Pump-Trap Combinations Sizing & Selection
Stall Condition: When the steam pressure in the HX is equal to the back pressure of 15 PSIG , the condensate will no longer drain out of the HX. The Pump-Trap will now need to operate in Pump Mode to remove the condensate from the HX. We need to calculate how much condensate will be produced when there is 15 PSIG in the HX.
Δ T = (T S – T i ) + (T S – T o ) 2 = (250 – 45) + (250 – 140) 2 Δ T = 157.5 F = Avg Temp. Difference
From the steam table, 15 PSIG steam has a temp of 250˚F
To find out how much energy will be transferred to the water, we use the Δ T calculated above in our heat transfer equation.
E = U x A x Δ T = 120 x 116 x 157.5 = 2,192,400 Btu/hr
To determine how much steam is required to heat the water, we use the following formula. (LH = Latent Heat.)
2,192,400
E
(For 15 psig steam, the LH is 946 Btu/lb)
lbs/hr =
Q s
=
LH 946 Steam Flow = 2,318 lbs/hr
When the HX stalls, we will be using 2,318 lbs/hr of steam and will need to pump 2,318 lbs/hr of condensate. The pump-trap must be sized to handle this condensate load since it is the maximum load under stall conditions (see table below).
Table based on a HX size of 116 ft 2 and back pressure of 15 PSIG The following table summarizes the above results and shows how the steam flow, pressure, temperature and latent heat vary as a function of the water flow rate. It can be seen that the system is operating in Trap Mode between water flow rates of 60 to ~46 GPM, and in Pump Mode between ~46 to 20 GPM (based on 15 PSIG back pressure). Also, at flow rates below 35 GPM, the steam pressure inside the HX is below atmospheric pressure (0 PSIG).
Flow Rate
Steam
Steam Latent
Trap
Steam Pressure Temp
Heat
Condensate Differential System
Water (GPM)
Usage (lbs/hr) 3,125 2,943 2,720 2,466
in HX (PSIG)
in HX of Steam Generated Pressure Condition
(°F)
(Btu/lb)
(lbs/hr)
(PSI)
60
50 40 30 20 15 10
298 287 274 259 250 239 212 192
912 920 929 940 946 953 970 983
3,125 2,943 2,720 2,466 2,318 2,140 1,715 1,409
35 25 15
(Maximum Heat Load)
57.0 53.2 48.8
Trap Mode
5 0
Steam Pressure = Back Pressure
46.2 2,318
(Stall Point)
42.9 35.0 29.2
2,140 1,715 1,409
--- --- ---
Pump Mode
0
-5
(Vacuum)
---
20
948
-10
161 1,002
948
(Minimum Heat Load)
43
Tel: 610-495-5131 • Pottstown PA • USA •• www.watsonmcdaniel.com
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