The students constructed a single pipe heat exchanger by burying a steel pipe in an active compost pile to study if compost heat could effectively heat water. Water was run through the pipe at two flow rates while temperature data was collected. The experiment resulted in water temperature increases of 1.4°C and 1.3°C for the different flow rates. Modeling in STELLA predicted the temperature increases could have been higher at around 9°C and 5°C if steady state conditions were reached. Faster flow rates led to higher heat transfer rates but slower rates allowed for greater temperature change according to the models.

1. Single Pipe Heat Exchanger in Compost
Courtney Kittel, Rachel Burger, Matthew Lawrence, & Dr.
Drapcho
BE 4120, Biosystems Engineering, Clemson University,
Clemson, SC, 29631
Introduction
Goal: To use compost as a water heater
We have seen how solar water heaters can use the solar radiation
from the sun to effectively heat water. Our concept was similar, but
instead we wanted to use compost. In the process of making
compost, some of the piles can reach 170 Fahrenheit. We wanted to
see if we could use the heat generated from compost to heat water
running through a pipe that is placed in the compost
Materials & Methods
The single pipe heat exchanger was constructed by burying the
steel pipe in the upper half of a 15’ trapezoidal section of active
windrow compost. Well water was run through a garden hose to a
valve that connected the hose and pipe. The valve was opened in two
variations to allow for different flow rates, and timed volumetric
measurements were conducted and averaged to determine the two
flow rates used. The temperature of the compost was measured in
various locations of the pile using a compost thermometer. After
measuring the constant influent water temperature, the HOBO
datalogger thermometer was used to measure the temperature of the
water effluent for 20 minutes per run for each of the two runs. Steady
state conditions were assumed after 20 minutes of run time.
Results & Discussion
All of the data collected at the compost site can be seen in Table 1.
Table 1. Raw data from experiment
Length of pipe in the compost (L)=15.64583 ft= 4.77m
Diameter of pipe (D)=2in=0.0508m
With this data, the internal convection coefficient of the water in the
pipe was calculated by using the forced internal convection equations.
Then to calculate the heat transferred from the compost to the water,
a thermal energy balance equation with a boundary condition of
constant surface temperature was used. The calculated data and the
equations used can be seen in Table 2.
Table 2. Calculated data from experiment.
From Table 2, it can be seen that the heat transfer rate (q) was a
little higher for the faster mass flow rate than for the slower mass flow
rate. The same goes for the internal convection coefficient. From this,
it can be concluded that the faster the mass flow rate, the higher the
heat transfer rate.
Modeling
To model what should have happened with our single pipe heat
exchanger, we modeled the experiment in STELLA. We modeled what
the outlet temperature should have been once it reached steady state.
We used the same mass flow rates and heat transfer rates that were
calculated from the data. The STELLA model can be seen in Figure 3
below.
Figure 3. STELLA model for steady state heat transfer to water from compost
Figure 4 is a graph that shows the modeled outlet temperature for
the first run and Figure 5 is for the second run.
We assumed the temperature of the water flowing out of the pipe
had reached steady state after 20 minutes but STELLA shows that it
should have happened much quicker.
Conclusions
The heat generated from the biological activity in compost piles can
be harnessed and used to heat water using a single pipe heat
exchanger. This experiment resulted in a 1.4℃ and 1.3℃ temperature
change from influent to effluent water in the 0.193 kg/s and 0.420 kg/s
mass flow rates, respectively. Under ideal conditions, the STELLA
models illustrated that steady state heat transfer conditions should
have reached ~9℃ and ~5℃ temperature changes in the water for
each flow rate, respectively. The experiment showed that he faster the
mass flow rate, the higher the heat transfer rate, but the models
illustrated that the slower flow rate could reach a higher change in
temperature compared to the higher flow rate. Possible improvements
to our design to generate more meaningful heat transfer include
ensuring proper depth of pipe for equal heat distribution over the
entire pipe surface, increasing the length of pipe and pile, decreasing
the flow rate of the water.
Variable 1st Run 2nd Run
Mass flow rate (m) 0.193 L/s = 0.193 kg/s 0.420 L/s = 0.420 kg/s
Inlet temperature (Tin
) 57.7°F = 14.3°C 57.7°F = 14.3°C
Compost temperature (TS
) 150°F = 65.6°C 150°F = 65.6°C
Outlet temperature after 20
minutes (Tout
)
60.29°F = 15.72°C 60.10°F = 15.61°C
Variable 1st Run 2nd Run Equation used
Average temperature
(Tave
)
15.01°C = 288K 14.955°C = 288K Tave
=(Tin
+Tout
)/2
Viscosity at Tave
(μ) 1080*10-6
Ns/m2
1080*10-6
Ns/m2
[-]
Thermal conductivity
at Tave
(k)
598*10-3
W/mK 598*10-3
W/mK [-]
Prandtl number at
Tave
(Pr)
7.56 [-] 7.56 [-] [-]
Reynold’s number
(Re)
4479 [-] 9747 [-] Re= (4m)/(πDμ)
Nusselt number (Nu) 43.06 [-] 80.21 [-] Nu=0.023Re4/5
Prn
n=0.4 since TS
>Tave
Internal convection
coefficient (hi
)
506.89 W/m2
K 944.2 W/m2
K h=(Nu*k)/D
Log mean
temperature (Tlm
)
50.59°C 50.64°C Tlm
=[(TS
-Tout
)-(TS
-Tin
)]/ln[(TS
-Tout
)/(TS
-Tin
)]
Thickness of pipe
wall (x)
0.083 in= 0.0021m 0.083in = 0.0021m [-]
Resistance (U) 182.33 W/m2
K 218.77 W/m2
K U=1/[(1/hi
)+(x/k)]
Heat transfer rate (q) 7021.9 W 8433.6 W q=UπDL(Tlm
)
● Well water
● 17’ galvanized steel
pipe
● 10 cu. yd. windrow
compost pile
● Garden hose
● PVC valve
● HOBO datalogger
● Compost thermometer
● Digital thermometer
● Timer
References: Drapcho C. (2021) Lecture 15 Forced convection internal. Unpublished Notes, BE 4120, Clemson University.
Acknowledgments: We would like to thank Dr. Drapcho, Cherry Crossing Research Center, and Dave VanDeventer for helping
us set up and execute this project.
Figure 1. Profile view of single pipe heat
exchanger in compost pile
Figure 2. Side view of single pipe heat
exchanger in compost pile
Figure 4. Outlet water temperature from
model for 0.193 kg/s flow rate
Figure 5. Outlet water temperature from
model for 0.420 kg/s flow rate