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Chemical and Process Engineering Resources

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Category: Heat Transfer Technology
Question: What's the best way to control an oversized, horizontally oriented shell and tube steam heater?
Keywords: v1i10,shell,tube,heat,exchanger,steam,heater,control,best,way
Answer: BACKGROUNDA used shell and tube heat exchanger is to be used in steam heating duty. The heat exchanger is larger than necessary and the control scheme to be employed is being investigated. The steam to be used will be 65 psia saturated steam. The process fluid is a liquid brine fluid.ANSWERSTwo opinions were offered on this topic:A. The actual pressure in the heater, while the steam is condensing is dependent on the condensing rate and the overall dirty U. Tubes can be plugged to reduce the amount of heat transfer area, as long as the process side (tube) velocity does not get too high. Calculate the needed area and then the required steam flow rate. An orifice can be sized to control the steam flow rate, however at reduced loads, the condenser may experience partial vacuum conditions so be sure that the shell is rated for full vacuum. When this partial vaccum condition does occur, choked flow will be experienced through the steam control valve. The Cv trim value would need to be sized such that the choked flow does not exceed what is needed. This is tricky and requires several trim size change outs.Source: Mr. Art MontemayorB. Another option is to control the steam side via level control on the shell of the heat exchanger. This way, the 65 psia pressure can be maintained and the condensate will rise up and cover "unnecessary" tubes. This method works well with vertical shell and tubes, however is a horizontally mounted unit, the level will change more slowly and is more difficult to measure.