Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve HEAT EXCHANGER FORMULAS & EXAMPLE
T) What is the flow of steam (steam capacity) thru the control valve at the minimum load? From the steam table (using extrapolation), the latent heat of steam (LH) corresponding to 146°F saturated
steam is 1,011 Btu/lb. Using Formula 6 , the steam capacity is: Q S = E min /LH = 1,125,000 Btu/hr / 1,011 Btu/lb = 1,113 lbs/hr
Figure 21:
Heat Exchanger System with Pump-Trap at Minimum Load
100 PSIG
Elevated Condensate Return Line
-11 PSIG
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
SUMMARY of HEAT EXCHANGER SYSTEM The following table summarizes the above results and shows how the heat load and the pressure, temperature, latent heat and flow of steam 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 100 and 71 GPM, and in Pump Mode between 71 and 25 GPM. Also, at flow rates below ~58 GPM, the steam pressure inside the HX is below atmospheric pressure (0 PSIG).
Flow Rate
Steam
Steam Latent
Trap
Heat Load
Steam Pressure Temp
Heat
Condensate Differential System
Water (GPM)
Usage in HX in HX of Steam Generated Pressure Condition
(Btu/hr)
(lbs/hr)
(PSIG)
(°F)
(Btu/lb)
(lbs/hr)
(PSI)
100 4,500,000 4,934 50 298 912 4,934
40 30 20 10
(Maximum Heat Load)
94.7 4,262,400 4,633 40 88.3 3,973,800 4,278 30 80.9 3,640,800 3,873 20
287 274 259
920 929 940
4,633 4,278 3,873
Trap Mode
Steam Pressure = Back Pressure
71.0 3,196,800 3,354 10 239 953 3,354
0
(Stall Point)
57.7 2,597,400 2,678 47.9 2,153,400 2,191
0
212 192
970 983
2,678 2,191
--- --- ---
Pump Mode
-5
(Vacuum)
(Minimum Heat Load)
25 1,125,000 1,113 -11 146 1,011 1,113
79
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