Heat Exchangers: Stall Prevention & Solutions

1. Heat Exchangers Dr. Sanjay Vashishtha Assistant Professor, CREED, BITS, Pilani

2. Heat exchangers • Shell and tube heat exchangers. • Plate heat exchangers. • Air heating coils or batteries in ductwork. • Pipe runs or pipe coils in process equipment, tanks, vats etc.

3. Heat Exchanger stall • Stall is the reduction or the cessation of condensate flow • Stall occurs when the pressure in the heat exchanger is equal to, or less than, the total backpressure imposed on the steam trap

4. Reasons of low pressure in a heat exchanger • The secondary fluid inlet temperature rising as a result of a falling heat load. • The secondary fluid flowrate falling as a result of a falling heat load. • The secondary fluid outlet temperature falling due to a lowering of the set point.

5. Heat Exchanger stall An air heater battery suffering the effects of stall

6. Example • A heat exchanger running at full-load uses saturated steam at 1 bar g (120°C) to heat water from 40°C to 60°C. Full-load therefore occurs when the water temperature rises by 20°C.

8. Effects of stall 1. Cold or cool steam trap.2. Hunting control valve.3. Fluctuating outlet temperature.4. Stratified heater temperatures.5. Waterhammer.6. Reduced heat output.7. Reduced product quality.8. Corroding heat exchangers.9. Leaking heat exchangers.

9. The Heat Load, Heat Exchanger and Steam Load Relationship

10. Typical temperature control of a steam /water shell and tube heat exchanger

11. Heat Exchanger Designing • A manufacturer is to design a heat exchanger in which the specification calls for steam at 4 bar g to heat secondary water from 10°C to 60°C. The water flow is to be constant at all loads at 1.5 L/s. It is assumed that 1 litre of water has a mass of 1 kg, so the mass flowrate = 1.5 L/s x 1 kg/L = 1.5 kg/s. The manufacturer uses a heat transfer coefficient ‘U’ for the heat exchanger of 2500 W/m2°C.Take the specific heat of water as 4.19 kJ/kg°C.

12. Determine: • (A) The design heat load.(B) The corresponding steam flowrate.(C) The minimum heating area required.Also, if the customer’s minimum heat load occurs when the inlet water temperature rises to 30°C, determine:(D) The minimum heat load.(E) The corresponding steam pressure in the heat exchanger.(F) The corresponding steam flowrate.

13. Example

15. Oversized Heat Exchanges

16. Over sizing of heat exchanger

20. Example • Supplier ‘X’ can provide a heat exchanger with a heating area of 2 m2, a ‘U’ value of 2 500 W/m2°C and duty of 350 kW when operating with steam at 4 bar g and with a water flow of 1 L/s. • Supplier ‘Y’ is able to provide a heat exchanger with a smaller heating area more suitable for the design heat load of 293 kW, when operating with steam at 4 bar g and with a water flow of 1 L/s. The ‘U’ value is 2500 W/m²°C. • The heat exchanger condensate line will lift 5 metres to a condensate return pipe that falls en route to a vented receiver, and having a total backpressure of 0.5 bar g.

22. This saturation temperature is equivalent to a steam pressure of 0.45 bar g. This pressure is less than the 0.5 bar g backpressure, and the system will permanently stall. Consider supplier ‘Y’It is first necessary to determine the rated LMTD for the heat exchanger with a steam space pressure is 4 bar g (TS = 152°C).

23. From his standard range, supplier ‘Y’ can provide a plate heat exchanger that meets the specification with a heating area of 1.198 m2. This is oversized (by about 5%) and steam pressure will therefore be less than 4 bar g at the full-load operating condition

27. The Stall Chart - Constant Flow / Varying Inlet Temperature

28. Constant secondary flowrate with varying inlet temperature Shell and tube heat exchanger with primary control valve

29. Stall chart • Consider a steam temperature of 120°C heating a constant flow of secondary water from 20°C to 80°C.

30. Constant flowrate / Varying inlet temperature Horizontal line represent the equivalent steam saturation temperature of the condensate backpressure.

31. Example: • The steam pressure in a heat exchanger at full-load is observed to be 7 bar g. Condensate pressure is 1 bar g, and there is a lift after the trap of 10 m. At full-load, the secondary fluid enters the heat exchanger at 25°C and leaves the heat exchanger at 80°C. 1. What is the percentage heat load at stall?2. What is the secondary inlet temperature at stall?

33. Practical Methods of Preventing Stall

34. Static head and vacuum breaker method of dealing with stall

35. Auxiliary drain method of dealing with stall

36. Combination pump and steam trap method of dealing with stall

37. Pump-trap method of dealing with stall

38. Pump-trap method of dealing with stall

39. Constant steam pressure - Secondary temperature control

40. Constant steam pressure - Condensate level control

41. Detailed installation of a pump-trap with plate heat exchanger