I make my own beer at home.  The end result of a brew day is a giant pot of boiling “wort,” or unfermented sugar-water.  This fluid needs to be cooled to 70 degrees Fahrenheit so that yeast can be added & fermentation can be begin.  It’s absolutely amazing how long 5 gallons of boiled wort will stay hot, even with the pot submersed in ice water.  It takes as long as 10 hours to accomplish via ice bath.  Ideally, you don’t want the wort sitting around long before adding yeast because bacteria from the air can start eating the sugars, causing bad off-flavors in the finished product.  So to demonstrate aspects of heat exchangers, show how simple they are to build, and cool my wort quickly, I built a counter-flow heat exchanger.  The one I build consists of a garden hose and inner copper tube.  The inner copper tube caries the boiling wort, and the garden hose carries tap water in the opposite direction.  On the ends are some copper fittings that seal everything up and allow connection to hoses.  Here’s a pictorial of its construction and the device in action:


Though my first test of the heat exchanger (aka “counterflow wort chiller”)  was brewing a batch of stout (it worked great!  It cooled 5 gallons from boiling to 70 degrees in about 15 minutes, note however I had the cooling water coming from the faucet at near full blast), I did some actual tests to find epsilon, or the effectiveness of the chiller.  I measured each volumetric flow rate by measuring how long it took to fill a 2 quart pitcher:
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With this data, we can now approximate NTU, and approximate the overall heat transfer coefficient via E-NTU relationships:


I can’t find any data to compare this “wort chiller” to.  But I think the effectiveness seems reasonable, and the the overall heat transfer coefficient seems wildly large compared to architectural materials (which are obviously designed to have low heat transfer coefficients).


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:

Drain heat 2 Drain heat 1

(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.

Heat pump(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.

Heat Exchangers: Interesting Applications Part I: HRV’s and ERV’s

Call me a nerd, but I actually find heat exchangers pretty cool.  My first exposure to them was in high school when my parents’ bathroom fan quit and I was tasked with purchasing and installing a new one.  When I went to a big-box home store to survey my replacement options, most of the fans were in the $50 to $150 range.  I was shocked to see one with a $500 price tag.  The unit was a “Panasonic FV-04VE1 WhisperComfortTM Spot ERV.”  The unit’s box was printed with the following image:

ERV-1(picture from http://www.panasonic.com)

It’s pretty clear from the picture what the ERV (energy recovery ventilator) does, at least regarding sensible heat.  In winter, warm air from inside a conditioned space heats fresh cold air drawn in from outside.  The reverse process occurs in summer.  This is most often accomplished by separating the fluids by a thin membrane with a very large surface area.  HRV’s (heat recovery ventilators) are units that only exchange sensible heat.  ERV’s like the one pictured above actually exchange latent heat as well,  recovering some of the energy lost due to a phase change of water.  This is commonly done via a moisture-permeable membrane separating the fluids.  Here’s a crude diagram of what’s going on:


Both ERV’s and HRV’s are a great way to decrease indoor air pollutants in today’s tight, low-infiltration homes, while not significantly increasing space-conditioning energy use, as would traditional forced HVAC ventilation or natural ventilation would.  However ERV’s are pretty much required (when compared with HRV’s) in extreme climate conditions.  When HRV’s are used in situations with a large temperature gradient between air streams & at least one of those air streams is even slightly humidified, condensation is sure to occur in the unit, which can result in mold and actually make bad indoor air worse.  ERV’s should also not be used without considering moisture issues; some conditions may overcome a unit’s ability to exchange moisture and condensation can still occur.  A great example of ERV misuse would be the story I told before; that Panasonic ERV should have never been by the bathroom fans, since using it in that application, as a device intended to evacuate shower steam, would likely be more moisture than it was designed to cope with.