The Professor: Diagnosing Air Conditioning Systems

May 5, 2008
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Figure 1. Pulley with connecting belt. (Photo courtesy of Ferris State University.)


This is the first of three columns on systematic air conditioning troubleshooting and diagnostics. It will deal with airside and the evaporator. Part 2 will deal with condensers, subcooling, liquid and suction line restrictions, and undercharges/overcharges and will appear in the June 2 issue of The NEWS. The final part, running in the July 7 issue, will deal with compressor inefficiencies, noncondensables in the system, low and high condenser entering air temperature, capillary tubes, and TXV metering devices.



AIRSIDE

In a/c system diagnostics, a service technician must realize that system problems fall mainly under two major categories: airside problems and refrigerant cycle problems. Air system problems also fall into two major categories: Too much air and too little air.

If we assume that the entire a/c unit was originally set up and operated properly, too much air probably will not be a problem area since air-handling systems do not just suddenly increase their air volume flow (cfm) without some outside help. When the fan is not directly driven by the motor, the motor will have a sheave or adjustable pulley with a connecting belt between it and the fan to drive it (Figure 1).

If the driven sheave on the blower motor becomes loose, the driven belt would ride lower on the driven sheave, thus decreasing the fan’s speed, not increasing it. It can also rule out the fan’s speed being too high since most a/c units call for the higher fan speeds to operate properly in order to move the higher densities of colder air associated with air conditioning.

This is why the majority of airflow problems will be not enough airflow rather than too much airflow. However, one must remember that a lot of times air conditioning systems are not set up properly in the first place. There can be bad duct design, which encompasses either oversized or undersized ducts, or simply leaky ducts.

To determine if you are dealing with an airflow or refrigerant flow problem, first record the air temperatures in and out of the evaporator coil and determine if it is higher or lower than it should be. Too low of an airflow will give you greater temperature differences across the coil than too much airflow. This greater temperature difference is from the air being in contact with the coil longer, thus decreasing its temperature coming out of the coil. By comparing the measured temperature difference to manufacturers’ required temperature differences, a technician can establish whether there is an airflow problem or a refrigerant flow problem.

Figure 2. Direct expansion evaporator coil. (Photo courtesy of Ferris State University.)

But, what should the temperature difference across the evaporator coil be? A direct expansion evaporator coil (Figure 2) is referred to as an “A” coil because of its shape. Air enters the “A” coil from the bottom and exits both sides. Coils can be of different configurations and shapes also.

To determine the required temperature difference across the coil, a technician must obtain the wet bulb temperature (WBT) and dry bulb temperature (DBT) of the air entering the coil. A psychrometer is the only instrument needed for these measurements.

A psychrometer has both wet and dry bulb thermometers in one package. Today, psychrometers are often of the digital type. However, a thermistor or thermocouple with a wet piece of cotton wrapped around it can give the WBT accurately enough for air conditioning work. Once both the WBT and DBT of the entering air is measured, the rh of the entering air can be obtained from a chart or table.

Now, referring to Chart 1, one can see that for a constant air entering DBT, the temperature difference across the coil increases with decreasing rh. The reason for this larger temperature difference with lower rh is the decreased moisture (latent) load that the a/c coil has to condense. If the coil doesn’t have to condense as much moisture out of the air, it can perform more sensible cooling because the coil’s temperature will be lower. Sensible cooling is exactly what we are measuring when we measure the temperature difference across a cooling coil with a DBT.

Now, if the temperature difference is greater than the required temperature difference across the coil, then we are dealing with an airflow problem. The problem would be too low of an airflow, causing the air to stay in contact with the coil much too long, giving a greater temperature difference across the coil. However, if the temperature difference across the coil is less than the required temperature difference, we would be dealing with a refrigerant flow problem instead of an airflow problem.

Chart 1. Chart to obtain relative humidity of entering air.

This is because of the fact that a/c systems hardly ever increase in airflow without some kind of human intervention. Listed below are some causes that may produce a decreased airflow in an air conditioning system:

1. Dirty air filters;

2. Faulty duct design;

3. Fan pulleys loose;

4. Fan belts slipping;

5. Blower motor running slow (burning out);

6. Restriction in the duct system;

7. Dirty evaporator coil;

8. Dirty or missing fan blades;

9. Direct-drive blower with wrong speed tap on the motor; and

10. Variable-frequency drive (VFD) malfunctioning and running the fan motor too slow.

THE REFRIGERANT SYSTEM

The low temperature difference across the evaporator coil was an indication to the technician that he is dealing with a refrigerant flow problem. The low temperature difference also indicates a capacity drop, meaning that the heat-handling capabilities of the system has failed. If we assume that the a/c system was started and has run for some time under these conditions, the service problem cannot be electrical.

The larger the coil surface area of the evaporator, the closer the coil temperature will be to the entering air temperature. This would force the coil temperature to be at a higher temperature. Thus the a/c unit would be running higher evaporating (suction) pressures and temperatures.

With this increase in vapor pressure and coil temperature, the a/c unit would experience higher efficiencies from the higher-pressure (more dense) refrigerant gases entering the reciprocating compressor each revolution of its crankshaft. The compression ratio would also decrease from the higher evaporator temperatures, and pressures causing the mass flow rate of refrigerant vapors through the compressor would increase.

These are the reasons why manufacturers have been manufacturing a/c coils larger and more efficient. The larger and more efficient the coil is, the smaller the coil’s temperature will be from the entering air temperature. This causes higher evaporator pressures, and thus more efficient a/c units.

This larger and more efficient coil does increase the manufacturing cost, but increased unit efficiency hopefully will offset these higher costs.

The term used to describe an a/c unit’s efficiency is the Seasonal Energy Efficiency Rating (SEER). However, one has to remember that coil surface area may vary from one manufacturer to another.

One manufacturer may choose large coil areas for high suction pressures and smaller compressors. Another may go with smaller surface area coils causing lower suction pressures and use larger compressors. As long as the a/c unit meets the required Btu requirements and SEER ratings, it is a trade-off.

Often coil surface area is a function of geographic regions. Large surface coils running higher suction pressures may not have low enough coil temperatures, thus not being able to remove enough moisture (latent heat). Their apparatus dew points (average coil temperatures) will be too high to condense the right amount of moisture from the air passing through the coil. This may cause high humidity, mold, and human discomfort problems.

As of Jan. 23, 2006, federal law requires that every central split cooling system manufactured in the United States must have a SEER of at least 13.

Publication date: 05/05/2008

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