Direct indication of the operating conditions of a compressor is by pressure gauges at suction, discharge and oil delivery. Such gauges are mounted on or near the compressor.
Since the pressure losses along the discharge and suction lines are comparatively small on most systems, these pressures will also approximate to the conditions in the condenser and evaporator, and the equivalent saturation temperatures will be the condensing and evaporating temperatures. To indicate these temperatures for the common refrigerants, pressure gauges will have further calibrations showing these equivalent temperatures (see Figure 9.5).
Figure 9.5 Refrigeration pressure gauge (Star Instruments)
Gauge mechanisms are mostly of the bourdon tube type, having a flattened tube element, which distorts under pressure change. Gas pulsations from the compressor will be transmitted along the short connecting tubes and may lead to early failure of the needle mechanism. These can be damped by restricting the tube with a valve or orifice, or oil filling the gauge as shown in Figure 9.5, or both. Gauge needles should not be allowed to flicker noticeably from gas pulsations. Pressure transducers are now used for integrated control circuits.
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