COMMERCIAL WASTE heat recovery systems have been around for a very long time. Their recovery efficiencies are a known fact, and their savings have been documented for quite some time. Commercial waste heat recovery potentials vary from operation to operation and I’ve had the opportunity to work with most of them. There are, however, some untapped savings waiting to be exploited.
The most common means of waste heat recovery uses hot refrigerant from the condenser of refrigeration systems. A typical fast-food restaurant has a large, walk-in type cooler-freezer assembly with a large, remotely mounted compressor/condenser package. This system extracts heat from the inside of the cooler and ejects it to the ambient air via the outdoor condenser.
Waste heat recovery can be as simple as installing a desuperheater in-line with the compressor and using the heat from the process for preheating the hot water used for cleaning and sanitation. An alternative is using all the heat available from the condensing unit for preheating the domestic hot water. Using a desuperheater consumes the least amount of space but also delivers the least amount of energy. Generally, available space is a major consideration in most restaurants, and unless taken into consideration in the initial design, the space is hard to get.
This method of heat recovery is also practiced quite a bit in the dairy industry. When the cows first come into the milking shed, their udders are washed with a warm water solution. This is a pretty sizeable DHW load. Once the cows are milked, their warm milk is immediately cooled, and the heat that has been removed is used to preheat the water used in the sanitizing process. Generally in these operations, the preheat storage/waste heat recovery tanks are large enough to handle the majority of the hot water needs without requiring much supplemental energy for the water heating process. The exception is water used for sanitization. That water is generally boosted to a higher temperature by a flash heater, but it uses the “free” preheated water in the process.
Commercial laundries are another area where energy recovery makes a lot of sense. It is entirely possible to reduce the water consumption and energy consumption of a commercial laundry by 30% to 50%.
For example, when a large laundry machine has finished its last washing cycle, the clothes are typically rinsed in hot water. This hot water is well-balanced, pH wise, and is usually just dumped into the sanitary sewer. Numerous laundry equipment manufacturers will recover this relatively clean, hot and properly adjusted, chemically balanced water in an insulated holding tank.
The water is pumped back into the washing machine for the next incoming load for what is referred to as the “break cycle.” A break cycle is simply where the linens or what-have-you are wetted and rolled around, getting them ready for the detergent cycle. In addition to the obvious energy and water savings, the laundries can realize a substantial reduction in chemical consumption.
Some systems don’t reclaim the water but recover the energy used heating the water. This involves the use of large-passageway, shell-and-tube heat exchangers. A wet-well sump located in the drain trenches serves the washing machines. When hot water is discharged from a machine, a sump pump turns on and pumps this hot water through the shell side of the heat exchanger. An automatic four-way reversing valve sends the wastewater through the heat exchanger in a different direction each time it is used. This avoids the possible buildup of lint, hair and other things found in washing machine drain water.
All cold water entering the DHW system serving the laundry is diverted through the tube side of the heat exchanger, thereby extracting a major chunk of energy that would have otherwise just been run down the drain. This type of system typically recovers 50% of the energy used for heating hot water.
Although not usually considered pure hydronics, the application of waste heat recovery from hot gas streams, such as pizza ovens and other baking processes, has a lot of potential for energy savings. The key to efficiency is making sure that the heat recovered can satisfy a substantial DHW load. In other words, if there is no place for the recovered energy to be used, then it probably doesn’t make sense to recover it. That’s rare.
In most cases, if food is being prepared at any level of production, there is usually a pretty substantial hot water load to accompany other heat consuming appliances. Waste heat recovery, while not usually found in the typical hydronic heating contractor’s bag of tricks, represents a substantial income stream that not only provides good jobs, but is also good for the environment.
That’s it for this month on commercial waste heat recovery. Tune in next month as we rediscover the potentials of solar. Until then, Happy Recovered Waste Heat Hydronicing!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.