Wednesday, May 7, 2014

Four by Sixteen Rule for Heat Exchangers

Subtitle: Easy Estimation for Heat Exchanger Area

An earlier post (see link) discussed a quick but accurate way to mentally determine process pipe size.  I was asked if there is a similar technique for sizing a process heat exchanger?  It turns out, Yes, there is.  This post describes the technique. 

Estimating Exchanger Size

The design of heat exchangers is quite complex, but a few quick and easy decisions can make the design process suitable for initial design, costing, and economic evaluation.  A practicing process engineer frequently needs to assess the process for the benefits and costs of installing a new heat exchanger.  This is true for oil refineries, petrochemical plants, chemical plants, water treatment, and other processes where fluids must be heated or cooled.   The good engineer will not waste time and computing resources doing a detailed heat exchanger evaluation for each potential application, but can speed things along by a quick but accurate estimation procedure.  

This procedure begins with a "Four" and a "Sixteen", just as the pipe-sizing method does.  In this case, the "Four" is a four-foot diameter shell for a shell-and-tube heat exchanger.   The "Sixteen" is for two elements: the tubes are sixteen feet long, and there are 1600 tubes in the tube bundle.  (The area is actually 5,027 square feet).  

This configuration, 1600 tubes of length 16 feet, for 3/4 inch OD tubes on triangular pitch with 1/4 inch spacing, provides a heat exchange area of almost exactly 5,000 square feet.  The tubes will fit nicely inside a 4-foot diameter shell, with about 23 percent unused area in the tubesheet. 

Knowing that 5,000 square feet is an exchanger 4 feet diameter and 16 feet long, one can use the process calculations to determine the required surface area.  Then, almost the same rules of pipes and sizing apply to heat exchangers.   There are a few caveats.  If one has, for example, a required area of 2,500 square feet, or half of the 5,000 in the base case, one can determine the shell size as 4 divided by 1.4, or approximately the square root of 2. Working this backwards, what number can be multiplied by 1.4 to yield 4?  The answer is just slightly less than 3, as 3 times 1.4 is 4.2.   We can use 34 inches, slightly less than 36 inches, and a quick check shows that 1.4 x 34 is 47.6, close enough to 48 inches.   The 2,500 square foot area can be obtained in a 34 inch shell with 16 foot tubes. 

But, what if the required area is greater than 5,000 square feet?  Perhaps 7,500 square feet is required.  Then, a caveat applies.  Heat exchangers, in general, are less costly with long tubes and small-diameter shells.  However, longer tubes require that the velocity in each tube is rather high, and pressure drop is also higher compared to shorter tubes.   If we maintain the 16 foot tube length, the shell diameter is then 4 times 1.22, or 4.88 feet or 58 inches (the 58.6 actual value is rounded down to 58).  It might be better to maintain the 4 foot shell, and use 24 foot tubes in this case.   How do we know that 24 foot tubes will work?  Because, the required area is 50 percent greater (7500 / 5000 is 1.5), and 1.5 times 16 feet long tubes is 24 feet long. 

Some caveats: this technique is for shell and tube exchangers, using English units of inches, feet, and square feet.  It does not work well for other designs, although a clever engineer can probably develop similar quick estimating procedures.   Also, for highly viscous fluids, shorter tubes are desirable to minimize pressure drop.   There are also various tube inserts to increase turbulence in low-velocity but shorter tubes. 

In hopes that this article helps the various process engineers, in whose shoes I also once walked. 

Roger E. Sowell, Esq.
Marina del Rey, California

1 comment:

Cooling Tower Australia said...

I can see that this rules are essential and will play a vital part in the process done by heat exchangers..