Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve HEAT EXCHANGER FORMULAS & EXAMPLE
Now, what happens when the water flow rate thru the HX reduces to the point that the steam pressure goes into vacuum? This occurs at water flow rates below about 58 GPM down to the minimum of 25 GPM. Since the HX will be operating in vacuum, the condensate would not effectively drain regardless of the steam trap size chosen. This is why a vacuum breaker must be added to the top of the HX. The vacuum breaker draws in air to neutralize the pressure in the HX which allows the condensate to drain (see Figure 19).
Heat Exchanger System with Steam Trap shown with and without Vacuum Breaker
Figure 19:
System without Vacuum Breaker
System with Vacuum Breaker
100 psig
Sub-atmospheric pressure
100 psig
0 psig
OPEN Vacuum Breaker
- 5 psig
Condensate Backs Up into Heat Exchanger
1 psig
2.31 ft. water coulmn
Condensate will not flow without a vacuum breaker
Condensate flows through trap
2.31 Ft. X 0.433 = 1 psig
The HX can be properly drained using a steam trap as long as there is no BACK PRESSURE in the condensate return line. If there is back pressure in the condensate return line, a Pumping Trap must be used.
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