Troubleshooting Inefficient Compressors

July 31, 2001
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There are a number of reasons why a reciprocating compressor may have bad valves or worn rings and become inefficient.

These include:

  • Slugging of refrigerant and/or oil;
  • Refrigerant migration problems;
  • Refrigerant flooding problems;
  • Insufficient lubrication;
  • Overheating of the compressor from system problems;

  • Acids and/or sludges in the system deteriorating parts; and

  • The TXV is set incorrectly — too little superheat is causing overheating.

    Figure 1 is a service checklist for an inefficient compressor with bad valves or worn rings. This checklist is for a compressor that is still cooling but is showing signs of inefficiency. (Note: The severity of the problem will cause the checklist to change.)



    SYSTEM SYMPTOMS

    There are a number of symptoms for an inefficient compressor, such as:

  • Higher than normal discharge temperatures;
  • Low condensing (head) pressures and temperatures;
  • Normal to high condenser subcooling;
  • Normal to high superheats;
  • High evaporator (suction) pressures; and
  • Low amp draw.
  • A system that has bad valves or worn rings will have both a high evaporator (suction) pressure along with a low condensing (head) pressure. This causes blow-by of gases around the piston rings or short-cycling of gases through valves, creating low head pressure with high suction pressure. So, anytime the technician installs his gauge and has low head with high suction pressures, there has to be either a valve or ring problem.

    Higher than normal discharge temperatures. A discharge valve that isn’t seating properly because it has been damaged will cause the head pressure to be low. Refrigerant vapor will be forced out of the cylinder and into the discharge line during the upstroke of the compressor. On the down-stroke, this same refrigerant that is now in the discharge line and compressed will be drawn back into the cylinder because of the discharge valve not seating properly. This short-cycling of refrigerant will cause heating of the discharge gases over and over again, causing higher than normal discharge temperatures. However, if the valve problem has progressed to where there is hardly any refrigerant flow rate through the system, there will be a lower discharge temperature from the low flow rate. Worn rings will have the same symptom.

    Low condensing (head) pressures. Because some of the discharge gases are being short-cycled, there will be a low refrigerant flow rate to the condenser. This will make for a reduced heat load on the condenser thus reduced condensing (head) pressures and temperatures.

    Normal to high condenser subcooling. There will be a reduced refrigerant flow through the condenser, thus through the entire system because of components being in series. Most of the refrigerant will be in the condenser and receiver. This may give the condenser a bit higher subcooling.

    Normal to high superheats. Because of the reduced refrigerant flow through the system, the TXV may not be getting the refrigerant flow rate it needs. High superheats will result. However, the superheats may be normal if the valve or ring problem is not real severe.

    High evaporator (suction) pressure. Refrigerant vapor will be drawn from the suction line into the compressor’s cylinder during the downstroke of the compressor. However, during the up-stroke, this same refrigerant may sneak back into the suction line if the valve is worn and/or not seating. If the rings are worn and leaking during the upstroke, the refrigerant will sneak back into the crankcase causing higher than normal suction pressures. In both cases, the result is higher than normal suction pressures.

    Low amp draw. Low amp draw is caused from the reduced refrigerant flow rate through the compressor. With bad valves, during the compression stroke some of the refrigerant will leak through the suction valve and back into the suction line reducing the refrigerant flow. During the suction stroke, some of the refrigerant will sneak through the discharge valve if it is not seating properly. The refrigerant will get back into the compressor’s cylinder. In both situations, there is a reduced refrigerant flow rate causing the amp draw to be lowered. The low head pressure that the compressor has to pump against will also reduce the amp draw. Worn rings will short-cycle refrigerant, also creating a low compression ratio and thus a lower than normal amp draw.

    Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI, and author of the book Troubleshooting and Servicing Modern Air Conditioning and Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696.

    Sidebar: Supermarkets Monitor Energy on the Web

    Albertson’s, a major supermarket chain with stores throughout energy-starved California, has taken energy conservation to the Internet.

    The chain teamed up with Notifact, a wireless equipment communications company, and Invensys Energy Solutions to provide Internet-based remote control of lighting systems at 250 locations in the state.

    This real-time communications capability will enable the company to turn the lights on or off at any or all locations instantly, allowing Albertson’s to meet the California Public Utility Commission’s (CPUC) 30-minute window for participating in the curtailable load rebate program. Albertson’s also will receive a grant for installation of the equipment through the CPUC’s demand load reduction program.

    “With two-way communications capabilities, we now offer the potential to control equipment remotely from a branded website, WAP phone, or Palm Pilot,” said Kevin Duffy, vice president, sales and marketing, Notifact.

    The Notifact wireless transceiver connects directly to the Invensys Energy Solutions energy management and control system located in the Albertson’s stores, and allows Albertson’s to remotely configure the system.

    Publication date: 08/06/2001

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