A Comparison of Wort Chiller Water and Time Usage as a Function of Coolant Temperature and Flow Rate

8/13/2019 A Comparison of Wort Chiller Water and Time Usage as a Function of Coolant Temperature and Flow Rate

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A Comparison of Wort ChillerWater and Time Usage

as a Function of CoolantTemperature and Flow Rate

By E. Baxstrom and P. Meleney

EmpiricAles.comOctober 16, 2012


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AbstractThe aim of this experiment was to compare cooling rates and water usage of commonly available immersion wortchillers and determine the most efficient equipment and method for chilling wort. All tests were conducted withfive gallons of boiling water as a wort analog. The wort was cooled for ten minutes while continuously stirringand measuring temperature every minute. Two 25 chillers composed of Copper and Stainless Steel were comparedwith both chillers cooling at nearly identical rates. The results of the 25 chiller trials were then compared againstseveral trials with a 50 Copper chiller run at different flow rates. The 50 chiller outperformed both 25 chillers inboth water usage and speed of cooling. A test of unstirred wort with the 50 Chiller had the worst performance,

resulting in both high water usage, and high wort temperature at the end of the ten minutes of cooling. Wortchilling was then modeled using Newtons Law of Cooling and it was determined that the most efficient coolingwas achieved through use of the 50 chiller run at medium flow rate while stirring the wort. The model also showsthe profound impact of coolant temperature on chilling efficiency. Lower coolant temperatures were significantlymore efficient with time and water usage.

IntroductionThe most common wort immersion chillers are made from either copper or stainless steel, with lengths of either 25feet or 50 feet. There has been some discussion regarding performance, with some claims that copper is moresuitable than stainless due to its superior heat conductivity (by a factor of more than 20) [1]. However, thermalproperties alone do not determine heat conduction rates, with surface area and conductor thickness both being

significant factors [2]. In the case of wort chillers, coolant flow rate, coolant turbulence and wort convection are allfactors affecting overall cooling rate. This study determines cooling rates empirically, models the chillers usingNewtons Law of Cooling, and provides recommendations for chilling quickly and efficiently.

Methods and MaterialsAll tests were conducted using 5 gallons of tap water (measured between each trial) in place of wort. The coolingwater was also tap water, and will be referred to as coolant while the water used as a wort analog will be referredto as wort. Three chillers were tested: a 25 x 3/8 Copper chiller, a 25 x 3/8 Stainless chiller, and a 50 x 1/4Copper chiller. For each test, the wort chiller was placed in the pot before the wort was brought to a boil. Worttemperature was measured using a calibrated dial thermometer (0.5 F) once per minute for ten minutes. The wortwas stirred using a long handled spoon at a rate of approximately one revolution of the spoon around the pot per

second. For the unstirred test, the wort was left completely unstirred until the 10 minute mark, at which time thewort was homogenized through stirring in order to achieve an accurate temperature reading.

Coolant flow rate was determined using a stopwatch to measure the time to fill a bucket of known volume.

Tests were conducted on two different days separated by approximately two months, and consequently ground-water temperatures were different for the two sets of test. Thus, some trials cannot be compared directly. Furtheranalysis is given in the Discussion section.

ResultsAll immersion wort chillers performed better than ice-bath cooling or no-chill methods which can take up to one or

24 hours respectively. Only the 50 Copper chiller running at a flow rate of (an unrealistically slow) 0.608 gpm (2.3L/min) failed to cool the wort below 95F (35C) within the ten minutes of cooling. All other chiller configurationscooled the wort below 81F (27C) within ten minutes, even with 72F (24C) coolant water for some configurations.

The two 25 chillers (Copper and Stainless) performed almost identically, with wort temperatures within a degree ofeach other after ten minutes of stirred chilling. The 50 Copper chiller with wort stirring outperformed all otherconfigurations, even with substantially reduced coolant flow rates compared to the 25 Chillers. Because the testwas conducted over several weeks, coolant water temperature (ground water temperature) varied significantly, sonot all results can be compared directly.


