Gas Water Heater Exhaust Heat Recuperation Device
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Transcript of Gas Water Heater Exhaust Heat Recuperation Device
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
For AUTOTECH 2TS3Dr Timber Yuen
Design, Construction and Testing of a Heat
Exchanger to Recuperate Heat Normally Lost from Exhaust Fumes of a Gas
Hot Water Heater
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
-Gas water heaters burn methane (CH4) which gives off CO and CO2
-These gases must be ventilated to outdoors to protect health
-Tremendous amounts of heat are lost due to the hot fumes being ventilated
-We wanted to build a device that uses the heat from these fumes to preheat cold water to save gas during the water heating process.
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
-Normal Hot Water Heater Configuration -Our planned configuration
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
-Cold water would normally come in at 11°C and be heated to about 55°C
-This heat exchanger would “preheat” the cold water to a slightly warmer temperature (ie, 20°C) and reduce the amount of energy required to get to 55°C
-The blower is only active when the tank is filling
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
Design-Helical pipes wound within air pipe
-In reality, pipes would go in and out sealed holes in outer casing
-To simplify construction, pipes protruded from open end of exhaust pipe.
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
Design-We needed a source of hot air that would simulate the exhaust of a gas water heater
-We approximated the temperature and air flow rate of a space heater to be that of a water heater exhaust unit (may have been inadequate)
-Using an approximate input air speed of 4 m/s, a fluid simulation was done to look at the speed of air as it passes the coils
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
Construction
-Copper was used for water tubes due to high thermal conductivity
-Copper tubes were wound around a smaller pipe to obtain the helical shape
-4 tubes were used (two purchased and cut in half)
-Difficulty with kinking, used ice to hold structure of tube
-We bent two of the coils in the wrong direction – had to re-coil
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015 Construction
-A space heater was used to simulate hot exhaust
-Duct fittings were used to route the air into the abs pipe
-Spray foam was used to insulate steel ducts from convective air
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
-Copper Manifold used to split flow to 4 coils
-Flare fittings to vinyl hose
-Only potential source of leaks
Construction
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015 Testing
- We wanted to know what effect the flow rate has on the rate of heat transfer
- We expected an optimal rate of fluid flow that balances the convection coefficient h and the time t each element of water spends in the elevated temperature environment
-The optimal rate of fluid flow will then be compared to what is expected from an average water heater
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
-Divide fluid flow range to four different rates
-At each rate, measure flow rate using graduated cylinder and timer
-Measure output temperature at 30 seconds and 2 minutes
-Turn space heater on fan only to clear out any heat between tests, cool down heater
-Repeat for every flow rate
Procedure
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015 Testing
-This experiment had considerable danger
-Space heater has exposed electrical components, any contact with water could transmit voltage to casing and heat exchanger components
-A GFCI outlet was used to ensure shutoff if water contacted any electrical parts
-Problems with temperature limit on space heater
-Reduced power to 1300W, reduced cycle time, removed cap of air pipe
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015 Testing
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
Results
10
12
14
16
18
Outlet Temperature vs. Flow Rate
Outlet Temperature at 30 SecondsOutlet Temperature at 2 minutes
Flow Rate(kg/s)
Tem
pera
ture
(°C)
Inlet Temperature = 11°C
Flow Rate (kg/s)
Outlet Temperature at 30 Seconds
Outlet Temperature at 2 minutes
Heat Transfer Rate at 30 Seconds
Heat Transfer Rate at 2 Minutes
0.007413 14 18 92.95902 216.904380.012893 14 16 161.67822 269.4637
0.0256 12 13 107.008 214.0160.054167 11 11 0 0
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
Results
0.007413 0.012893 0.0256 0.0541670
50
100
150
200
250
300
Heat Transfer Rate vs. Flow Rate
Heat Transfer Rate at 30 SecondsHeat Transfe Rate at 2 Minutes
Flow Rate (kg/s)
Heat
Tra
nsfe
r (W
)
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
0 0.02 0.04 0.060%
5%
10%
15%
20%
25%Electrical Power to Water Heating Power Efficiency
Efficiency @2min (1300W)Efficiency @ 30s (1300W)
Flow Rate (kg/s)
Effici
ency
Results
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
- We observed a maximum heat transfer rate at 0.013 kg/s
-Expected flow rate to supply a hot water heater is 0.134 kg/s (2012 Ontario Building Code)
-At that rate, our heat exchanger would be virtually ineffective
-We would need to increase the number of coils, or increase tube diameter, or a combination
-Designing an experiment to find an optimum configuration of tubes would be difficult
Analysis
National Research Council of Canada, 2009
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015
- National Research Council of Canada did a study that finds the average family of four uses 167 L/day of hot water - We wanted to calculate how much energy we can save heating 167 litres of water:- We used 14¢ as the cost for 1kWh
- = 12,846 s
- Using our best energy transfer rate 269.46W:
- 12,846s * 269.49W = 3461kJ
- = 13.44¢/day = $49/year in savings
- This value is highly theoretical because it uses the heat transfer rate when the flow is very small
Analysis
Domestic Water Pre-Heater using Gas Water Heater Exhaust Heat
Jack Gillies & Joshua MolinaMarch 31, 2015 Conclusion
-The heat exchanger would have to be redesigned to accommodate higher flow rates without sacrificing surface area
-Although we don’t know the actual savings, we know that a device like this has potential to save considerable amounts of energy
-Devices such as this would not be difficult to retro-fit into existing residential plumbing
-Other heat recovery systems are becoming popular ie, drain heat recovery
-The testing showed that there is in fact an optimum flow rate for heat exchangers, as our hypothesis predicted
Questions?