Phase-change heat exchanger efficiency??

[That_guy]

Hi science community,

 

I am attempting to calculate the efficiency of an oil cooler that takes the form of a counterflow shell and tube heat exchanger.

 

In the shell is the oil which is being cooled from around 70*C to 40*C

 

In the tubes is pure Ammonia (R-717 refrigerant) which enters as a liquid and leaves as a gas.

 

I am very confused by the phase-change in the Ammonia.. do I use the heat of vaporization to find the Heat generation (Q_dot). (With this equation: Q_dot = m_dot * h_fg)?

 

I have been attempting to use this formula to find the efficiency:

 

Q_dot = e * C_min * (T_hot_in - T_cold_in)

 

with e being the efficiency.

 

Am I on the right track at all??

[cypress]

Hi science community,

 

I am attempting to calculate the efficiency of an oil cooler that takes the form of a counterflow shell and tube heat exchanger.

 

In the shell is the oil which is being cooled from around 70*C to 40*C

 

In the tubes is pure Ammonia (R-717 refrigerant) which enters as a liquid and leaves as a gas.

 

I am very confused by the phase-change in the Ammonia.. do I use the heat of vaporization to find the Heat generation (Q_dot). (With this equation: Q_dot = m_dot * h_fg)?

 

You could if you knew that 100% of the ammonia mass flow evaporated but was not super heated so that only latent heat is considered.

 

Total heat may be easier to measure from the oil side using heat capacity mass flow and temperature drop. If you can measure accurately, either way works, your formula or the oil side.

 

I have been attempting to use this formula to find the efficiency:

 

Q_dot = e * C_min * (T_hot_in - T_cold_in)

 

with e being the efficiency.

 

Am I on the right track at all??

 

That formula looks correct except you are missing mass flow of the oil. It is because Q_dot is the actual energy transferred and C_min*(T_h - T_c) [* m_oil] (don't forget the mass flow rate) is the theoretical maximum energy transfer possible. Efficiency being the ratio of actual to maximum possible.

[That_guy]

I think the missing mass flow rate might just do it... I will have to check on Monday and repost here depending on my results.

 

Thanks!

[cypress]

Your welcome. Good luck.