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It is also worth noting that the 50 Chiller could not achieve a flow rate higher than 2.05 gpm, while both 25 chillerspeaked at 3 gpm. Also, coolant temperatures for both days varied less than 1.5 F as measured between trials.

Chiller Typeand

Flow Rate

25' Stainless:3gpm (11.4

L/min)

25' Copper:3gpm (11.4

L/min)

50' Copper:0.608 gmp (2.3

L/min)

50' Copper:1.33gpm (5

L/min)

50' Copper:2.05gpm (7.8

L/min)

50' CopperUnstirred:

2.05gpm (7.8L/min)

Coolant temp: 72F (23.9C) 72F (23.9C) 62.6F (17C) 72F (23.9C) 62.6F (17C) 62.6F (17C)

Time

0 212 212 212 212 212 212

1 186 175 183 177 165

2 165 149 169 154 133

3 142 130 155 136 112

4 125 116 145 122 98

5 111 104 136 112 88

6 101 97 127 102 81

7 93 92 120 95 74

8 88 86 114 89 71

9 84 81 109 85 68

10 79 78 97 81 67 80Table 1. Wort temperature vs. time for various chiller configurations. For the unstirred trial, wort temperaturebetween time 0 and 10 minutes was heterogeneous and therefore could not be measured.

Discussion25 Copper vs. 25 StainlessThe 25 Copper and 25 Stainless chillers behaved almost identically, with the temperatures being within 1 F afterten minutes of cooling (with equal coolant temperature and flow rate). Coppers superior heat conductivity is oftencited as evidence of its superioriority as a wort chiller material. However, other factors appear to make up forstainless inferior heat conductivity, with the two chillers performing equally well. These other factors determining

overall cooling rate likely include tubing thickness, interior surface area (the amount of water in contact with thetubing interior), and fluid flow characteristics, i.e. turbulence which is affected by fluid velocity, interior surfaceroughness, and pipe diameter.

50 CopperThe 50 Copper Chiller outperformed both 25 chillers, cooling the wort as well as the other two chillers, whileusing 55% less coolant. It is worth noting that none of the chillers brought the coolant into equilibrium with worttemperature. Sporadic effluent (wort chiller outflow) temperature measurements indicated the effluent temperatureto be more than five degrees colder than wort temperatures, even for the 50 Chiller at the lowest flow rate.

Stirred vs. Unstirred

Between the stirred 50 Copper and unstirred 50 Copper chiller trials, the stirred method performed significantlybetter. With the same coolant temperature and flow rate, the stirred wort was 13 F cooler than the unstirred wortat the end of ten minutes. It took 10 minutes to cool the unstirred wort to 80 F, while it took the stirred wort justover six minutes to reach the same temperature.

Time vs. Water UsageIn order to compare time and water usage for different coolant temperatures, further analysis is required. Thecooling is modeled using Newtons Law of Cooling. There are more engineering-centric models that take intoaccount parameters such as chiller surface area, specific heat, as well as wort and coolant mass. By using Newtons


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Law of Cooling, all these parameters are lumped into a single parameter, the thermal conductivity, k. The k value isthen used to calculate and compare cooling times and water usage for various chiller configurations.

Newtons Law of Cooling states that the change in temperature with respect to time is proportional to thetemperature difference between the coolant and the material being cooled (Figure 1).

Figure 1. Newtons Law of Cooling. T=the temperature of the wort, Tcoolantis the temperature of the coolant, k isa constant determined empirically, and t=time.

It should be noted that this is not the intended application of Newtons Law of Cooling. However, the modelprovides an easy and reasonably accurate1means of comparing the cooling properties of the various chillerconfigurations for typical wort chilling parameters.

