HEAT EXCHANGER FORMULAS & EXAMPLE Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve
CONTROL VALVE SIZING
E) What is the flow of steam (steam capacity) thru the control valve at the design heat load? Formula 6 gives the mass flow rate of steam based on the heat load and the latent heat of saturated steam (LH). From the saturated steam table, the latent heat of 50 PSIG steam is 912 Btu/lb. Therefore, the steam capacity is: Q S = E D / LH = 4,500,000 Btu/hr / 912 Btu/lb = 4,934 lbs/hr F) How must the control valve be sized? The valve must be sized for the maximum steam capacity of the application, which occurs at the maximum (design) heat load of the heat exchanger. We first need to determine if the pressure drop across the valve at the maximum flow rate is above or below the critical pressure drop, so that we can apply the correct formula:
Valve Inlet Pressure (P 1 ) = Steam Supply Pressure = 100 PSIG + 14.7 = 114.7 PSIA Valve Outlet Pressure (P 2 ) = Heat Exchanger Pressure = 50 PSIG + 14.7 = 64.7 PSIA Differential Pressure ( Δ P) = P 1 – P 2 = 114.7 – 64.7 = 50 PSI Critical Pressure Drop ( Δ P critical ) = 0.42 P 1 = 0.42 (114.7) = 48.2 PSI
Since the pressure drop across the valve (50 PSI) is greater than the critical pressure drop (48.2 PSI), the steam flow thru the valve is critical. Therefore, we apply Formula 12b to calculate the valve coefficient:
C v = Q S / ( 1.71 x P 1 ) = 4,934 / ( 1.71 x 114.7) = 4,934 / 196.1 = 25.2 Therefore, the control valve must have a flow coefficient of at least 26.
G) What Watson-McDaniel Control Valve should be selected for this application? Refer to the Control Valves section of this catalog. The Watson McDaniel HB-Series 2-Way Pneumatic Control Valve is used for heating and cooling applications. Since this is a heating application with steam, a Normally-Closed, Air-To-Open (ATO) valve should be selected. (This is a fail-safe feature in case the air signal to the valve actuator becomes interrupted. If the air signal is lost, the valve will automatically close and block the flow of steam.) Referring to the HB Control Valve Selection chart, a full-port valve with Cv value of 42 should be selected. The Model Number for this valve with NPT ports is HB-17-N-ATO . This valve has a 2” NPT connection size, stainless steel body and trim, and a pressure-temperature rating of 300 PSIG @ 450°F.
72
www.watsonmcdaniel.com •• Pottstown PA • USA • Tel: 610-495-5131
Made with FlippingBook - Online magazine maker