 degrees the system is restricted. The point of restriction can be found by taking temperature measurements across the liquid line and/or each of the liquid line devices (driers, valves, etc.). If the temperature drop from one end of the liquid to the other is less than 5 degrees F., the liquid line and its devices are not restricted. Note: A temperature drop across a heat exchanger is normal. If the temperature drop across the liquid line is more than 5 degrees F., check for a temperature drop across each of the liquid line devices. If the liquid line devices are not restricted (no temperature drop), the restriction is in the liquid line. Check for kinks in the line or an undersized line or a coupling soldered shut. If the temperature drop across the liquid line is less then 5 degrees F. then the metering device (expansion valve) may be restricted or airflow across the coil may be restricted.
IX Restricted Metering Device Once we have traced the restriction to the metering device we must determine whether the device is, in fact, restricted or out of adjustment. In a cap tube system it is definitely a restriction, but in a expansion valve system the inlet screen should be examined before blaming the valve. If the screen is clear the problem is in the expansion valve replace the valve. If the Acurator is clogged clean or replace it. Compressor Vacuum Test A compressor efficiency test involves shutting off the flow of refrigerant at the high side valve, then running the compressor to see how far into a vacuum it can pump. The question is not whether the compressor is efficient but whether it is efficient enough to do its job. If the compressor cannot pump at least 15 inches of vacuum, it is not efficient enough for a normal heat load and should be replaced. If it pumps 20 inches, it is efficient. This test will identify most compressor pumping problems, but not all. We have tested its efficiency, but not capacity.