Heat Exchangers: Introduction and Basic Concepts
Saturday, February 03, 2007
A heat exchanger does exactly that -- exchanges heat between two streams, heating one and cooling the other. Levenspiel (1998) divides heat exchangers into three groups:
1. direct contact exchangers
2. recuperators
3. regenerators
Direct contact exchangers are self-explanatory. The hot and cold streams are brought into direct contact (mixed) and heat is transfered. These are particularly common when one stream is solid or entrained with a solid (air dryers, etc.) or for vapor-liquid sstems where only the liquid product is of value (spray dryers, cooling towers, etc.). Use of liquid-liquid systems is limited to immiscible pairs.
A regenerating exchanger transfers heat in steps: first from the hot fluid to a storage medium and subsequently from the storage medium to the cold fluid. A sand tank or rotary slab may be used as the storage phase.
In this class, we will primarily work with recuperating exchangers, since they are probably of the most industrial interest. In this arrangement, the hot and cold fluids are separated by a wall and heat is transferred by conduction through the wall. This class includes double pipe (hairpin), shell and tube, and compact (plate and frame, etc.) exchangers.
Terminology
Flow patterns
- cocurrent (a.k.a. parallel)
- countercurrent
- crossflow (crosscurrent)
If there is no phase change, the temperature of a fluid in a heat exchanger will vary with position, so mean values are typically used. The bulk temperature is a "mixing cup" average and is used extensively.
When vapor is condensing (or liquid boiling), the temperature can generally be taken to be uniform throughout that region of the exchanger, since normal operation is usually at constant pressure. The big exception is in variable area exchangers where the heat transfer surface is covered and uncovered as the liquid phase rises and falls. Another example might be a surface condenser with a subcooling well.
The temperature approach or approach difference is the difference between the two entering or two exiting streams. Note that these may be very different, depending on the flow pattern. Often, design criteria will specify a minimum approach (for instance, 10 degrees).
Heat transfer is much better in turbulent flow than in laminar, so it is common to specify minimum fluid velocities. Brodkey and Hershey (1988, p. 532) suggest a minimum of 3 m/sec.
Most exchangers transfer heat radially in a cylindrical geometry, hence the cross-sectional heat transfer area varies logrithmically with radius.
Heat Exchanger Design
Process design of a heat exchanger comes down to three main questions:
What is the required heat load (heat duty)?
What configuration (double pipe, shell and tube) will be used?
What is the overall heat transfer coefficient?
Once these are determined, the designer can determine the required heat transfer area.
References:
Brodkey, R.S. and H.C. Hershey, Transport Phenomena: A Unified Approach, McGraw-Hill, 1988, pp. 532.
Levenspiel, O., Engineering Flow and Heat Exchange, Revised Edition, Plenum Press, 1998, pp. 173-74, 243-47.
McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (5th Edition), McGraw-Hill, 1993, pp. 309-11.
McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (6th Edition), McGraw-Hill, 2001, pp. 315-17.
Adopted from: http://www.cbu.edu/~rprice/lectures/hxbasics.html
Labels: Chemical Engineering, Heat Exchanger
posted by Kipas Repair JB @ 9:11 PM,
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