Differance Of Opinion

[SIGNSTU]

:) I HAVE ALWAYS BEEN TOLD WITH AN EVAP. CONDENCER YOU SHOULD ALWAYS RUN THE WATER FIRST THEN THE FANS AND THAT MAKES SENCE. I WAS JUST TOLD BY ANOTHER ENGINEER FAN FIRST THEN WATER.,IAM ALMOST SURE THAT IS WRONG. ANY HELP ON THIS?

[aawood1]

Hi, I like water first and the fans start on a presure switch at about 8.5 bar. on Ammonia systems. All the systems that I know about and have worked on work this way around.
All the best Arthur.

[US Iceman]

Water first & fans last...

The majority of the condenser capacity is created by the water flow. The fans add only just a bit of capacity.

If you turn on the fans first, the condensing temperature can raise fairly quick. Then if the water pumps start, the discharge pressure drops quickly. Then the water pump shuts off and the coil dries off. Very soon the water pumps start again and the coil is wetted once more. This results in very rapid scale buildup on the coils if the water is not perfect and results in very poor control of the discharge pressure.

If the water starts first the discharge pressure tends to stabalize unless the entering wet bulb temperature increases. In cold weather you have a steam plume that some people object to, but I rarely hear complaints on this point.

[NH3LVR]

The majority of the condenser capacity is created by the water flow. The fans add only just a bit of capacity.

Here I have to (gently) disagree with my esteemed colleague.

When a Evap Condenser is operated without water or fans, the only condensing is done by convection. Not very effective.
With the water on and no fans, the water temperature rises quite quickly. There is some evaporation (evidenced by the steam plume), but the water temperature rises.
With the fans on the water temperature is reduced, and the air and water moving over the coil forces condensing below dry bulb.
I agree with the water first. I have a customer who insists in cycling the pump. As I was leaving the last time he was acid washing the condenser. In fact I think that except in cases of very disproportionately sized components or very cold climates, the water pump should NOT be cycled.
My disagreement is with the contribution of the fans. I believe they play a major role in the efficient of the condenser. However this is from experience. I have no hard figures to back that up.

[US Iceman]

Here I have to (gently) disagree with my esteemed colleague.


Ouch! :D

I agree with what you said, but I'm not changing my mind.:rolleyes: Let me explain.

My thoughts and comments were based on the condenser operation in the cooler weather which is where we see fan (& pump) cycling occur. In cooler weather with the warm(er) water flowing over the condenser coils the natural convection currents are pretty high.

Which is why you can see systems operate at stable discharge pressures in cool weather without the fans.

What I was thinking of is the system reaction to pumps starting or fans starting to control discharge pressure. In the winter, starting the fans tends to bring the discharge pressure down slowly. This of course depends on how many fans you start too. However, if you start the pumps (with the fans off) the discharge pressure drops rapidly.

This is where my comment was drawn from about the fans having less capacity/impact than the pumps. I did not explain myself too well the first time I guess.:o

When the load or entering wet bulb temperature begin to increase this mode of operation (fans only, no pumps) tends to be insufficient to prevent the discharge pressures from raising. This is where the fan operation comes into play. The fans simply provide forced convection currents so that the high humidity air in the condenser box is removed and replaced with lower humidity air.

I hope I clearned that up (with egg on my face).;)

[NH3LVR]

Ouch! :D (with egg on my face).;)

Do not worry about egg on the face. (I have not had any on mine since last Thursday)

[Lc_shi]

Water pump and fan start at the same time,and then start compressor after a designated seconds. No need to separate them. It's my understanding:)

[US Iceman]

Water pump and fan start at the same time,and then start compressor after a designated seconds. No need to separate them. It's my understanding:)


If the normal operation occurs in the summer or moderate weather I think this is OK to do. If you start the pumps and fans together during the winter, you will have big problems.:eek: