Energy usage calculations of heat exchanger 1

[wilsonudevil]

Hi all,

I'm trying to calculate the energy usage of a developmental hybrid ventilation system.

It's a small domestic sized unit with a water/air heat exchanger. Fresh air from outside flows through the unit where it is tempered before entering a room.

I've rigged it up so that air is blowing over the heat ex at 40 l/s. Warm water is flowing through the heat ex at 4 l/min in a closed loop. I'm heating the water with a domestic immersion heater with an energy meter attached to it.

Letting the system stabilise and then run for an hour, if I use Q=mCpDT to calc the energy given to the air crossing the heat ex it is always much higher than the experimental reading I take from the energy used by the water heater. I thought it would be the other way around! I'm pretty sure my flow rates are correct.

Any ideas?

Many thanks.

 

[DrRTU]

Did you include pump heat, fan heat and thermal loss of sheet metal and ductwork. Run the calculations with the +/- accuracy of the air flow meter and temperature sensors. How did you measure electric heat input to the immersion heater (true watts)?

 

[wilsonudevil]

I didn't include pump heat so that will contribute. Thanks!

Thermal losses through ductwork etc should act to increase my energy reading shouldn't they?

I measured the electric heat input using a cheap off-the-shelf power meter, but have checked it using a commercial unit and it's actually quite accurate. What's 'true watts'?

 

[DrRTU]

Your KWH meter should be certified to a known standard such as ANSI C12.1 and C12.16 Electronic Meter National Accuracy Standards. You then can project the accuracy (%) of the readings.

 

[Waramanga]

Cp of water changes with temperature but this is a relatively small factor. The flow rate is the only thing that could get you far from the mark. How many watts are the pump and the heater?