How to Make It Hot Without Touching It?

With cross-flow fluids, one of the fluids is usually a liquid while the other fluid is almost always air.  This type of heat exchanger is not included in this article.

This diagram shows counter-flowing fluids.  The blue color represents the fluid at the colder temperature.  The red color represents the fluid at the hotter temperature.  The gold is the blue fluid which has gained temperature from the red fluid or the red fluid which has lost temperature to the blue fluid.  Note that the temperature of the hotter fluid as it leaves the heat exchanger may be equal to the temperature of the colder fluid after it is warmed, or it may be different.  Among the deciding factors is how long the two fluids remained in contact with each other, separated only by the chamber walls in the heat exchanger.

The material makeup of heat exchangers may be comprised of different metals, but plastic and wood are never used.  Since the purpose of the heat exchanger is to transfer heat from one fluid to another, the materials used must have low resistance to the flow of heat, as do metals with no insulation.  The specific materials used depend on the characteristics of the fluid which is being conducted, particularly its corrosiveness.  For example, for each type of corrosive fluid, a different type of stainless steel could be preferred.

Little or no maintenance is required (limited only to filtering the fluids to prevent sediment buildup inside the device) unless the fluids contained corrode through one of the plates.  If that occurs, the best thing is to replace the unit.  If one plate fails or a connection fails, you can be certain without any additional analysis that more failures are on the way.  So, just break down and replace the unit.  If you have a maintenance contract with a reputable maintenance contractor, they will replace the unit at their prescribed times to minimize their cost to repair the unit.  For example, while performing other tasks, your contractor may deem it timely to replace the heat exchanger.  This is a wise practice not only because it saves money, but it may also save you downtime.  There is no getting around that old adage, “Time is Money," so just replace it.

The useful life is the number of years the item will provide satisfactory performance without significant repairs.  When an item is at the end of its useful life, it will cease to perform optimally without frequent repairs.  When cost analyses are being performed on two or more choices, the initial cost of the item, the cost to install the item, all the maintenance costs associated with the item, and the cost to replace the items at the same interval must be analyzed.  The period of analysis is usually taken to be the minimum time it takes for the longest life item to be replaced at least once.  If utility costs are known, they also must be counted in the analysis.  In the case of plate-type heat exchangers, since there are no moving parts, depending on whether or not there are corrosive materials involved, the useful life of this type of heat exchanger can be taken as 25 years.

There are three basic types as defined by their construction.  Frame and plate heat exchangers have a framework in which the plates are located.  The plates are spaced apart with spacers and the whole assembly is fastened together by bolts.  Hence it is frame…and…plate.  The next type of construction is welded plate, and finally brazed plate.  The configuration of each type of construction is the same, and flows will still be as mentioned earlier.  The differing fluid pressures determine the different construction.  Each type has its own set of functions.

Frame and plate serves its function in radiant floor heating systems, solar heating systems, snow melt systems, domestic water heating (as in the case of heating water with the waste heat from another process), swimming pool and spa water heating, outdoor boilers, cooling tower isolation, high rise pressure breakers, some refrigerant-based systems, and free cooling in commercial applications.  Welded plate serves its function primarily in processing plants.  Brazed plate serves its function in refrigeration, whether it utilizes ammonia, R410A, R407C or any of the ever-changing family of refrigerants.  Master Format, Division 23, is primarily focused on frame and plate heat exchangers and brazed plate.

All heat exchangers are sized based on several factors, including but not limited to: the temperature difference of the two fluids, the length of time allowed for the change, the flow rate of the fluids, the space requirements, and any cost related issues.

For additional information specific to a certain installations, be sure to consult your maintenance contractor, engineer, or manufacturer’s representative with questions regarding frequency of maintenance.  For design questions not answered here, consult your engineer.