Chapter hvac engineering Fundamentals: Part 1 Introduction

9.3
Compressors
In the refrigeration cycle, a compressor is a pump, providing the work
energy to move the refrigerant from the low-pressure region to the
Equipment: Part 1
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Chapter Nine
Figure 9.4
Reciprocating compressor.
high-pressure region through the system. Compressors come in two
general types: positive displacement and centrifugal. Positive displace-
ment compressors include reciprocating, rotary, scroll, and helical ro-
tary (screw) types.
9.3.1
Reciprocating compressors
Reciprocating compressors are usually the single-acting piston type.
Figure 9.4 shows one possible arrangement. The volume swept by the
piston is the displacement. The remaining volume under the cylinder
head is the clearance. The theoretical volumetric efficiency is a func-
tion of the ratio of these two volumes together with the compression
ratio of the system. Higher compression ratios result in lower volu-
metric efficiencies. The actual volumetric efficiency will be somewhat
less due to pressure drops across valves and other inefficiencies. The
capacity of the compressor is a function of the volumetric efficiency,
the properties of the refrigerant, and the operating pressures.
A compressor is always designed for a specific refrigerant at some
narrowly defined range of operating pressure.
A compressor may have from 1 to 12 cylinders. Some older machines
had as many as 24 cylinders. Most compressors for comfort cooling are
direct-driven by electric motors. Historically, compressors were belt-
driven, often by steam engines and at slow speeds.
Equipment: Part 1
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