Engineering Manual o.. - HVAC.Amickracing

VALVE SELECTION AND SIZINGb. The pressure drop across an open valve should beabout half of the pressure difference betweensystem supply and return, enough so that thevalve, not the friction through the coil or radiator,controls the volume of water flow or the valvepressure drop should be equal to or greater thanthe pressure drop through the coil or radiator,plus the pipe and fittings connecting them to thesupply and return mains.c. Verify allowable full open and full closedpressure drops for all proportional and twopositionwater valves with appropriatemanufacturer literature.d. Make an analysis of the system at maximum andminimum rates of flow to determine whether ornot the pressure difference between the supplyand return mains stays within the limits that areacceptable from the stand point of controlstability and close-off rating.2. For two- and three-way valves consider the followingguidelines for valve pressure drop:a. In load bypass applications (Fig. 13) such asradiators, coils, and air conditioning units, thepressure drop should be 50 to 70 percent of theminimum difference between the supply andreturn main pressure at design operatingconditions.b. A manual balancing valve may be installed in thebypass to equalize the load drop and the bypassdrop.3. When selecting pressure drops for three-way mixingvalves in boiler bypass applications (Fig. 13), considerthe following:a. Determine the design pressure drop through theboiler including all of the piping, valves, andfittings from the bypass connection through theboiler and up to the three-way valve input.b. The valve pressure drop should be equal to orgreater than the drop through the boiler and thefittings. If the valve drop is much smaller thanthe boiler pressure drop at design, effectivecontrol is obtained only when the disc is near oneof the two seats. The mid-portion of the valve liftwill be relatively ineffective.c. A manual balancing valve may be installed in theboiler bypass to equalize the boiler drop and thebypass drop.WATER VALVE SIZING EXAMPLESEXAMPLE 1:A two-way linear valve is needed to control flow of 7°Cchilled water to a cooling coil. The coil manufacturer hasspecified an eight-row coil having a water flow pressuredrop of 22 kPa. Further, specifications say that the coil willproduce 13°C leaving air with a water flow of 3.32 m 3 /h.Supply main is maintained at 275 kPa, return is at 200 kPa.Select required capacity index (Kv) of the valve.Use the water valve Kv formula to determine capacityindex for Valve V1 as follows:Where:Q= Flow of fluid in Cubic meters per hourrequired is 3.32 m 3 /h.ρ = Density of water is 1000.∆PSubstituting:Kv = Q= Pressure drop across the valve. Thedifference between the supply and return is75 kPa. 50% to 70% x 75 kPa = 37.5 to52.7 kPa. Use40 kPa for the correct valve pressure drop.Note that 40 kPa is also greater than the coilpressure drop of 22 kPa.Kv = 3.32ρ∆P • 10100040 • 10 = 5.2Select a linear valve providing close control with acapacity index of 5.4 and meeting the required pressureand temperature ratings.EXAMPLE 2:A bypass valve is required to prevent flow through thechiller from dropping below 90 percent of design flow.When sizing valves for pump or chiller bypassapplications (Fig. 16), system conditions that cause thevalve to open or close completely must be consideredbefore a pressure drop can be selected.ENGINEERING MANUAL OF AUTOMATIC CONTROL439