2000 Hook-up Book

How to Size Temperature and Pressure Control Valves

Having determined the heating or cooling load required by the equipment, a valve must be selected to handle it. As the valve itself is only part of the complete control, we must be acquainted with certain terminology used in the controls field: Flow Coefficient. The means of comparing the flow capacities of control valves by reference to a “coefficient of capacity.” The term Cv is used to express this rela tionship between pressure drop and flow rate. Cv is the rate of flow of water in GPM at 60°F, at a pressure drop of 1 psi across the fully open valve. Differential Pressure. The differ ence in pressure between the inlet and outlet ports when the valve is closed. For three-port valves, it is the difference between the open and closed ports. Maximum Differential Pressure. The pressure difference between inlet and outlet ports of a valve, above which the actuator will not be able to close the valve fully, or above which damage may be caused to the valve, whichever is the smaller. Pressure Drop. The difference between the inlet and outlet pres sures when the valve is passing the stated quantity. A self-acting

control may or may not be fully open. For three-port valves, it is the difference in pressure between the two open ports. Working Pressure. The pressure exerted on the interior of a valve under normal working conditions. In water systems, it is the algebra ic sum of the static pressure and the pressure created by pumps. Set Point. Pressure or tempera ture at which controller is set. Accuracy of Regulation or “Droop”. Pressure reducing valve drop in set point pressure necessary to obtain the published capacity. Usually stated for pilot operated PRV’s in psi, and as a % of set pressure for direct-acting types. Hunting or Cycling. Persistent periodic change in the controlled pressure or temperature. Control Point. Actual value of the controlled variable (e.g. air temperature) which the sensor is trying to maintain. Deviation. The difference between the set point and the measured value of the controlled variable. (Example: When set point is 70°F and air temperature is 68°F, the deviation is 2°F.) Offset. Sustained deviation caused by a proportional control

taking corrective action to satisfy a load condition. (Example: If the set point is 70°F and measured room temperature is 68°F over a period, the offset is 2°F and indicates the action of a proportional control cor recting for an increase in heat loss.) Proportional Band or Throttling Band. Range of values which cause a proportional temperature control to move its valve from fully open to fully closed or to throttle the valve at some reduced motion to fully closed. Time Constant. Time required for a thermal system actuator to travel 63.2% of the total move ment resulting from any temperature change at the sen sor. Time increase when using separable well must be included. Dead Zone. The range of values of the controlled variable over which a control will take up no corrective action. Rangeability. The ratio between the maximum and minimum con trollable flow between which the characteristics of the valve will be maintained. Turn-Down Ratio. The ratio between the maximum normal flow and minimum controllable flow. Valve Authority. Ratio of a fully open control valve pressure drop to system total pressure drop.

SYSTEM DESIGN

Case in Action: Log Bath-Furniture Manufacturing

Solution Two temperature control valves to be open during start-up with one closing as it approaches the desired cooking tem perature. The second smaller valve continues to provide steam to the system until the set-point is reached. As addi tional steam is required, the smaller valve supplies it. A sparge pipe was also sized and installed. Benefits: • Payback of this system was less than 2 weeks on materials and labor. • Substantial cost savings due to improved energy use. • Increased profitability by increasing productivity in the steam system.

At a furniture manufacturing facility, the water used for bathing logs to prepare them for production was “rolling” in the front of its containment tanks. The production manag er had thought that the temperature had to be at least 212 °F. Further examination showed the water’s temperature to be 180°F. The water was “rolling” because the steam, entering the side of the tank, could not be absorbed by the water before it rose to the surface in the front of the tank. Cedar logs are cooked for 48 hours, in open top tanks before going through a veneer machine. The logs absorb the hot water, making it easier to slice the wood into strips. The six log baths did not have any temperature controls. Twenty-five psig steam flowed through a 2" coupling into the side of the tank to heat the water. With the tank size being 12' x 12' x 6', the 105 cedar logs approximately 10' long occupy most of the space in the tank. River water or “condenser water” off of the turbine at 90°F is fed into the tank.

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