Not all heat exchangers are created equal. While the difference is not always that of an F-18 versus a Piper Cub, sometimes it is. Of course, the comparisons may not follow the lines of reasoning you think they should, and the F-18 is not always going to come out on top. When comparing these planes, you would look at a number of different data sets beyond their typical specs:
- What is the plane’s purpose?
- What parts are needed to maintain them?
- What fuel do they run on?
- How efficient are they?
You would ask the same questions about heat exchangers. Prior to selecting a type or style, you need to be sure you have a clear idea of the purposes, what is needed to maintain them, what fluids are required, and how efficient they will be when considering your objectives.
To start with, heat exchangers are classified in several different ways, including:
- Transfers process: Direct or indirect
- Number of fluids: Two-fluid, three-fluid, and so forth
- Construction: Tubular, plate-type, rotary, fin, etc.
- Flow arrangement: Single pass, multi-pass
- Transfer mechanism: Single or double-phase convection on single or double sides
Each classification has its pros and cons. From size, sanitation, corrosion-resistance, functionality in various climates, and maintenance, the specific choice of heat exchanger will make a significant difference when it comes to functionality.
While you may think this is basic information and simple common sense, consider the following: When comparing plate-fin, tube-fin heat exchangers, and oil coolers, the plate-fin exchanger would come out on top when using water as a cooling fluid, and the tube fin would finish a distant third. If this is the only comparison you looked at, the choice would be simple. But does a plate-fin exchanger fit the space, the climate, the process, etc.? Would changing the cooling fluid make a difference? Would it make a difference if the exchangers were made out of different metals? The answer is “yes” to all questions.
For example, copper is often used when conductivity is important. It is also used when corrosion is a concern. The problem is that copper should not be used with corrosive fluids, deionized waters, oil, or fluids like polyalphaolefin (PAO). When copper is not appropriate, steel, stainless steel, iron, aluminum, or other metals or alloys may be chosen. Each of these also has its limits.
Aluminum works very well when oil or glycol solutions are used, but it should not be used with corrosive fluids. Stainless steel is the go-to metal when corrosive fluids or de-ionized water is to be used as the heat-transfer liquid. Stainless steel is also a great metal when there are hygienic concerns (food, beverages, biopharmaceuticals, and so forth). Yet, stainless steel should never be used in an environment where it is subject to saltwater contact, as it will corrode.
If different fluids, metals, and designs have such a major impact on performance, is there a simple checklist that can be used to determine the right heat exchanger? While there is not one checklist that will answer every question, the following six questions will give you a very good start:
- What is going to be heated or cooled?
- What is your heat-transfer fluid?
- What are the temperatures at the inlet and outlet?
- What is the flow-rate?
- What are your BTU requirements?
- How much space do you have?
To understand why these questions are so important, consider the need to heat or cool highly viscous, sticky, and/or heat-sensitive fluids. A fin-style heat exchanger would only be gummed up. In this case, you would need what is known as a scraped surface heat exchanger.
Plate heat exchangers typically take up less space so they can be used when the allowable footprint is minimal. When the plates are laser welded, they are the best option for utilizing caustic materials and, since there are no gaskets, they are excellent for use in high temperatures and at high pressure.
Heat transfer fluids are often directly linked to the environment in which the heat exchanger will be used. Water is excellent for heat transfer, is safe, and has a low cost. However, water freezes at a relatively high temperature and expands upon freezing. This can lead to ruptures and ruined equipment.
In order to use water as your heat-transfer fluid in an environment where temperatures may near 32 F or 0C, glycol or another “freeze inhibitor” needs to be added. Adding antifreeze to the water reduces the heat transfer efficiency, reduces thermal conductivity, increases viscosity (which increases power consumption), and makes the water toxic.
In addition to freezing, you need to worry about corrosion. In any environment where oxygen is going to be present (either from air or dissolved O2) corrosion is a very real threat. This is especially true when any sort of salt is also present. Even glycol is corrosive without a corrosion inhibitor. Corrosion is also impacted by temperature, the metal used in the device, and the fluid PH.
When you put this all together, you need to do your research before investing in a heat exchanger. Or, if you are not an expert in metals, chemicals, and thermal units, you may want to submit the answers to the questions above to an expert in the heat exchanger field, and they can match your needs with the proper device.
Going back to the initial illustration, an F-18 will not run very well on standard car gas. A Piper Cub’s engine will burn out on JP8. Aviation fuel has anti-icing additives because it gets cold up high. The right fuel is essential. While we wouldn’t want the Cub in a dogfight, we wouldn’t use the Hornet for day-tripping in the Alaskan bush. While this makes sense, put the same logic to use when choosing your heat exchanger.
When the correct metal, fluid, and heat exchanger design matches your requirements, the cost savings from improved power efficiency will be great. Properly maintained, these heat exchangers can continue to provide the same savings well beyond their probable useful life based on the current level of technological advancement. In short, when you choose wisely, your need for them will wear out before they do.