In this 10 tips article, learn how understanding your heat exchanger can help get more heat transfer bang for your buck.
Understanding the concept of approach temperaturewill help ensure that an air-cooled heat exchanger operates properly.

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Understanding the concept of approach temperature will help ensure that an air-cooled heat exchanger operates properly.

We all understand the importance of keeping costs low while maintaining efficiency and productivity, and most businesses are quite adept at keeping their internal costs down. They are able to do this because they are experts in their respective fields. This same type of benefit is available to engineers that skillfully make use of a supplier’s sales engineering staff.

As design engineers taking calls at a company that designs and manufactures heat exchangers on a build-to-order basis, experience through the years has shown that some people consistently get more value for their dollar than others. They are not necessarily heat transfer experts, but they understand the fundamentals well enough to know how their specifications affect the heat exchanger’s cost. They also are masters at tapping the knowledge of their suppliers and working together with them to come up with appropriate designs.

Would you like to become a savvier shopper as well? Here are 10 tips on heat exchanger fundamentals that you can use to get the most out of your supplier.

1. Say What You Want

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Understanding the concept of approach temperature will help ensure that an air-cooled heat exchanger operates properly.

Tell your supplier how you expect the heat exchanger to benefit the process. Most heat exchangers are complex, intricately engineered products and, if you speak directly with a design engineer who knows the product and its application backward and forward, they are likely to have solved the same type of problem many times before. Their experience and knowledge generally come free of charge.

2. Master the Approach

Understand the concept of “approach temperature.” The approach temperature refers to the difference in temperature between the exiting process fluid and the entering service fluid. For example, if an ambient-air fan-cooled heat exchanger cools hot water to 40°F in a 30°F ambient, the approach is 40°F - 30°F = 10°F. In other words, the approach temperature represents how closely the water temperature approaches the air temperature. The approach is fairly constant for many heat exchangers, so it is a handy way to think about their performance under varying conditions.

3. Know the Conditions

Understand the concept of a “design condition.” Few things in this world are the same every day, including the flow, composition and temperature of the fluids flowing through heat exchangers. Although heat exchangers can function properly with all kinds of variability, it is not possible to design to a range. A good design condition factors in the variability the heat exchanger will experience, so it will perform adequately under all required conditions.

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4. Don't Overlook the Basics

Know the basics of what makes a heat exchanger work. A fundamental relationship of heat exchanger performance is described in terms of energy transferred (Q), by just two equations (click images to enlarge):

If there is phase change involved - either condensation or evaporation - things become a little more complicated, but the relationships are very similar.

5. Factor In Fouling

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If you want some “safety factor” in a heat exchanger, fouling factors must be considered. There are other methods used to attempt to generate safety factors; however, they will very likely come with unintended (and sometimes catastrophic) consequences. Here are some examples of what not to do.

  • Specifying Greater Flow. This method requires larger flow-area to maintain acceptable pressure drop. If the actual flow is a lesser value, the fluid temperature will be a lesser value as well, resulting in a reduction in U value. The decline in U may not be offset by the decline in Q. In other words, a heat exchanger designed for 150 percent of the actual flow may not work properly with 100 percent flow.
     
  • Specifying Greater Fluid Temperature Change. Because heat exchangers are largely defined by their mean temperature difference, a decision to add safety factor by designing to cool an extra 10°F will skew not only the load Q = (ṁ • cρ • ΔT), but also the Δmean; therefore, the area A = Q / (U • ΔTmean).
     
  • Specifying a Percentage Excess Area. This case has the least propensity to cause error, but if the concern is based on silty cooling water or oil mist in a process airstream, this approach treats both the hot and cold side the same and fails to properly address the concern with any precision. The result is likely a design that will be short on performance and long on cost.

6. Avoid Bottlenecks

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A good design condition factors in the variability the heat exchanger will experience, so it will perform adequately under all required conditions.

In heat exchanger design, bottlenecks move. Depending on the heat exchanger construction and process conditions, it could be surface limited, flow limited or ΔTmean limited. Consider a heat exchanger designed for a very close approach: changing the amount of surface area or service flow has a much smaller effect than increasing the approach. It is “approach limited.” With other operating conditions, the same equipment could be surface limited.

7. Keep an Eye on Materials

Materials of construction affect cost, U value and pressure drop. While it is obvious that some metals are more costly than others, there are a several less obvious factors that enter into heat exchanger design. The thermal conductivity and cost of fabrication are different for various metals, so the cost of a heat exchanger made from different metals will often be driven by these factors.

8. Think Through Installation

Though the type of installation - electrical, indoors, outdoors, ventilated, air-conditioned, heated, nozzles sizes/types, etc. - may not seem pertinent to the process, it really is. For example, consider a water-cooled heat exchanger outdoors in a Minnesota winter. Enough said.

9. Understand Their Strengths

If you need to heat or cool air or another gas, understand that heat exchangers are designed for mass flow. This is not much of an issue for liquids, which have nearly constant densities, but it is not the same case for gases. In process systems with blowers and heat exchangers, gas density is constantly changing and flow specifications in volume terms (cfm or m3/hr, for instance) only have meaning if they include a density or reference temperature and pressure.

10. Don't Overlook Humidity

If your application involves cooling humid air or any gas with humidity that condenses, it is important to have a good understanding of humidity. During a typical 24-hour period, relative humidity will go from 100 percent when the dew falls when it’s 60°F (15.5°C) in early morning to 30 percent during the afternoon when it is 90°F (32°F). Because the daily range of humidity is 30 to 100 percent and the temperature range is 90 to 80°F (32 to 27°C), do not make the mistake of thinking a good design condition is 100 percent at 90°F. Those conditions may both happen, but not at the same time. A far better way to think of humidity is in terms of dewpoint. Unlike relative humidity, the dewpoint does not change with respect to temperature. It is only relative to pressure.

So take these fundamentals and better understand how your specifications affect the heat exchanger’s cost.

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