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Reciprocating Compressors

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Most cooling systems in use today rely on reciprocating piston-type compressors. Reciprocating compressors are manufactured in three types:

  1. Hermetic - compressor-motor assembly contained in a welded steel case, typically used in household refrigerators, residential air conditioners, smaller commercial air conditioning and refrigeration units.
  2. Semi-hermetic - compressor-motor assembly contained in a casting with no penetration by a rotating shaft and with gasketed cover plates for access to key parts such as valves and connecting rods.
  3. Open - compressor only with shaft seal and external shaft for coupling connection to belt - or direct-drive using as electric motor or natural gas engine. These are largely used for ammonia refrigeration applications as hermetic designs cannot be used with ammonia refrigerant, and for engine-driven units.

As the piston nears the bottom of its stroke within the cylinder, the intake valve opens and the refrigerant vapor enters. As the piston rises, the increased pressure closes the intake valve. Then as the piston nears the top of its stroke, the exhaust valve opens permitting the vapor at the higher pressure to exit. Reciprocating compressor capacity is a function of the bore and stroke of the piston-cylinder configuration as well as the speed of the machine, and the clearance tolerances. Compressor capacity is also related to the compression ratio.

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The mechanical design is rugged and reliable but has one significant limitation. Reciprocating compressors are designed to handle vapors, not liquids. When liquid enters the cylinder on the intake stroke, it tends to damage the valves on the compression stroke and possibly the compressor itself. This is why chillers incorporate liquid-to-suction heat exchangers, which assure some level of vapor superheat at the compressor suction. Capacity is controlled by multiple staging of smaller compressors or in large multiple cylinder reciprocating compressors by unloading banks of cylinders on the compressor. This tends to make the machine most efficient at full load. Therefore, for maximum efficiency recips should generally be operated at full load. This is the reason small compressors are cycled on and off in most residential and small commercial applications.

The mechanical design is rugged and reliable but has one significant limitation. Reciprocating compressors are designed to handle vapors, not liquids. When liquid enters the cylinder on the intake stroke, it tends to damage the valves on the compression stroke and possibly the compressor itself. This is why chillers incorporate liquid-to-suction heat exchangers, which assure some level of vapor superheat at the compressor suction.

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Reciprocating compressors - power requirements

Due to better valve designs and configurations that reduce pressure losses, power requirements for reciprocating chillers have been improving over the years. Overall mechanical and compression efficiencies vary with the compression ratio, but are generally in the 72 to 78% range including the hermetic-type refrigerant-cooled 1,750 rpm motor. Compression ratio is computed by dividing the absolute discharge pressure by the suction pressure both measured in psia.

At ARI Standard rating conditions (44°F leaving chilled water, 85°F entering condenser water), typical chillers operate around 40°F evaporating and 100°F condensing temperatures equivalent to pressures. A modern reciprocating compressor has an energy efficiency ratio (EER) of about 15, equal to 0.79 kW per ton. However, in air-cooled conditions the condensing pressure is likely to run up to a 130°F temperature corresponding to pressure, with EERs ranging from about 10.4 up to 11.3, which equate to 1.15 to 1.06 kW per ton.

Assembled into chiller packages in the 20 to 200-plus ton capacities, air-cooled units will typically have EERs ranging from 9.0 to 10.9, equal to 1.33 to 1.10 kW per ton with an average of about 1.22 kW per ton. Similar water-cooled chiller packages will have EERs ranging from 13.1 to as high as 15.8, which equates to 0.92 to 0.76 kW per ton with an average of about 0.82 kW per ton.

Manufacturers continue to develop more efficient models. In some cases, scroll compressors are being used, in place of reciprocating.

Reciprocating compressors - first cost

While they are the least efficient of the chiller package options, reciprocating or scroll compressor chillers have a definite first cost advantage in the smaller chiller sizes. The first cost of reciprocating chiller packages is the lowest of the various electric chiller options, certainly when expressed in $ per ton. The compressors are competitively priced since they are used in many different chiller models. Plus, many more reciprocating chillers are produced than larger centrifugal and screw type chillers. These economies of scale result in a lower unit cost, especially for models up to about 200 tons.

Compare - Installed Costs - Chillers

Reciprocating compressors - operating costs

For example, an air-cooled chiller serving a hospital operating room suite that operates year-round could well have a lower annual electric cost than a comparable water-cooled unit, due to the large number of operating hours the unit will be operating at part-load and low-ambient temperature conditions. Only a careful energy use analysis of each application performed by a qualified professional can identify the most economical equipment choice.

Maintenance costs must also be factored in. Here are some typical mid-1995 $ per ton annual values.

Chiller Type 20 Tons 50 Tons 75 Tons 100 Tons 150 Tons 200 Tons
Water Cooled $79 $67 $58 $51 $40 $35
Air Cooled $70 $50 $45 $43 $35 $31

If the chiller is driven by a natural gas engine, the added maintenance costs of the engine must also be included. Typically this amounts to about $0.012 per ton per operating hour.

Reciprocating compressors - emissions

Reciprocating chiller emissions fall into two major categories: direct (or on-site), and indirect (or emissions resulting from the production of the energy used to operate the equipment).

Direct on-site emissions are confined to the release of refrigerant due to leaks or servicing. Federal law now mandates no intentional release. It is the responsibility of the user and service agency to minimize leaks and service release. Good preventive maintenance practices are imperative. Other factors affecting emissions include chiller age, application (whether it's a single package or split system), compressor type (open compressors with shaft seals tend to leak more than hermetic designs).

A typical semi-hermetic type chiller might lose about 3 to 5 percent of its charge annually. With the refrigerant charge running about 3 pounds per ton, the emission of refrigerant might total about 0.12 pounds per ton per year. An open chiller might lose 5 to 7 percent, and thus emit about 0.18 pounds per ton-year. To allow for less than ideal conditions, a conservative estimate of emissions might be 0.25 pounds per ton-year.

Natural Gas engine-driven reciprocating chillers must use open-type compressors. In addition to the same refrigerant emissions as an electric chiller, they also produce emissions from the combustion of the natural gas. Also, the leakage of natural gas into the atmosphere although small, is believed to contribute comparable greenhouse gases as refrigerant leakage.

These emissions can be estimated, based on the annual gas consumption. Typical gas engine driven chillers use about 9,300 Btu per ton-hour of natural gas (on a HHV basis). Using the annual ton-hours of cooling, the emissions of CO2 and the criteria gases can be estimated using these relative values of pounds per million Btu of fuel burned. The emissions of all gases other than NOx are relatively constant throughout the loading range. NOx emissions will vary considerably, depending on the annual load profile.

On-site emissions in pounds per million Btu of Natural Gas burned
CO2 CO NOx SOx VCC Particulates*
118 20% 44% 0 .31 .00%

*Particulates are 10 microns or less. Volatile organic compounds (VOC) includes hydrocarbons (HC).

While so-called "lean-burn engines" emit less NOX than conventional engines at full load, they emit more at part load conditions. Since chillers operate largely at part load, the added expense of a lean-burn engine is usually not justifiable.

Indirect emissions occur at the power plants generating the electricity used to power chillers. Remember that comparing different chillers (for example, electric versus gas) must include the effect of the chiller and system auxiliary energy consumption - not just the chiller's power use. These emissions can be estimated from the annual power consumption in kWh and the local electric utility's emission data.

Most utilities know their typical emissions of the various gases andparticulates on a "per kWh" basis.