More Refrigerant Flow Problems

January 31, 2004
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My last column focused on refrigerant flow problems. (See "Analyzing Refrigerant Flow Problems," Jan. 12.) In it, we discussed how cooling coil sizes, refrigerant subcooling, liquid and suction line restrictions, undercharging, and overcharging can affect refrigerant flow.

Let's continue by looking at compressor inefficiencies, noncondensables, and high and low entering air temperatures.

Compressor Inefficiencies

Inefficient compressors certainly decrease the heat transferability of an air conditioning system; after all, they are responsible for circulating the refrigerant through the system. Leaky valves and worn piston rings are two major problems that lead to inefficiencies associated with compressors.

A symptom of an inefficient compressor is high suction pressure along with low discharge pressures.

As we discussed in the previous article, the evaporator cannot handle the load due to a decreased refrigerant flow; the conditioned space temperature will start to rise. This rise in return air temperature will overload the evaporator - heat will cause high suction pressures and higher-than-normal superheats.

Piston ring blow-by and reed valve leakage can also cause high suction pressures. The condenser will also see a reduced load from the decreased mass flow rate of refrigerant being circulated through it.

The reduced condenser load results in low condensing pressure. The compressor amp draw will be lowered because less work is being expended. You can expect that subcooling in the condenser will be a bit low due to the reduced heat load on the condenser.

Symptoms of compressor inefficiencies include:

  • High suction pressures.

  • Low head pressures.

  • Low compressor amp draw.

  • High superheat.

  • High return air temperature.

  • Low to normal condenser subcooling.

    Noncondensables In The System

    Air and water vapor are probably the best-known noncondensables that may be found in refrigeration or air conditioning systems.

    A noncondensable usually enters the system through poor service practices and/or leaks. For instance, a technician might forget to purge hoses, thus letting air and water vapor into a system. The air and water vapor pass through the evaporator and compressor because the compressor is essentially a vapor pump.

    Once the air gets to the condenser, it remains at its top and will not condense. The subcooled liquid seal at the condenser's bottom prevents the air from passing out of the condenser. This air and water vapor take up valuable condenser surface area and cause high head pressures.

    Subcooling will be high because the high head pressures cause a greater temperature difference between the liquid temperature in the condenser and the ambient.

    It's important to note that noncondensables in a system and an overcharge of refrigerant have very similar symptoms when the system has a thermostatic expansion valve (TXV) metering device.

    Symptoms of noncondensables in a system include:

  • High head (condensing) pressures.

  • High subcooling.

  • High compression ratios.

  • High discharge temperatures.

    Low Condenser Entering Air Temperature

    Low condenser entering air temperature will cause low head pressure from the excessive heat transfer between this cool ambient and the condenser coil. Low head pressures may reduce flow through some metering devices, particularly those that have capacity ratings dependant on the pressure differences across them.

    A 30-psi pressure difference is usually the minimum across TXVs. This reduced refrigerant flow causes a starved evaporator that will in turn cause low suction pressures and high superheats. However, this may be offset by increased subcooling at low ambients.

    The entire drop in capacity may decrease the air conditioner's heat-removal abilities if it is not designed for it. If not designed properly, liquid will start to back up in the condenser. But, because of a low heat transfer rate caused by the lower condenser temperature, the liquid temperature in the bottom of the condenser will be low, causing liquid subcooling in the condenser to be increased.

    Less refrigerant circulated means less work for the entire system to perform, so the amp draws of the compressor will be lowered.

    If the system is set up for this reduced condenser air entering temperature, the head pressure can be designed to "float" or change with the charging ambient temperature. This will give lower head pressures and in-creased efficiencies. A TXV that is properly matched to handle reduced pressure drops across its orifice may need to be incorporated into the design.

    Symptoms of low condenser entering air temperature include:

  • Low entering air temperature.

  • Low suction pressure (if not designed for low head pressure at the TXV).

  • Low discharge (condensing) pressure.

  • High superheat (if not designed for low head pressure at TXV).

  • Low amp draw.

  • High subcooling.

    High Condenser Entering Air Temperature

    High ambient temperatures have much different effects on an air conditioning system. The higher outdoor ambient temperatures will cause head pressures to elevate in order to complete the heat-rejection task. The temperature difference (TD) between the condensing temperature and the hotter ambient will decrease.

    The refrigerant gas will not condense until the condensing pressure and temperature rise. The condenser cannot reject as much heat at this lower TD and thus will accumulate the heat. The accumulated heat forces the condensing temperature to elevate to a TD where the heat can be rejected.

    Remember, the temperature difference drives the heat transfer. However, this heat rejection happens at a higher condensing temperature, forcing the system to have high compression ratios and low efficiencies.

    High head pressures cause the compression ratio to increase; this causes low volumetric efficiencies when higher-pressure vapors re-expand in the clearance volume of the piston cylinder on each downward stroke. As volumetric efficiencies decrease, mass flow rates decrease and the compressor is less efficient.

    High head pressures also elevate liquid temperature entering the metering device, which will increase evaporator flash gas and thus decrease the net refrigeration effect. Because of these inefficiencies, the suction pressure may be a bit higher.

    Symptoms of high condenser entering air temperature include:

  • Higher suction pressures.

  • The system will have a hard time maintaining temperature and humidity of the conditioned space.

  • Evaporator superheats varying depending on the type of metering device.

    Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at tomczykj@tucker-usa.com.

    Publication date: 02/02/2004

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