Pump-Trap Combinations Sizing & Selection Pump-Trap Sizing:
When the steam pressure in the heat exchanger is higher than the return line back pressure, the PUMP-TRAP functions like a standard float-operated TRAP, allowing the steam pressure in the heat exchanger to discharge the condensate. Under these conditions, the unit is in TRAP mode. When the steam pressure in the heat exchanger falls below the back pressure, the condensate backs up into the body of the pump-trap, raising the float and opening the motive steam inlet valve, which then pumps the condensate into the return line. Under these conditions, the unit is in PUMP mode. We therefore have two separate and distinct capacities; the PUMP CAPACITY (when operating in Pump Mode) and the TRAP CAPACITY (when operating in Trap Mode) . In the example below, the system will be analyzed to determine when the Pump-Trap is in Trap Mode and when it is in Pump Mode, and the specific capacity requirement of the pump. If the total back-pressure of the condensate return line is known, the Pump-Trap should be selected with sufficient pump capacity to handle the condensate load at the system stall point. (i.e.; when the steam pressure is equal to the total back-pressure). Alternatively, if the total back-pressure is not known, it is best to select a pump-trap with enough pump capacity to handle the maximum condensate load of the application. (i.e., at maximum steam pressure and flow). Refer to Sizing Charts. Reservoir Sizing: (Refer to chart on previous page) When using a Pump-Trap, a condensate holding reservoir should be installed above the pump-trap and below the heat exchanger (shown below). This will enable the condensate to collect while the pump is in the discharge cycle, thus preventing condensate backup. When back pressure against the pump outlet is less than 50% of the steam pressure to the heat exchanger, the pipe lengths given in the chart can be reduced by half.
Heat Exchanger (HX) using Steam to heat Hot Water
The following example describes a Heat Exchanger (HX) using Steam to heat domestic hot water for a medium size apartment complex. Note that the hot water usage varies significantly depending on the time of day. The physical size of the heat exchanger needed (sq. ft. of surface area) is based on the following criteria: (1) MAXIMUM water usage (GPM), (2) the temperature rise of the water, and (3) what pressure steam will be used to heat the water during maximum demand. Note: The selection of the steam pressure (which determines the steam temperature), to heat the water at maximum demand (flow rate), is the primary factor in heat exchanger sizing. The application is requiring water to be heated from 45˚F to 140˚F in a HX using Steam. The maximum flow rate has been determined to be 60 GPM . The Steam Trap will be discharging into a condensate return line that may have a Total Back Pressure of 15 PSIG and the flow rate of heated water could be as low as 20 GPM . The facility engineer has chosen to base the HX size on using 50 PSIG of steam pressure. Therefore, the size of the heat exchanger was selected based on heating 60 GPM of water using 50 PSIG of steam.
PUMP Mode In response to a reduction in demand of hot water, the pressure in the HX has now dropped to 10 PSIG . Based on this particular size HX, 10 PSIG steam will heat 43 GPM of water. Since back pressure is 15 PSIG , the system is stalled and condensate backs up into the system; the float will continue to rise to activate the pump and discharge the condensate.
TRAP Mode The system is operating with 30 PSIG inlet pressure to the heat exchanger. The Pump-Trap unit functions like a standard float operated trap. Condensate is pushed thru into the return line by the steam pressure in the HX. Based on this particular size HX, 30 PSIG steam will heat 53 GPM of water.
30 PSIG
10 PSIG
Fill Head
15 PSIG
Fill Head
15 PSIG
Δ P Trap = 30 psig - 15 psig = 15 psi
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