Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve HEAT EXCHANGER FORMULAS & EXAMPLE
Comparison of Methods: Both methods gave the same result for the water flow rate at which stall will occur: 71 GPM. This means that at flows at or below 71 GPM, the steam pressure in the system is insufficient to push the condensate thru the steam trap and into the return line. The condensate will therefore back up into the heat exchanger unless something is done to prevent it. The main difference between the two methods is that the heat exchanger size was needed to calculate the stall
flow rate using Method 1 , but not needed using Method 2 . N) How can stall (condensate back-up) be prevented?
Stall can be prevented by replacing the steam trap with a pump-trap (i.e., a pressure motive pump and steam trap combination). Pump-traps are available with either internal or external steam traps, depending on capacity requirements. When there is sufficient steam pressure to push the condensate thru the trap, the pump is not used and the pump- trap is operating in trap mode . The condensate will pass thru the pump body and thru the trap. The trap must be sized to handle the condensate at full-load conditions as well as when the trap differential pressure is slightly above 0 PSI (i.e., just above the stall point). In addition, the orifice size of the trap should be optimized to handle the high instantaneous discharge flow rate when the pump is operating. This will reduce the discharge time of the pump and its overall fill/discharge cycle. Watson McDaniel pump-traps have the trap size optimized for all conditions. When the steam pressure drops to or below the back pressure, the condensate will start to fill the pump tank. When the float in the tank reaches the upper trip point, the mechanism will open the steam valve while simultaneously closing the vent valve. High pressure steam will then force the condensate thru the trap and into the condensate return line. Check valves are used with the pump to prevent the backflow of condensate. When the pump is emptied, the float mechanism will then simultaneously close the steam valve and open the vent valve so the pump can fill on the next cycle. When the pump is being used, the pump-trap is operating in pump mode . The pump must be sized to handle the condensate load at the stall point. That is, when the steam pressure is equal to the back pressure. When sizing Pressure Motive Pumps in closed-loop return systems, a condensate reservoir should be installed on the inlet side of the pump and below the HX, as shown in Figure 20. This will enable the condensate to collect while the pump is in the discharge cycle, thus preventing liquid backup into the HX. The reservoir should be located 12” above the top of the pump tank to provide adequate filling head. The reservoir must have sufficient size (volume) to provide adequate vapor space for the condensate to collect during the pump’s discharge cycle and for the pump to vent during its filling cycle. The vent line also acts as a balancing line to allow condensate to drain into the pump tank while the HX is operating in vacuum.
Heat Exchanger System with Pump-Trap at Stall Load
Figure 20:
10 PSIG
100 PSIG
Elevated Condensate Return Line
Air Vent
50 PSIG
Pressure Regulator used to reduce Motive Steam Pressure
Motive Steam Pressure
Vent Line
Hot Water Outlet
10 PSIG
Cold Water Inlet
Total Back Pressure from Condensate Return Line
Check Valve
Check Valve
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Tel: 610-495-5131 • Pottstown PA • USA •• www.watsonmcdaniel.com
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