HEAT EXCHANGER FORMULAS & EXAMPLE Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve
O) If a pump-trap is used to prevent stall, what capacity must the pump have? The maximum condensate load that the pump must discharge occurs at the stall point (i.e., when the steam pressure is equal to the total back pressure of the condensate return line). This can be determined from the steam load at the stall point, using Formula 6 . The heat load at stall was determined in Part M to be 3,196,800 Btu/hr. The steam temperature at stall was also determined in Part M to be 239˚F. From the steam table, the latent heat of steam at 239˚F is 953 Btu/lb. The steam capacity is: Q S = E stall /LH = 3,196,800 Btu/hr / 953 Btu/lb = 3,354 lbs/hr The maximum condensate load at stall conditions is therefore 3,354 lbs/hr and the pump must be sized to remove condensate at this rate. P) What Watson-McDaniel Pump-Trap should be selected for this application? Referring to the pump-trap capacity chart when operating in Pump Mode , it can be seen that model WPT3 (pump with external trap mounted on common base) can meet the condensate load at stall (3,354 lbs/hr) when the motive steam pressure is 50 PSIG and the total back pressure is 10 PSIG. Under these conditions, this model has a maximum capacity of 4,080 lbs/hr. Since the steam supply pressure is 100 PSIG, a pressure regulator can be used to reduce the pressure to 50 PSIG for the motive steam line.
MINIMUM LOAD & OPERATION IN VACUUM
Q) What is the minimum heat load of the application? The minimum heat load occurs at the minimum water flow of 25 GPM. Using Formula 5 : E min = Q w x 500 x C p x Δ T w = 25 GPM x 500 lbs/hr x 1.0 Btu/(lb-°F) x (140°F – 50°F) 1 GPM = 12,500 lbs/hr x 1.0 Btu/(lb-°F) x 90°F = 1,125,000 Btu/hr R) What is the steam temperature in the heat exchanger at the minimum load? Use Formula 1 to calculate the mean temperature difference between the steam and water: Δ T M = E min /(U A) = 1,125,000 Btu/hr / (120 Btu/(hr-ft 2 -°F) x 185 ft 2 ) = 50.7°F The steam temperature is then given by Formula 4 : T S = Δ T M + T WM = Δ T M + T o + T i = 50.7 + 140 + 50 = 50.7 + 190 = 50.7 + 95 = 146°F 2 2 2
S) What is the steam pressure in the heat exchanger at the minimum load? From the steam table (using extrapolation), the steam pressure corresponding to 146°F saturated steam is 22.7 in Hg Vacuum which is equivalent to -11.1 PSIG. Therefore, the steam pressure inside the heat exchanger is below atmospheric pressure. This is another advantage in the use of a pump-trap. If a steam trap is used to drain condensate, the system could not operate in vacuum since the condensate would never drain out. Therefore, a vacuum breaker is used which essentially mixes the steam with air to achieve the proper temperature differential for a particular size HX. This added air facilitates corrosion by forming carbonic acids. Some of this air is eventually discharged thru the air vent on top of the HX but some mixes with the condensate. A pump-trap can discharge condensate when the HX is operating in vacuum, which precludes the use of a vacuum breaker and thus air is prevented from entering the system.
78
www.watsonmcdaniel.com •• Pottstown PA • USA • Tel: 610-495-5131
Made with FlippingBook - Online magazine maker