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



-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.



-Normal Hot Water Heater Configuration -Our planned configuration



-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



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.



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



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


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



-Copper Manifold used to split flow to 4 coils

-Flare fittings to vinyl hose

-Only potential source of leaks

Construction


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



-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



-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



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



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

)



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



- 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



- 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


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?

gas water heater exhaust heat recuperation device

Engineering

domestic water preheater

gas hot water heater

water heater exhaust

water tubes

cold water

gas water heaters

space heater

water heating process