Metal Finishing Guide Book


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Steam pressure (psig) Steam temperature (oF) Heat of evaporation (BTU/lb) 5 226 960 10 240 950 15 250 945 20 260 940 25 266 935 30 274 930 Table IV: Steam Table Nominal Pipe Size (in.) Steam Required (lb/hr) 1 Up to 100 1½ 100-300 2 300-500 3 300-1,000 Table V: Nominal Pipe Size for Various Steam Requirements where ln = Naperian (natural) logarithms. Steam pressure produces specific temperatures that will be used in the calculation of the LMTD. Typical values are given in Table IV. As an example, assume 10 psig steam is to be used to heat a solution from 65OF (ambient shop temperature) to 140OF (solution operating temperature). Steam temperature (from Table IV): 240OF ∆T1 = 240 - 65 = 175OF ∆T2=240 - 140 = 100OF LMTD = (175 - 200)/[ln(175/100)] = 75/0.55 = 134OF The heater area required to steam heat a process solution equals the design heating requirement, divided by the overall heat transfer coefficient, times the log mean temperature. Design heating requirement (BTUH)/Overall heating requirement LMTD As with any immersion heater, the heater surface temperature will be higher than the solution temperature. Obviously, it cannot exceed the steam temperature. If the solution has a high temperature limit below available steam temperatures, you may require a custom electric immersion heater or a hot water (or thermal fluid) heater with a lower heating temperature. Although the heater temperature is limited to the steam temperature, damage to process tanks and accessories can result from overtemperature or low liquid levels. It is wise to equip your process tank with overtemperature and low liquid level cutoffs. Once a coil size is selected, piping size should be investigated. The quantity of steam used for a specific coil size varies with the steam pressure (see Table V) and the heat released is the heat of evaporation (latent heat) only. The values in the table are in BTUs per pound of steam. So the quantity of steam required equals the design heating requirement, divided by the heat of evaporation of the steam. Design heating requirement(BTUH)/Heat of evaporation (from Table IV) The result, in pounds of steam per hour, can be equated to pipe size as shown in Table V. The condensate generated (condensed steam) must be "trapped," that is, equipped with a steam trap. Steam traps are sized based on pounds per hour times a safety factor. Since the amount of condensate varies with the temperature of the solution, it is wise to use a safety factor of four or better. Trap capacity equals the steam required times four. 693

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