On several occasions during plant energy assessments, people have asked me to calculate the efficiency of a particular heat exchanger. But what does that really mean? For us mechanical and chemical engineers, despite everything we studied in school, we never discussed the energy efficiency of a heat exchanger. How could we have missed something that simple?
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But actually, we didn’t miss it — efficiency, as the true definition goes, is the ratio of output divided by input. For all practical purposes, other than a very small amount of heat loss (probably less than 1%), heat exchanger thermal efficiency is 100%. Hence, very little improvement is possible and so we don’t worry about it. In fact, every time we do a heat balance, we inherently assume the thermal efficiency is 100% when we say the “hot side is equal to the cold side.” I also have seen people sometimes arbitrarily use a factor of 0.95, but I have no theoretical basis for it.
So, is there a way to assess the performance of any heat exchanger? This really is what the original question should have been. Engineers can appraise heat exchanger performance several ways — effectiveness, UA analysis, approach, estimation of fouling, comparisons with design, etc. My intention isn’t to describe these methods, but I would advise you to pick up a textbook on heat transfer or heat exchanger fundamentals and develop an understanding for the methods to be used in your specific applications. Other good reference sources are articles published in Chemical Processing. (“Keep Out of Hot Water, “Prevent Tube Pullout,” and “Heat Exchanger Cleaning Requires a Rethink, are just a few such articles — a search on ChemicalProcessing.com will reveal many more.)
Heat exchanger performance can vary, so how do you correlate it to operating cost or energy savings? This is a much more difficult question but extremely pertinent and important because the answer will determine whether a process shutdown is required for cleaning the heat exchanger or if operation can continue until a turnaround or a planned shutdown. Most decisions for this come down to operating capacity and bottlenecks. Nevertheless, in my energy assessments, I try to identify heat exchangers that are operating ineffectively and then quantify the energy and cost savings possible if those heat exchangers were operating at proper design levels.
Let me share a simple example: Consider a reboiler using saturated steam as the heat source. The typical setup involves steam flow controlled by the process outlet temperature with condensate removed by a large steam trap. All the design conditions are available; you can compare the operating parameters to design to confirm the operations. However, as the heat exchanger starts fouling, the process outlet temperature setpoint can’t be met and the control valve opens to let more steam into the reboiler. (A similar scenario exists where the process flow changes through the reboiler, but let’s assume that this flow is fixed.) Opening the control valve increases the shell (or chest) pressure on the steam side. This has the effect of increasing the saturation (or condensing) steam temperature in the heat exchanger. Now we have a higher driving force and the reboiler can meet the process setpoint. We are back in business!
As we investigate further, we find that the condensate leaving now is at a higher temperature. This means that it has a higher enthalpy than before and that energy wasn’t transferred to the process in the reboiler. Hence, to maintain the heat duty of the reboiler, we now need more steam flow. A thermodynamic energy balance can help calculate the extra steam needed. This steam comes at a cost ($/klb). Now you have everything you need to complete your energy assessment quantification.
This same analysis can be performed for the cooling side with appropriate costs accounted for. I hope I have given you some ideas to evaluate your heat exchangers and develop a methodology to correlate heat exchanger operation and performance with lost opportunity cost.
Riyaz Papar, PE, CEM, is director, Global Energy Services, at Hudson Technologies Company, Pearl River, N.Y. He has more than 20 years of experience in industrial energy systems and with best practices. He also is a U.S. Department of Energy (DOE) Steam Best Practices senior instructor and a DOE steam energy expert. He has provided energy consulting services in 100+ industrial plants in the U.S. and internationally. You can email him at [email protected].