TecKnow of the day
General info on REFRIGERATION AND A/C
There is one horsepower (1 hp) for each ton of cooling. It takes a l-hp compressor to provide 1 ton of comfort cooling. Therefore, a 25-ton system has a 25-hp compressor motor.
Note: This applies to comfort cooling only. A lower-suction-pressure application such as refrigeration will have a higher hp/ton ratio. The lower the suction pressure, the higher the hp/ton required.
Thermostatic expansion valve superheat should be 8Â° to 12Â°F.
The valve controls evaporator superheat, which means it controls the amount of liquid refrigerant boiling in the evaporator. The lower the superheat, the more liquid in the evaporator.
However, superheat that is too low can cause the valve to lose control of the superheat altogether. The valve only controls evaporator superheat. If the valve is properly controlling superheat, do not adjust it to change evaporator temperature or pressure. The valve is not an evaporator temperature or pressure control, so do not attempt an adjustment without measuring the superheat. The 8Â° to 12Â° superheat is normal for air conditioning systems. Low-temperature systems often use different valves and lower superheat settings.
Remember, superheat is a temperature differential, not a single temperature measurement.
Normal system superheat is 20Â° to 30Â°. System superheat is not the same as thermostatic expansion valve superheat. System superheat consists of the temperature differential from the point in the evaporator where all the refrigerant has changed to a gas to the suction line — about 6 in. from the compressor service valve.
Normally, charged and operating systems will have a system superheat of 20Â° to 30Â°. System superheat greater than 30Â° may indicate that the low side of the system is starved for refrigerant. System superheat less than 20Â° may indicate that the low side is overcharged.
Again, system superheat is a temperature differential, not a single temperature measurement. The valve superheat is part of the system superheat.
High-side subcooling is 5Â° to 15Â°.
Refrigerant in the condenser changes from a gas to a liquid and then begins to subcool. Subcooling takes place in the bottom of the condenser and in the liquid line. The amount of subcooling taking place in the condenser is 5Â° to 15Â°. It is never greater than 15Â°.
Since only liquid subcools, the amount of subcooling is an indication of the amount of liquid in the high side of the system. Overcharged systems have higher-than-normal amounts of subcooling. Undercharged systems have low subcooling.
Comparing high-side subcooling and low-side system superheat will usually solve most refrigeration cycle problems of overcharge, undercharge, and restrictions.
Normal evaporator air temperature drop is 18Â° to 20Â°.
Measuring the temperature drop, or difference of the air as it moves through the evaporator, is one method of approximating correct airflow. Assuming 400 cfm per ton of cooling, when the airflow is correct there will be an 18Â° to 20Â° drop in air temperature.
Abnormally low airflow will remain in contact with the evaporator longer, and will be chilled to a lower temperature and greater temperature difference.
Normal condenser air temperature rise is not to exceed 30Â°. Lack of air over the condenser results in high head pressure, lower system capacity, and increased power consumption. Air is heated as it passes through the condenser.
Low air across a condenser is indicated by an air temperature rise greater than 30Â°. The smaller quantity of air over the condenser must absorb the same amount of heat; therefore, the temperature rise is greater.