Operating Evaporative-Condensers in Extreme Winter Conditions.

Dear RE-Forumers,

Appreciate you advise on the following matter. Where I am the temperatures in winter range from -20 deg C to -40 deg C. I am now trying to optimise the settings for summer/winter operations on our BAC evaporative-condenser, in order not to get too high discharge pressure on our NH3 compressors. We have the condenser outside, with a remote sump (no heating).

There are 2 methods of cooling - using water and air (pump and fan). And also two modes of cooling:
a. summer (water spray first, then fan)
b. winter (fan only)
For summer mode, pressure to turn on fan is 10 bar, while winter mode is 8 bar.

I have currently set the switching from "summer" to winter mode to -28 deg C. I noticed that above -28 deg C, that temperature the evap-cond cannot cope sufficiently to liquify the NH3 gas.

What I'd like to know, learning from your experience:
1. at what lowest temperature can water be used to spray onto the evaporative condenser? Any possibility of condenser or water pipes freezing? Our water pipe diameter is DN80.
2. in extreme winter conditions, -25 deg C, what is the practice of running the evaporative condenser - water spray first, then fan; or fan first, then water if the pressure escalates?

Appreciate any other advice on this subject matter.

You will have to determine control set points for your application by trial and error.

It's not a good idea to add spray water with the fan on. CFM gets high in the absence of water. Adding water with the fan running will cause water to spit out the top for 10-15 seconds. In summer it's a mess; In winter, instant ice formation on eliminators, etc..

Maybe a speed drive on the fan would allow you to operate 'wet' all the time.

For future purchases, BAC offers fins on the bottom few rows of the coil. This makes for an efficient air cooled condenser. You'd still switch to wet operation but at a much higher temperature.

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Originally Posted by Don Davis

For future purchases, BAC offers fins on the bottom few rows of the coil.

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I disagree. The use of secondary heat transfer surface such as fins on an evaporative condenser coil would make water quality a critical chore. It's hard enough to keep condenser coils clean without adding other hard to clean surfaces.

Fins can be spaced far apart- Not like an air cooled condenser.

Yes, if you generate a lot of scale, fins could add to your problems but doesn't it all come back to good water treatment?

On that I agree, it all comes down to water treatment. My bias is showing a little on this, I have just seen too many condensers where the water treatment is not adequate. So from my perspective, why tempt fate?

stepperider,

The majority of the condenser capacity occurs when the coils are wet. If you have more heat rejection than the condenser can accommodate in dry operation, your only option is run the water until the air temperature is low enough to reject sufficient heat.

If you operate the condenser dry with fans only and it does not remove enough heat, the discharge pressure may rise too high.

For dry operation the fan motor may pull too many amps for two reasons; 1) the cold air is more dense than warm air. This can increase the power input requirements. 2) Without water running on the coil the static pressure the fan works against is much lower. This also raises the power input.

You might try leaving the pump and fan on. Then when the discharge pressure begins to drop too low either shut the fan off (or cycle it or use a variable frequency drive to vary the fan speed).

As the air temperature becomes even more cold, shut the fan off completely. The condenser with water flowing on the coils can provide a lot heat rejection in cold weather. Yes, some ice will form in some locations but if the heat rejection is great enough this will help to minimize the ice growth also.

In very cold weather, the fans only should be able to provide sufficient heat rejection when the coils are dry.

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Originally Posted by stepperider

Dear RE-Forumers,

Appreciate you advise on the following matter. Where I am the temperatures in winter range from -20 deg C to -40 deg C. I am now trying to optimise the settings for summer/winter operations on our BAC evaporative-condenser, in order not to get too high discharge pressure on our NH3 compressors. We have the condenser outside, with a remote sump (no heating).

There are 2 methods of cooling - using water and air (pump and fan). And also two modes of cooling:
a. summer (water spray first, then fan)
b. winter (fan only)
For summer mode, pressure to turn on fan is 10 bar, while winter mode is 8 bar.

I have currently set the switching from "summer" to winter mode to -28 deg C. I noticed that above -28 deg C, that temperature the evap-cond cannot cope sufficiently to liquify the NH3 gas.

What I'd like to know, learning from your experience:
1. at what lowest temperature can water be used to spray onto the evaporative condenser? Any possibility of condenser or water pipes freezing? Our water pipe diameter is DN80.
2. in extreme winter conditions, -25 deg C, what is the practice of running the evaporative condenser - water spray first, then fan; or fan first, then water if the pressure escalates?

Appreciate any other advice on this subject matter.

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Can you give more information about your plant?
How many condensers do you have? That type(axial, centrifugal) of condenser fans do you have? What is the goal of you optimization(energy, better operation...)? Is this production facility or cold storage?

