Understanding the trade-off between compressor work and condenser work

Hi everyone,

I hope you guys don't mind me posting a question but I've been reading articles on the forum for a while and am still struggling with this issue.

Our system:

The compressor (twin screw ammonia) suction pressure is determined by the efficiency of the evaporator, but the the discharge pressure is mainly controlled by the evaporative condensers.

For a given heat load (in our system, approximately 2MW), increasing the fan speed on the evaporative condensers reduces the discharge pressure and therefore the required compressor power. WHY?

Or perhaps a better question; If an evaporative condenser has a given heat dissipation value for a constant wet bulb temperature, what does the extra fan energy achieve? Is it increasing the amount of sub-cooling in the ammonia?

I hope this isn't too much of a stupid question. I understand the rest of the system, but this bit still causes me problems.

Thank you very much in advance!

Kind regards,

Simon

Keeping it simple and basic.
Think of the compressor as something that is lifting a heavy weight, The faster and higher the weight you have to lift, the more power you need to do it. So your suction is the start and you discharge is the height, so lowing the discharge pressure, means you are not lifting as high so less power is required.

You evap cond, will be designed for one condition, if the design condition changes so does the performance of the evap cond, normally actual design conditions, are normally ever hit for a few hours a year.
As long as the fan power is less than the compressor savings and the systems as a whole remains reliable then leaving the fan on is a good thing.

Is the ammonia sub cooled further, if you have a liquid receiver, then no, but if it is flooded maybe

dont know much about R717 but i would expect you would get the same reaction as any other system the closer the suction pressure and discharge pressure are together the less the amp draw from the compressor hence less load = less power required.

Thank you very much for your comments. I understand the importance of getting the suction pressure as high as possible for reducing compressor load and we're achieving this by renewing our evaporators.

Its the mechanics of the condenser i'm really curious about.

If I have a system which generates a 10Bar discharge pressure (gauge) with two evaporative condensers running their fans at low speed (heavily oversized). Why does the discharge pressure reduce if I run the fans at full speed?

If the mass flow of ammonia in the system remains constant, theoretically, the heat extracted from the ammonia during the condensation process remains constant? So what does running the fans harder actually do the ammonia gas to cause the pressure reduction?

Sorry to keep going on about the same point and thank you very much for your help.

Kind regards,

Simon

You're basically removing the heat faster from the Evaporative condenser by increasing the fan speed....to simple a reply?

Keeping it simple! (no factual figures)

Dry coil first
lets 1000Kw at 10Bar, with 1000M3 of air which heated by 10C
So if we double the air to 2000M3 it will only need to be heated by 5C.
Because the mechanics of the heat exchanger have NOT changed, the 10Bar will fall to meet the new equilibrium. Where the air will be heated between 5-10C

Evap Cond, more air can absorb more water (vapour), or the same amount of water more easily.

This is optimization of the refrigeration plant operation. It is complicated issue, but I'll try to keep it simple.
Assume that initial head pressure is 10 bars. You increased fan speed and spent additional 10 KW of condenser(fan) power. Head pressure was reduced to 9.5 bars. Now your compressors use less energy. Question is how much less? If your compressor power reduced by 20 KW, you are going to the right direction. If your compressor power reduced by 5 Kw, you are going to the wrong direction. The goal is to keep total(compressors+condensers) power at minimum level. Usually, sophisticated PLCs have wet bulb approach feature(evaporative condensers) to control balance between compressor and condenser capacities to keep total energy use at minimum level.
On the suction side it will more complicated. Higher suction pressure will improve compressor efficiency, but it will increase evaporator fan power. This additional evaporator fan energy will be released in refrigerated space as parasitic refrigeration load. However, at certain optimum(not maximum) suction pressure efficiency of the refrigeration plant will be maximum.