Optimal pressure drop in condensers and evaporators

[MrReds]

hallo to everybody !
Please, I'd like to ask you if you know which is the optimal pressure drop in an
a) air cooled condenser
b) air cooler

with different refrigerants.
Question is: do the optimal pressure drop changes as funcion of refrigerant?
Ex: optimal pressure drop for R404a is ?and for R410A is ?

thanks in advance

Mr. Reds

[RapidRun]

Do you want optimum temperature drop or optimum efficiency (COP)

I think you should research entrophy and COP graphs/charts

On the R404a units I am familiar with I typically see pressures of 1.5 - 2 bar (relative to atmosphere) suction pressure and 15 - 17 bar discharge.

[RapidRun]

sorry. enthalpy not entrophy

[MrReds]

Many thanks Mr. Rapid Run.
Nevertheless my question referred to the sole condenser and evaporator.
Do you have any figure concerning pressure drop ?
Maybe you have to obtain same temperature drop for each refrigerant ? therefore, which is this optimal temperature drop in
a) condensation
b) evaporation ?

Thanks in advance

[Brian_UK]

Ideally the optimal pressure drop would be zero.

Think in temperature and the changes that will have on the refrigerant.

[US Iceman]

I think Brian hit it right. The optimal pressure drop would be based on the impact of the pressure drop/temperature loss on the coil performance. I don't think you can use only one value as far as pressure is concerned.

For every refrigerant there is a different vapor/pressure curve. The pressure loss results in an equivalent temperature loss. This varies as the evaporating pressure changes.

I would suggest you think in terms of temperature as Brian suggested. Let's say you wanted to limit the temperature loss, so that no negative impact to the heat transfer performance is exceeded.

At -40C the equivalent pressure loss would be different, than if the coil was operating at say +5C.

[MrReds]

Many thanks to everybody.
I agree to think in terms of temperature drop.
I don't agree that ideal pressure drop should be 0.
In this case velocity of fluid would be practically 0, so that I won't be in turbolent flow, but laminar one, also if we are in a phase of state change (evaporation of condensation).
May I ask you what do you think about it ?

[Erik Detroit]

I agree with MrReds, there is an optimum pressure drop. Unfortunately I think it's such a complicated subject that there is no single answer like "0.56 bar" (wish there was)

Pressure drop in a heat exchanger implies turbulence, better heat transfer, better mixing (for evaps). If you do a real world test you will find that a higher pressure drop heat exchanger can outperform a lower pressure drop. It's a balance between the negative effects of temperature change with pressure drop, and the positive effects of turbulence. Clearly there is an optimum somewhere in such a system.

I used to do R&D engineering on automotive evaporators, and there was always an optimum pressure drop for each evap design and mass flow rate, but that optimum changed with different conditions, different evaporators, etc. It must surely be different with different refrigerants also. That's one of the main lessons I took from that work. The more you dig into the fine details, the more complicated things are.

[MrReds]

Dear Mr. Erik,

I'd like to thank you for you attention.
Please, could you suggest me some figures (with R134a, I suppose), or some literature concerning this issue ?

thanks in advance