SPECIFYING PLATE HEAT EXCHANGERS.
UIT the energy conservation concerns that peaked in the 1970s, HVAC designers began to encounter heat transfer processes for which the familiar shell&tube heat exchanger was not practical. For example, heat recovery from waste streams, such as laundry drains, required closer temperature approaches than shell & tube exchangers could handle, or free cooling systems which typically involved a temperature cross (see diagram at right).
THE PLATE EXCHANGER SOLUTION.
What designers needs was a heat exchanger that operated in pure counter flow yet did not take up the space that would be required by a single-pass (counter flow) shell& tube heat exchanger. Such exchangers had been available since the 1920s, but were used mainly for process work where they remained largely unknown to HVAC designers. Their non familiar compact design consisting of thin pressed non ferrous, corrugated plates between which the two streams flow in countercurrent pattern. When their value in HVAC design became recognized they began to appear in the ASHRAE shows, and plate exchanger applications soon followed.
FREE COOLING EXAMPLE
A free cooling system from our job files, designed by Greg Gershkovich, PE, is a good example of the plate exchanger advantage in special situations. This was a retrofit project for a major downtown office building in San Francisco with very limited equipment space. The conditions were:
2300 gpm 67F à 58F
1720 gpm 65F à 53F
This temperature profile contains two characteristics that are anathema to shell&tube exchangers. One is the close 2 degree approach, the other is the temperature cross (65F>58F)
SIZING AND SELECTION
The heat transfer surfaces used in plate exchangers are thin, non ferrous plates pressed with a variety of patterns such as washboard or chevron and others. This surfaces encourage turbulent flow with high U factors. The manufacturer will often use combinations of plate designs in the same frame to achieve the specified performance. This involves a vast number of variables which make selection tables – such as those used for shell and tube exchangers – impractical.
Thus selections are routinely made by computer programs that are customized for each manufacturer.
Plates are available in 304 or 316 stainless steel, hasteloys, incaloys, titaniums and other pressable non-ferrous materials. A glance at the illustration on the front page shows that the length of the gasketing material around the edges of the plates is considerably longer than would be required on a shell & tube exchanger. Thus it is especially important that the gasket material be compatible with the exchanger fluids and that the gasketing system be designed for easy maintenance. Gaskets are available in nitirle and EPDM elastomers, or various grades of viton, depending on temperature and fluid properties. For most water to water applications not exceeding 275F, nitrile is used. Glueless gasketing in Plate exchangers is used to ease installation and removal.
Plate exchangers have the unique characteristic of expandable capacity simply by having additional plates installed when the need arises. If a future increase in capacity will be required, space is left between the follower and the end support to accommodate the required number of additional plates.
PLATE MAINTENANCE & REPLACEMENT
Due to highly turbulent flow between plates, scaling is held to a minimum and cleaning is rarely needed in less than six years. When plates need to be cleaned or replaced, the procedure is easily handled by building maintenance staff. Unlike shell and tube maintenance requiring pulling of the entire tube bundle, plates can be cleaned or replaced one at a time.
PLATE OR SHELL & TUBE?
Plate exchangers are generally a good choice for operating conditions where shell & tube exchangers are used except when pressures exceed 400 psig or temperature exceed 350F. They are especially good choice if conditions require a temperature cross or close temperature approach. When conditions require both sides of an exchanger to be stainless steel or other non-ferrous material, plate exchanger will almost always be the less expensive choice.
If space is a consideration (and when isn’t it?), a plate exchanger is especially appropriate. Its footprint is generally a good deal smaller than that of a shell&tube exchanger of equal capacity, and it doesn’t the tube-pull space that adds to shell & tube space requirements.
10 gpm 190F à 71F
50 gpm 73.8F <- 50F
Effectiveness 119/140 = 85%
The practical maximum effectiveness for a multipass shell& tube exchanger is about 75%. So its performance in this case would be:
10 gpm 190F à 85F
50 gpm 71F <- 50F
Effectiveness = 105/140 = 75%
In terms of heat recovery, there is this case an advantage of (73.8-71)(50)(500) = 70,000 Btu/h using the plate exchanger.
People sometimes speak of plate exchanger as being more “efficient” than shell & tube heat exchangers. This can be misleading if one thinks of “efficiency” in engineering terms, that is, the ratio of energy out to energy in. Any heat exchanger is essentially 100% efficient in this sense. A better term is “effectiveness” which(in the case of water to water) is the ratio of the larger temperature change to change that is theoretically possible. By this definition , plate exchangers are definitely more effective than shell and tube exchangers.
WASTE HEAT RECOVERY
As an example of effectiveness, consider the problem of condensate at 190F with 50 gpm of condensate at 190F with 50gpm of water at 50F. The theoretical maximum temperature rise is (190-50) =140F. Of course that’s not practically obtainable, but 85% of it is not an unreasonable expectation for a plate exchanger. This will result in a the following temperature profile:
Published by James Breese & Co. for HVAC&PLUMBING SPECIFIERS
6 Dorman Ave. San Francisco, CA 94121