| Step | Expected Result/Action |
|---|---|
| 1. Drill a hole in the return air duct to measure the evaporator entering air temperature as shown in Figure SP-14-3, View A. Drill a second hote in the supply air duct to measure the evaporator leaving air temperature. | Holes drilled in the supply and return air ducts in preparation for measurement. |
| 2. Turn on the power lo the system. Make sure that all the supply and return registers are open. Allow the system to run for about ten minutes to stabilize the temperatures. Set the thermostat low enough so that the system runs continuously during the measurement. | System on and temperatures stabilized. |
| 3. Using the electronic thermometer, measure and record the evaporator entering air wet bulb and dry bulb temperatures. Make sure the thermometer readings have stabilized for the measurement. | Evaporator entering air wet bulb and dry bulb temperatures measured and recorded. For our example assume: Entering air wet bulb temperature = 6 4 ° F Entering air dry bulb temperature = 7 6 ° F |
| 4. Using the electronic thermometer, measure and record the evaporator leaving air dry bulb temperature. Make sure the thermometer reading has stabilized for the measurement. | Evaporator leaving air dry bulb temperature measured and recorded. For our example assume: Leaving air dry bulb temperature = 57° F |
| 5. Using the proper airflow calculator, set the pointer to the indoor entering air dry bulb °F measured in step 3. | For our example assume: The pointer is a t7 6 ° F as shown in Figure SP-14-3, ViewB. |
| 6. On the calculator, find the value for the indoor entering air wet bulb temperature measured in step 3, then read the proper evaporator leavingair temperature directly below it. | For our example assume: The evaporator (indoor) wet bulb temperature of 64° F is found on the calculator. Directly below it is the correct evaporator leaving air dry bulb temperature, as shown in Figure SP-14-3, View C. The proper evaporator leaving air dry bulb temperature is 5 7 ° F and the actual evaporator leaving air temperature measured in step 4 is 5 7 ° F. This means that the airflow is within the 4 0 0 to 450 CFM per ton range and no adjustment to the blower speed is needed. A tolerance of ± 3 ° F is allowed. If the evaporator leaving air temperature measured in step 4 is above 60° F [proper evaporator leaving air dry bulb temperature of 57° F read from calculator plus 3° F = 60° F), decrease the blower speed. If the evaporator leaving air temperature measured in step 4 is below 54° F (proper evaporator leaving air dry bulb temperature of 57° F read from calculator minus 3° F = 54° F), increase the bfower speed. |
Purpose — This procedure describes how to determine if airflow across the evaporator in a cooling system is adequate. For proper operation of a cooling system, the blower should be moving from 400 to 450 CFM of air across the evaporator coil for each ton of cooling capacity. For example, on a 2-ton cooling unit, the volume of air should be at least 800 CFM. Too much or too little air can cause indoor comfort problems as well as equipment problems. The airflow is normally measured when a cooling system is initially installed, being charged with refrigerant, or when troubleshooting.
Airflow Problems
Too much air across the evaporator coil results in poor humidity control. If air moves too fast across the evaporator, moisture is not effectively removed. To correct the problem of too much air, decrease the blower speed.
While too much air can be a problem, the problem of too little air is more widely seen. The usual symptom is a frozen evaporator coil. Refrigerant flooding can result from low airflow and this in turn may cause compressor failures. To correct the problem of too little air, increase blower speed. However, before adjusting the blower speed, always make sure that:
• The system air filter is clean.
• The blower wheel is clean.
• The evaporator coil is clean.
• There are no loose or worn belts on belt-driven blowers.
• The blower is rotating in the right direction.
• The system has the correct refrigerant charge.
Airflow Measurement
The procedure for measuring airflow given here uses the “Proper Airflow Range” section of the Required Superheat/Subcooling C a lc u la to r p rev iou sly used in S e rv ic e Procedure SP-4- This calculator is designed to provide a quick check to see if the airflow across the evaporator is adequate for proper cooling system operation. It cannot be used to find the actual air quantity in CFM. Methods for measuring actual CFM are shown in Service Procedures SP-15 through SP-17.
A detailed procedure and illustrated example for determining airflow using the Airflow Calculator is provided later in this section. A brief overview of the procedure and the use of the calculator follows.
After system operation has stabilized, the following temperatures are measured:
• Wet bulb and dry bulb of air entering the evaporator coil.
• Dry bulb of air leaving the evaporator coil.
The measured dry bulb and wet bulb temperatures for the air entering the evaporator are used with the calculator to find the proper evaporator coil leaving air dry bulb temperature. This step gives the correct leaving air temperature, assuming the system refrigerant charge is correct and the airflow across the coil is within the 400 to 450 CFM per ton range.
Following this, the actual dry bulb temperature of the air leaving the evaporator is compared with the value indicated by the calculator. Ideally, they should be the same. A tolerance of ± 3 ° F is allowed before any adjustment is required (Figure SP-14-2). If the actual air temperature leaving the evaporator is more than 3 ° F lower than the proper airflow temperature, the evaporator blower speed should be increased. If the actual air temperature leaving the evaporator is more than 3 ° F higher than the proper air temperature, the evaporator blower speed should be decreased.
