LESS HEAT TRANSFER

If a condenser becomes damaged, dirty, or fouled, less heat transfer can take place from the refrigerant to the surrounding ambient air. If less heat can be rejected to the surrounding air with an air-cooled condenser due to fouling, the heat will start to accumulate in the condenser, making the condensing temperature rise.

Once the condensing temperature starts to rise, there will come a point at which the temperature difference between the condensing temperature and the surrounding ambient air (delta T) is great enough to reject heat from the dirty or fouled condenser. Remember, a temperature difference is the driving potential for heat transfer to take place between anything: The greater the temperature difference, the greater the heat transfer. With a dirty condenser, the condenser will reject enough heat at the elevated delta T to keep the system running; however, the system will run inefficiently because of the higher condensing temperature and pressure, causing high compression ratios.

Even the subcooled liquid temperature coming out of the condenser will be at a higher temperature when the condenser is damaged, fouled, or dirty. This means that the liquid temperature out of the condenser will be further from the evaporating temperature, which will cause more flash gas at the metering device and a lower net refrigeration effect.

The compressor’s discharge temperature will also run hotter because of the higher condensing temperature and pressure, which will cause a higher compression ratio. The compressor will have to put more energy in compressing the suction pressure vapors to the higher condensing or discharge pressure. This added energy is reflected in higher discharge temperatures and higher amperage draws.

HIGH CONDENSING TEMPERATURE

When the condensing temperature is high due to a fouled condenser, the compressor must compress the refrigerant from the low-side (evaporating) pressure to an elevated high-side (condensing) pressure. This added work of compression will make the heat of compression higher; thus, the compressor’s discharge temperature will be higher.

Remember, the condensing temperature is the temperature at which the refrigerant is changing phase from a vapor to a liquid in the condenser. There is a pressure/temperature relationship with the condensing temperature because of the phase change. A gauge reading on the high side of the system is all that is needed to find the condensing temperature. This pressure can be measured anywhere on the high side of the refrigeration system, as long as line and valve pressure drops and losses are negligible. Convert this pressure to a temperature using a pressure/temperature chart, and this will be the condensing temperature. (An exception to this is a near-azeotropic blend — the ASHRAE 400 Series blends — of refrigerants. With these blends, there is a temperature glide or range of temperatures when the blend is phase-changing.)

There are many causes for high condensing temperatures, which also cause high discharge temperatures. These include:

• Recirculated air over the condenser;
• Wrong refrigerant;
• Damaged condenser fan blade;
• Wrong condenser fan blade;
• Burned out condenser fan motor;
• Restricted airflow over condenser;
• Refrigerant overcharge;
• High heat load on evaporator;
• Dirty condenser;
• High ambient temperature;
• Non-condensable (air) in the system;
• Broken fan belts; and/or
• Undersized condenser coils.

COMPRESSION RATIOS

High compression ratios are a result of high condensing pressures, low evaporator pressures, or both. Any time there are high condensing pressures or low evaporator pressures — or both — there will be high compression ratios. Thus, more work will be added to the compression stroke of the compressor. More work on the compression stroke will cause the heat of compression to increase, causing the compressor to have a higher discharge temperature.

Compression ratio, or pressure ratio, is defined as the absolute discharge pressure divided by the absolute suction pressure. A compression ratio of 7 to 1 is expressed as 7:1 and simply means that the discharge pressure is seven times the magnitude of the suction pressure. Again, a compression ratio of 10:1 simply indicates to the technician that the absolute or true discharge pressure is 10 times as great as the absolute suction pressure.

The compressor’s discharge temperature is often overlooked when troubleshooting a refrigeration or air conditioning system. However, the discharge temperature is a very important temperature because it is an indication of the amount of heat absorbed in the evaporator and suction line and any heat of compression generated by the compression process.

Since the compressor’s discharge temperature is superheated, a pressure/temperature relationship does not exist, and it must be read directly on the discharge line by some sort of temperature measuring device. This discharge temperature should never exceed 225°F because carbonization and oil breakdown can occur if this compressor discharge temperature is exceeded.

The three causes for high discharge temperatures are:

• High condensing temp;
• Low evaporator temps; and
• High compression ratios.

VOLUMETRIC EFFICIENCY

When there is a high compression ratio with piston-type compressors, the volumetric efficiency will be lowered.

Volumetric efficiency is expressed as a percentage from 0 to 100 percent and is defined as the ratio of the actual volume of the refrigerant gas pumped by the compressor to the volume displaced by the compressor pistons.

A high volumetric efficiency means that more of the piston’s cylinder volume is being filled with new refrigerant from the suction line and not re-expanded clearance volume gases. The higher the volumetric efficiency, the greater the amount of new refrigerant that will be introduced into the cylinder with each down-stroke of the piston. Thus, more refrigerant will be circulated during each revolution of the crankshaft. This results in the system having better capacity and a higher efficiency.

So, the lower the discharge pressure, the less re-expansion of discharge gases to suction pressure. Also, the higher the suction pressure, the less re-expansion of discharge gases. This happens because the discharge gases will experience less re-expansion to the higher suction pressure, causing the suction valve to open sooner.

The compressor’s volumetric efficiency depends mainly on system pressures. In fact, the farther apart the discharge pressure’s magnitude is from the suction pressure’s magnitude, the lower the volumetric efficiency will be.

As stated earlier, compression ratio is the ratio that measures how many times greater the discharge pressure is than the suction pressure — in other words, their relative magnitudes. Lower compression ratios will cause higher volumetric efficiencies and lower discharge temperatures. The goal is to keep those compression ratios as low as possible by keeping condensing pressures low, evaporator pressures high, or both.

As can be seen here, dirty or fouled condenser coils can cause a whole host of problems.

Keeping coils clean is a good way to ensure a system can continue to run at peak efficiency.

Publication date: 11/5/2018

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