Engineering Manual o.. - HVAC.Amickracing

VALVE SELECTION AND SIZINGCASE A: 345 kPaCASE B: 425 kPa80°CHOT WATERSUPPLYFig. 17. Equal Percentage Valve Hot Water Application.100%FLOW AT CONSTANTPRESSURE DROP30%PRESSUREDROP0% 100%STEM TRAVEL70%PRESSUREDROPIDEAL EQUALPERCENTAGEVALVECHARACTERISTICFig 18. Effect of Pressure Dropin Hot Water Valve Sizing.C2340EXAMPLE 4:A three-way mixing valve is needed for a heat exchangerapplication with a bypass line. Water flow is specified atthe rate of 16 m 3 /h. Manufacturer data for the exchangerindicates a pressure drop of 4.2 kPa through the exchangercoils.Use the water valve Kv formula to determine capacityindex for Valve V1 as follows:Where:QKv = Q= Flow of fluid required to pass through thevalve is 16 m 3 /h.ρ = Density of water is 1000.∆PVALVE VI30% PRESSURE DROP, Kv = 1070% PRESSURE DROP, Kv = 4.4ρ∆P • 10HEATINGCOILLOCALPIPING15 kPaDROP4.54 m 3/hAT DESIGN,30 kPa DROP275 kPaHOT WATERRETURNC4369= Pressure drop across the valve. Plans of theheating system indicate 80-mm supply andreturn mains. From an elbow equivalenttable and pipe friction chart found in theASHRAE Handbook or other referencemanuals, the calculated pressure dropthrough a 80-mm tee and the piping from thevalve and the tee to the exchanger is0.62 kPa. Heat exchanger pressure drop is4.2 kPa. Total pressure drop from bypassconnection through the heat exchanger andto the hot-water input of the three-way valveis 4.2 kPa + 0.62 kPa or 4.82 kPa.Since the valve pressure drop (∆P) should be equal to orgreater than the drop through the heat exchanger andfittings, 4.82 kPa is used as the valve pressure drop.For optimum control, a manual balancing valve isinstalled in the bypass line to equalize the pressure dropsin the exchanger and bypass circuits.Substituting the flow of water, density of water, andpressure drop in the Kv formula shows that the valveshould have a Kv of 72.8 or 73.Select a linear valve providing close control with acapacity index of 73STEAM VALVESCalculate the required capacity index (Kv) for a valve usedin a saturated steam application, using the formula:Where:Q= Quantity of steam in kilograms per hourrequired to pass through the valve.0.224 = A scaling constant.∆P = Pressure drop in kPa.P o= Absolute outlet pressure of valve inkilopascals (P 1 – ∆P).Calculate the required capacity index (Kv) for a valve usedin a superheated steam application, using the formula:Where:QKv = 16Kv =Kv =∆PP 1V 110004.82 • 10 = 72.8Q0.224 ∆P • P oQ3.15 – ( 1.55 • ∆PP1)V1∆P= Quantity of steam in kilograms per hourrequired to pass through the valve.= Pressure drop in kPa.= Absolute inlet in kilopascals.= Inlet specific volume in cubic centimetersper kilogram.Determining the Kv for a steam valve requires knowing, thequantity of steam (Q) through the valve, the pressure drop (∆P)across the valve, and the degrees of superheat. See QUANTITYOF STEAM and STEAM VALVE PRESSURE DROP. Thenselect the appropriate valve based on Kv, temperature range,action, body ratings, etc., per VALVE SELECTION guidelines.ENGINEERING MANUAL OF AUTOMATIC CONTROL441