by Harry Hartigan

The last thing your customers need is their mechanical system failing because of repeat compressor failures. There are many factors that can cause a compressor to fail and, as a service contractor, no amount of technology or skill with tools can beat good detective work in the field. Let’s examine four areas of concern:

  • Superheat
  • Capacity Control
  • Compressor Condemning Process
  • New Compressor Start-up.

Superheat

Suction superheat (or lack thereof) significantly impacts compressor life. An improper superheat setting and poor refrigerant control will cause a slow death from repeated starts with poor lubrication. In extreme cases, poor superheat control is the setup for a catastrophic internal failure caused by oil slugging.

No matter how you look at it, control over suction superheat at all times is critical. The recommended superheat setting for air conditioning duty (assuming the system has a thermostatic expansion valve (TXV), and operates in the range of 90 to 125F saturated condensing temperature and 30 to 45F saturated suction temperature), is a minimum of 15F and a preferred 20F (after deducting the expected error as illustrated in the chart below from Brainerd Compressor).

Maintain this setting at all times (loaded and unloaded) to ensure that only dry gas returns to the compressor.

Consideration should be given to the following:

  • Application of a given TXV (or multiple TXVs over the capacity range of the refrigeration circuit) must be checked over the full range of expected operation to ensure control.
  • TXVs should be selected using the manufacturers’ extended capacity tables. The selection must then be checked over the entire expected operating range. TXV selection should never be made by the counter person at the supply house based on the nominal rating of the valve (typically stamped on the box). Selection in this manner typically leads to an oversized TXV even at full load, which significantly
    affects control at part load.
  • Variables that will affect how a given TXV performs include:

Outdoor ambient temperature (which affects the condensing temperature and subsequently, the inlet pressure to the valve).

The degree of refrigerant sub-cooling entering the TXV (more sub-
cooling requires fewer pounds of
refrigerant to be circulated to obtain a given refrigeration effect).

Evaporator temperature, also known as saturated suction temperature.

Compressor(s) unloading capability (could be a single compressor with significant unloading capability or a circuit with multiple compressors).

Don’t assume that a TXV will control superheat below about 70% capacity for single ported valves and about 50% for balanced ported valves. Although some manufacturers advertise that balanced ported valves will control down to 20% of their maximum capacity, they’ll likely not do so reliably for extended periods of time.

Also, if a single TXV is likely not to control over the entire range, consider using multiple TXVs with individual solenoid valves. As the load drops, close the solenoid valve serving the larger TXV, and open the solenoid valve serving the smaller TXV.

This has the affect of keeping the valves operating in the normal, expected controllable range.

What should the superheat be? One manufacturer says that if the field superheat reading is less than 12F, the compressor is bringing back liquid. Another says a minimum of 15F is needed to ensure a dry vapor at the inlet to the compressor, and recommends a minimum 20F superheat at the compressor.

Since suction line temperature measurements are affected by ambient temperature, it’s good practice to consider the effect of ambient temperature on the accuracy of the reading.

Keep in mind that superheated refrigerant isn’t needed (or desired) in the evaporator, as it uses valuable surface area that could otherwise be used for latent heat exchange; however, superheat is what protects the compressor.

In a laboratory environment, superheat is equal to the actual temperature of the refrigerant vapor (measured in the gas stream) minus the saturated suction temperature (pressure in the suction line at the temperature probe location, converted to temperature). In the field, "practical" superheat is equal to the measured suction line surface temperature minus the saturated suction temperature (suction pressure converted to temperature) minus field error. Field error accounts for such things as:

  • The temperature difference between the fluid temperature inside the pipe and the outside surface temperature of the pipe.
  • The limited surface contact area of the temperature probe used.
  • The effect of the outside ambient temperature on the temperature probe.

The recommended method for measuring superheat is as follows:

  1. Attach a good quality (and recently calibrated) electronic temperature probe to a clean section of suction line (not a fitting) within 12 in. of the suction service valve using copper straps (or the brass straps that come with most TXVs). Copper straps conduct heat all the way around the probe and provide a more accurate reading. Electrical tape, duct tape, tie -wraps, etc. should be avoided.
  2. Insulate the probe extending at least six inches in each direction, first with Armaflex type foam tape and then with Armaflex sheet, fiberglass or some other good quality insulating material.
  3. Install a gauge manifold (also recently calibrated) as close as possible to the temperature probe location. It is important not to have any significant pressure drop between the pressure gauge and the probe or the reading will be in error.
  4. Record the suction pressure and suction line temperature.
  5. Subtract from the suction line surface temperature, the saturated suction temperature plus the error in degrees to obtain the reasonable approximation of superheat.

Capacity Control

In a direct expansion (DX) cooling system, the load on the cooling coil doesn't match the full load capacity of the compressor except on a design day. At all other times (typically 97.5% to 99.6% of the time, based on ASHRAE standards), the compressor capacity exceeds the required cooling and some method of capacity control must be implemented to prevent the coil from freezing.

Methods include:

  • Cylinder unloading where one or more banks of cylinders are rendered inoperative by various methods such as:
    Suction cutoff unloading where the suction valves are blocked from the suction manifold on a particular head, preventing compression of new gas.
    Discharge bypass where a solenoid is installed on the compressor head that bypasses the compressed refrigerant from the discharge side of the head back to the suction side of the head preventing compression of new gas.
    Hydraulic unloaders (larger semi-hermetic and open drive compressors) where oil (typically) is used to raise the suction valve off its seat, preventing compression.
  • Hot gas bypass where compressor discharge gas is bypassed from the compressor outlet to either the coil inlet or compressor suction line.
  • Applying multiple smaller compressors to a single refrigeration circuit and then turning them on and off to vary circuit capacity.
  • Multi-speed or variable speed compressors (variable speed is typically seen on larger centrifugal units).
  • A combination of the above .

NOTE: Some technical information for this article was taken from reference material provided by Brainerd Compressor Co., Sporlan Valve Co., The Trane Co., and Carlyle Compressor.

This ends part one. Next month, we’ll go into more detail on capacity control, and present information on compressor condemning and new compressor start-up.

Harry Hartigan is the president of Mechanical Service Corp., a 27-year old commercial/industrial contracting firm based in Whippany, NJ. The company specializes in process cooling, facility automation, and HVAC service. This article is based an a technical bulletin Hartigan put together for his service technicians. He can be reached via e-mail at hhartigan@mscnj.com.