A value for k was calculated from the temperature drop in each time step (one minute). The overall k value foreach chiller was then determined using the mean of selected k values. Individual k values were excluded if thesignal-to-noise ratio was less than three. Additionally, the k values from the first two measurements of the stainlesschiller were excluded due to inconsistencies in measurement technique at the outset of the experiment. The

resulting average k values and standard deviations are given in Table 2.

25' Stainless:3gpm (11.4

L/min)

25' Copper:3gpm (11.4

L/min)

50' Copper:0.608 gmp(2.3 L/min)

50' Copper:1.33gpm (5

L/min)

50' Copper:2.05gpm (7.8

L/min)

50' CopperUnstirred:

2.05gpm (7.8L/min)

Average k value 0.27 0.26 0.15 0.24 0.3 0.22

k value Std Dev 0.03 0.01 0.03 0.02 0.01

Table 2. Newtons Law of Cooling k values for different chiller types and flow rates.

The 50 Copper unstirred test required a different approach, since only the starting and ending temperature areknown. The k value was calculated by solving Newtons Law of Cooling as a differential equation. Given the testparameters, k was calculated with algebra.

Coolant Temp F Time Gallons

50' Copper: 50 deg Coolant 50 13 8

8 10

6 13


10 13

8 16


14 19

11 23

25' Chiller: 50 deg Coolant 50 7 21

50' Copper Unstirred: 50 deg Coolant 50 9 18

Table 3. Coolant times and water usage for various wort chiller configurations. Each configuration assumes thewort begins at boiling and is cooled to a final temperature of 75F.

1In order for Newtons Law of Cooling to be a valid model, the rate of cooling has to be linearly related to the temperature differencebetween the medium and the coolant. In other words, the natural log of temperature versus time should be linear. The trend line ofLn(temperature) vs. time has R2values greater than 0.91 for all temperature sets, which is sufficient for this model.


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Treating Newtons Law of Cooling as a differential equation and finding the solution also allows cooling to bemodeled for various parameters. There are several ways to approach the information given by the solution to thedifferential equation. The most instructive approach for comparing time and water usage is to use fixed values forthe initial and final temperatures and calculate the resources required for each of the three flow rates.

The model assumes that the wort temperature begins at boiling and will be chilled to 75 F (24 C). Although 75 Fis higher than typical yeast pitching or fermentation temperatures, it is closer to the range of temperatures measuredin the experiment. Additionally, if the model used a final wort temperature close to the coolant temperature, it

would make the modeled results less meaningful since most of the total cooling time would occur in the last fewdegrees as the wort temperature approached the coolant temperature.

The results of the model for given parameters is shown in Table 3.

Graphing the results makes the trends more apparent. Each line in the graph represents the resources required tocool the wort to 75 F for the given coolant temperature. The water usage was determined using flow ratemultiplied by cooling time. It becomes readily apparent that there is a tradeoff between cooling time and waterusage and that the tradeoff is non-linear. With high flow rates, the coolant cannot reach equilibrium with the wortand therefore uses water inefficiently. For extremely low flow rates, the coolant comes closer to equilibrium withthe wort temperature, but at the expense of greater time usage.

Chart 1. Cooling resources required to chill 5 gallons of wort from 212 F to 75 F. The lines represent time andcoolant expenditure to chill the wort for various coolant temperatures using a 50 Chiller while stirring the wort.The two points are the time and coolant expenditure to chill the wort with 50 F coolant using a 25 Chiller (copperor stainless), and using a 50 Chiller without stirring.


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Bibliography

1. Wikipedia. List of Thermal Conductivities Retrieved 10/8/12http://en.wikipedia.org/wiki/List_of_thermal_conductivities

2. Taftan Data. "Fouriers Law of Conduction Retrieved 10/8/12http://www.taftan.com/thermodynamics/FOURIER.HTM

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A Comparison of Wort Chiller Water and Time Usage as a Function of Coolant Temperature and Flow Rate