Discharge throttling

In order to avoid the need to measure velocity head, the loop piping must be sized to have a velocity pressure less than 5% of the static pressure. Flow conditions at the required overload capacity should be checked for critical pressure drop to ensure that valves are adequately sized. For ease of control, the loop gas cooler is usually placed downstream of the discharge throttle valve. Care should be taken to check that choke flow will not occur in the cooler tubes. Another cause of concern is cooler heat capacity and/or cooling water approach temperature. A check of these items, especially with regard to expected ambient condi-... 

If the same compressor is modulated by discharge throttling and the flow is reduced in a mostly friction process, the compressor must follow Curve I and therefore operate at point (4) at 72% efficiency. However, the mostly friction system curve at this capacity requires only the discharge pressure corresponding to point (3) and therefore the pressure difference between points (2) and (3) will be wasted in the form of a pressure drop across the discharge valve. 

Brake horsepower is also plotted on the curve. Note that brake horsepower decreases with decreasing flow, but does not reach zero at zero flow. There will be a noticeable and sometimes troublesome "heat effect" in cetrifugal pumps operating with the discharge throttled to abnormally low flow rates. Pump efficiency is also indicated on the curve. 

But we are working with an ordinary AC (alternating-current) motor—which is a fixed-speed device. There are then three methods available to control the temperature in the evaporator spillback, discharge throttling, and suction throttling. 

The means available for controlling compressor capacity are suction throttling, discharge throttling, recirculation, variable guide vanes and motor speed control. Most of these controls are used in practice, and which is best depends on the application. Some of the pros and cons of these alternative control methods are as follows ... 

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