The selection and performance of wort chillers (heat exchangers) is a generally well-documented topic in homebrewing. However, most of the information available on wort chillers applies to the use of immersion, coiled counterflow and brazed plate chillers. An alternative design commonly used industrially that is not widespread in homebrewing is the double pipe heat exchanger.
In double pipe heat exchangers, one fluid flows inside a pipe and a second fluid flows in another pipe that surrounds the first in a concentric tube construction. This arrangement is very similar to the commonly used coiled counterflow chiller; however, there are important differences. Instead of using a continuous length of a double pipe, the length of the heat exchanger is split in short, straight sections interconnected by “return tubes.” This configuration allows the opening of the concentric tubes, which in this application carry wort. This means a wort chiller of this style can be cleaned and sanitized using just a pipe brush and mild chemicals. If a recirculation pump is available, then a clean in place (CIP) process can be used, but having a wort chiller that can easily be opened still gives you the ability to verify the effectiveness of your CIP process. Removable return tubes also allow you to completely drain the liquid for storage. Complete drainage is important; as it has been documented in other BYO articles that moisture left in wort chillers may lead to corrosion and microbial growth.
The performance of this double pipe wort chiller is comparable to the coiled-counterflow designs. The total surface area available for heat transfer in this wort chiller is 3.6 square feet (0.33 square meters). One disadvantage of a double pipe wort chiller may be its bigger physical size, but mounting it on a wall makes the space it takes up negligible. The temperature of the cooled wort after a single pass through this wort chiller depends on a variety of factors; however, for estimation purposes, an average of 0.9 cooling efficiency can be assumed for the wort chiller presented in this project. This assumes 0.9 gallons per minute of cooled wort (fed by gravity) with cooling water running at 6 gallons per minute. The greater the temperature differential between the hot wort and cooling water, the higher the efficiency. With this efficiency number it is possible to make a rough estimate of the temperature of the cooled wort as follows:
Cooled wort temp = temp of hot wort – [efficiency x (temp of hot wort – temp of cooling water)]
Assuming your cooling water temperature is 58 °F (14 °C) and the hot wort is 212 °F (100 °C), then:
Cooled wort temp = 212 – [0.9 (212-58)] = 73 °F. Or, for our metric-measuring friends: 100 – [0.9 (100 – 14)] = 23 °C.
If your estimated cooled wort temperature is higher than desired then you should build a unit with more surface area (more cooling pipes). Keep the length of the pipes below five feet so they can easily be cleaned with a regular pipe cleaning brush.
Use the following equations to determine how much surface area you need to use for a given cooling load.
Surface Area = Cooling Load (in BTUs/min) / U x LMTD
Cooling Load/min = (weight of wort x heat capacity* x temperature differential**)/cooling time (in minutes)
*assume heat capacity of wort to be 1 BTU/lb °F
**Temperature differential = Temperature of hot wort – desired temperature of cooled wort
U (overall heat transfer) = 7.56 BTU/min Ft2 °F
LMTD (log mean temperature differential) = 37 °F (3 °C)
The conceptual aspects of these equations are explained in more detail in the October 2006 BYO edition of Mr. Wizard. You can use these equations to find a balance between cooling time, required surface area, cooling water volume and temperature.
In the following example (given only in Imperial conversions as metric conversions will differ), these equations are used to determine how much surface area is required to cool 10 gallons of boiling wort from 212 °F/100 °C down to 70 °F/21 °C (using 58 °F/ 14 °C cooling water at 6 gpm) in 5 minutes
10 gallons of wort = ~88 lbs
Temperature Differential = 212 °F – 70 °F = 142 °F
Cooling load/min = [(88 lbs of wort) (1 BTU/lb °F) (142 °F)] / 5 minutes
Cooling load/min = 12,496 BTUs / 5 = 2,499.2 BTU/minute
Surface Area = (2,499 BTU/minute) / [(7.56 BTU/minute Ft2 °F) (37 °F)]
Surface Area = 8.93 Ft2
Each cooling pipe in the current configuration provides 0.72 Ft2, therefore 8.93 Ft2 would translate into 12 cooling pipes. Please note that the U-value in this example is only valid to estimate the required surface area of a wort chiller with the configuration presented in this project. Each wort chiller has its own U-value, which is affected by the materials of construction and physical configuration.
The wort chiller presented in this project is constructed of an outer PVC shell that cooling water flows through, which I painted with a plastic compatible paint for aesthetics. Wort flows through a concentric Type M copper tube that is held in place by plastic compression fittings. These plastic compression fittings are made out of polypropylene rated to 212 °F (100 °C) and use an O-ring to make a seal between the copper and PVC tubes. The concentric tubes are connected to each other with a high-temperature, food-grade silicone hose. The pressure rating of high temperature silicone hose is 10 pounds per square inch (psi) so it is important to have a free flow of wort out of the wort chiller. If you are using a pump to run the hot wort and need to regulate the flow-rate, do so by placing a throttling valve on the outlet of the pump before it gets to the heat exchanger. This will keep a low pressure in the heat exchanger. However, if the wort needs to be pumped over a long distance and a high restriction is expected, then a hose with a higher-pressure rating should be used. To regulate the wort flow in gravity fed configurations, place a valve on the outlet of the boil kettle and feed wort to the chiller from the bottom to top in order to keep concentric tubes fully submersed in liquid and maximize heat transfer. As for the connectors for the cooling water inlet/outlet, I glued a 1-inch slip by ¾-inch female pipe thread into each end of the PVC. Finally, the entire assembly may be mounted on plastic unistrut rails using “quick clamps” that attach to each section of PVC pipe. This makes the unit very stable and allows you to mount it on a wall.
26 feet of 1-inch PVC schedule 40 pipe
10 PVC T-fittings
30 feet of 1/2-inch ID copper tube type M (5/8-inch OD)
10 plastic Parker compression fittings (3/4-inch NPT x 5/8-inch OD)
10 reducing bushings (1-inch slip x 3/4 NPT)
5 feet of 1/2-inch high-temperature food-grade silicone tube
PVC primer and cement
Pipe cleaning brush