Lessons · Engineering · Moving heat between two streams
Moving heat from one stream into another
The heat a stream gives up or takes in is its mass flow times specific heat capacity times the temperature change. In a heat exchanger, what one stream loses the other gains, minus losses -- so knowing three quantities on one side and two on the other fixes the rest.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the arithmetic behind every radiator, cooler and heat recovery unit, and it is the quickest way to check whether a claimed duty is possible at all before anybody sizes hardware.
How to think about it
Write the energy balance as one equation per stream and set them equal. Keep mass flow in kilograms per second and specific heat in joules per kilogram per kelvin, and the answer comes out in watts without any conversion.
Worked example
Q = mass flow x specific heat x temperature change.One stream at a time.
What one side loses, the other gains.Minus losses to surroundings.
SI units give watts directly.kg/s, J/kg·K, K.
A balance that does not close means a missing stream.Or a wrong figure.
Your turn
Water at 2 kg/s, specific heat 4200 J/kg·K, rises 10 K. Write the heat duty in watts.
2 x 4200 x 10 = W
Solve one, graded on the server
The trap
Mixing litres per minute with kilograms per second. The formula needs mass per second, and the conversion is where the factor-of-sixty errors come from.