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Chemical and Process Engineering Resources

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Category: Plant Basics
Question: What is the best way to heat 150 GPM of water at 30 psig and 50 0F to 200 0F and 150 psig for use in washing a distillation column?
Keywords: heat,water,cleaning,column,pick,heater
Answer: Let's assume that you need a supply of 200 0F wash water and that you have initially chosen to heat the wash water with a generic adiabatic mixer (the well-known "Pick" heater). Let's also assume that you are undecided about which type is best for your needs and your budget. The answer to your problem then is as follows: 1. You should have already calculated the amount of steam that is required to heat the 150 gpm of wash water. This will be a lot of steam. To calculate the steam load, let's say that you are going to use relatively low-pressure saturated steam at 40 psig and inject it into the 30 psig wash water which will go to a 200 0F holding tank where a pump will pick the product hot water and elevate it to your required 150 psig. You could also pump the 30 psig to 150 psig and inject 175 psig steam directly into the pump's discharge stream. This method may not be recommended because you have no capacitance in your system and the high pressure steam and Pick heater come at a costlier price. Having set this basis and making a mass and energy balance around the system, you should get the following equations: Heat Added = (150)(500)(150) = 11,250,000 Btu/hr (a large heat flow) Low Pressure Steam Required = (75,000)(h at 200 0F - h at 50 0F)/(h at 287 0F - h at 200 0) Low Pressure Steam = 11,160 lbs/hr = 22.3 gpm condensate This is a lot of steam....you probably don't want to use high-priced, high pressure steam on this application if you can avoid it. 2. At this point, stop and reflect on the basic ingredients of what this project is starting to look like. The 150 gpm is easily a 3" piping loop with the steam piping coming in about at the same size. The Pick heater will have to be 3-4". Whether you go low pressure steam or high pressure steam doesn't matter to the boiler supplier. All they care about is that they have to makeup the 11,150 lb/hr of treated, de-aerated feedwater that you've converted into wash water. Depending on how long you need the 150 gpm, this could be a high cost operation and possibly one you can't afford. My main point in all of this is that before you select a type of heater, you must visit the economics scenario for each type. You inevitably will have to justify the capital and operating costs, so the sooner you confront the trade-offs, the better off the project will be. 3. Pick Heaters are a great solution, where they fit well economically. However, at these high levels of steam consumption the economics generally favor a surface type of heat exchanger such as a BEU TEMA type (always with condensate return). You may pick up a used unit somewhere or in your own "Bone Yard" if your project is in a processing plant. 4. Finally, you can look at buying or renting a direct-fired heater based on using natural gas or LPG. This is a little bit more complicated and requires safety controls and possibly a permit. But it's another option. Low pressure steam is cheap; no steam supply is "free"; all steam supply requires condensate return or compensation for feed water treatment and makeup.