HRV’s and ERV’s, which ventilate interior spaces without throwing energy out the window, are only one use of heat exchangers in buildings homes. Another permutation keeps energy from going down the drain, and is is called drain-heat recovery. The idea is to use hot water from a shower to preheat cold water going into the water heater. Here’s a schematic from one manufacturer:
(imagse from gfxtechnology.com)
The design of these heat exchangers is simple, consisting of a coil of copper tubing wrapped around a typical sanitary waste pipe. They take advantage of the fact waste water flows in a film on the inner surface of a vertical waste pipe, making for a large surface area and large convection heat transfer coefficient between the pipe’s walls and the drain water. According to the manufacturer (GFX), this configuration can capture as much as 60% of waste heat! This can translate into a big energy savings, especially in applications where a shower is getting a lot of use (imagine how much hot water goes down the drain in locker-room at a health club). It’s important to note however that drain heat recovery can only occur when fresh cold water is going into the water heater at the same time hot waste water is flowing; thus if a persons in a household usually bathe, the device is rendered useless.
Another place heat exchangers find application is homes is the energy-consuming task of drying laundry. The same concept of an HRV can be applied to a clothes dryer; the hot waste gasses heat incoming cold air, recycling heat. In this application, large amounts of condensation form on the moisture-laden waste-gas side of the exchanger, which is collected and drained off. Dryers of this type only recently and very briefly came to market. They were displaced practically overnight by another even more efficient dryer design, the heat pump dryer. Heat pump clothes dryers can be though of as utilizing a an “active” heat exchanger. An evaporator coil recovers both sensible and latent heat from the hot waste gasses leaving the dryer, returning it via a condenser coil to the incoming cold air. The COP of a heat pump in this configuration is very large, since it is moving heat in the same direction as a large temperature gradient. The reason this process is more efficient than a passive fluid-fluid heat exchanger is because the evaporator coil can get very cold, thus forcing more water to condense out of the waste air and recovering much more latent heat.
(image from http://www.winningappliancesblog.com)
What makes heat exchangers so interesting to me is that in spite of being so useful and seemingly innovative, they usually very simple and easily fabricated. Fluid-fluid, concentric pipe type exchangers are made easily with materials available anywhere. A great example of this obtainable utility, though not technically related to building science (forgive me), is a water pasteurizer utilizing a concentric pipe heat exchanger to regenerate waste heat. Water pasteurization is the process of heating water to around 170 degrees Fahrenheit for a short period of time, killing virtually all microbes. This is a very obtainable method of water treatment for developing areas of the world. All that is needed is a large metal vessel (like a re-purposed metal drum), a heat source (this can be supplied utilizing makeshift solar arrays), and two sizes of pipe or food-grade tubing. Water in the drum is heated to the appropriate temperature, then drained through a heat exchanger, heating the incoming water. Here is a rough scheme for the process:
Just to show how simple it is to fabricate a well-engineered heat exchanger, I designed and built one of my own. It’s not for anything practical, but certainly my favorite heat-exchanger application: making beer. See my next post for a full report and analysis.