Control valves energy losses with

The first method is used most frequently. The next preference is for the last method, mostly used in small compressors due to problems with speed control of electrical motors. Other means of capacity control are very seldom utilized due to thermodynamic inefficiencies and design difficulties. Energy losses in a compressor, when capacity regulation is provided by lifting the suc tion valves, are due to fric tion of gas flowing in and out the unloaded cylinder. This is shown in Fig. 11-84 where the comparison is made for ideal partial load operation, reciprocating, and screw compressors. 

The control valve is a variable orifice device in which the size of the orifice is adjusted to control a process variable. Consequently, the manufacturer, type, or even the size of a control valve has no effect on the energy dissipated in the control of a selected stream once the process pressure, line size, and pumps have been selected. This energy-independence of the control valve assures that continuous throttling of the flow stream is required to control a process variable. In those cases where a valve is used for shut-off or override control (not a continuous throttling device), energy savings can be realized by selecting a valve with a minimum pressure loss in the full-open position. 

In a process, energy losses consist of both thermal and mechanical losses. Thermal losses are typically originated from column overhead condensers, product mn down coolers, furnace stack, steam leaks, poor insulation of heat exchangers/piping and vassals and so on. Mechanical losses could also be significant, which usually occurs in rotating equipment, pressure letdown valves, control valves, pump spill back, heat exchangers, pipelines, and so on. Some of the thermal and mechanical losses are recoverable with a decent payback of investment but many others do not. 

Pumps are driven by either fixed-speed or variable-speed motors. Variable-speed motors are becoming increasingly common. Flow can be controlled by varying the pump speed with the motor and thus eliminating parasitic energy losses across a flow control valve. However, because of the extra electronic components needed for a variable-speed AC motor, 95 percent of the motors we work with are fixed-speed alternating-current motors. I will only discuss fixed-speed, alternating-current (AC) motors in this chapter. 

In addition to the energy savings typical with hydronic systems, the approach reduces head loss hy eliminating many control and balancing valves, as well as some pipe. The result is lower pump head and less energy consumption to move the water. 

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