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Originally Posted by stepperider

Dear RE-Forumers,

Appreciate you advise on the following matter. Where I am the temperatures in winter range from -20 deg C to -40 deg C. I am now trying to optimise the settings for summer/winter operations on our BAC evaporative-condenser, in order not to get too high discharge pressure on our NH3 compressors. We have the condenser outside, with a remote sump (no heating).

There are 2 methods of cooling - using water and air (pump and fan). And also two modes of cooling:
a. summer (water spray first, then fan)
b. winter (fan only)
For summer mode, pressure to turn on fan is 10 bar, while winter mode is 8 bar.

I have currently set the switching from "summer" to winter mode to -28 deg C. I noticed that above -28 deg C, that temperature the evap-cond cannot cope sufficiently to liquify the NH3 gas.

What I'd like to know, learning from your experience:
1. at what lowest temperature can water be used to spray onto the evaporative condenser? Any possibility of condenser or water pipes freezing? Our water pipe diameter is DN80.
2. in extreme winter conditions, -25 deg C, what is the practice of running the evaporative condenser - water spray first, then fan; or fan first, then water if the pressure escalates?

Appreciate any other advice on this subject matter.

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Hi Stepperider:)

not something we have much experience of.

The coldest I rember was -13 deg c. We are having a cold snap now, it's as cold as -4 deg c.

In our ambients we keep the pump going and speed drive the fan.

I know that your supposed to keep the tubes wetted to reduce corrosion.

I would suggest a smaller motor that cuts in low ambients and a drive on this fan motor. It may also be necessary to speed control the pump, using a two speed motor.

Kind Regards Andy:)

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Originally Posted by Andy

It may also be necessary to speed control the pump, using a two speed motor.

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I would suggest some caution in doing this Andy. If the water pump flow varies sufficiently to lower the wetting of the condenser coils you could inadvertently cause partial coil wetting. That does contribute to increased scale formation and/or corrosion.

Having said that, it may be possible to slow the pump down from 100% speed to something like 80-90% speed without affecting the water volume flow too much.

It depends on the surplus pump capacity and where the pump curve falls with the system curve of the condenser water piping and nozzles.

I agree with Mike. All condenser surface should be wetted. Certain nozzle inlet pressure(10psig) should be kept to wet all surface. Partial wetting create opportunity for scale formation.
I just don't understand the reason to have a target condensing pressures like 8 bar for winter and 10 bar for summer. At -30C capacity of air cooled condenser will be close to water cooled at design conditions. However, winter refrigeration load is lower than summer refrigeration load. Probably, at -25C he doesn't need water.

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Originally Posted by Segei

I agree with Mike. All condenser surface should be wetted. Certain nozzle inlet pressure(10psig) should be kept to wet all surface. Partial wetting create opportunity for scale formation.
I just don't understand the reason to have a target condensing pressures like 8 bar for winter and 10 bar for summer. At -30C capacity of air cooled condenser will be close to water cooled at design conditions. However, winter refrigeration load is lower than summer refrigeration load. Probably, at -25C he doesn't need water.

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Surely if the condenser is being run dry this will aid corrosion also.

Kind Regards Andy:)

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Originally Posted by Andy

Surely if the condenser is being run dry this will aid corrosion also.

Kind Regards Andy:)

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I'm a little bit skeptical about corrosion at described conditions. For good corrosion we need oxygen, moisture and temperature. -25C outside won't help to corrosion..

If the coils are hot-dipped galvanized or stainless steel being dry at low ambient temperatures should not hurt them at all.

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Originally Posted by US Iceman

If the coils are hot-dipped galvanized or stainless steel being dry at low ambient temperatures should not hurt them at all.

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Originally Posted by Segei

Can you give more information about your plant?
How many condensers do you have? That type(axial, centrifugal) of condenser fans do you have? What is the goal of you optimization(energy, better operation...)? Is this production facility or cold storage?

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Dear all,

Thanks for your input.

Segei: We have an (ONE) evaporative condenser, BAC type VXC-125 - Centrifugal fan, forced draft. The ultimate goal is of course energy conservation (from running compressor at lower discharge pressures) and also to know the correct way to operate the evaporative condenser in extreme winter conditions. (I am from the tropics). We are a brewery in Ulaanbaatar, Mongolia.

Andy/Iceman: Our coils are hot dipped galvanised, so no problems.

The control of the condenser in dry (fan only) or wet (pump + fan) mode is based on the the outside air temperature. We can set the temperature where it changes automatically.

Actually, I am trying to find the right set point for dry/wet operations during extreme winter conditions (lowest -40 deg C), at the lowest discharge pressure for our two compressors at full load, without damaging the evaporative condenser water piping/nozzels. Goal is beating the previous month's electricity consumption by taking advantage of natural environment's cooling.

What I learned from you all so far, and still questioning myself and experimenting:
1. cooling the coils using cold air uses a lot of power (fan motor is 12.5kW).
2. always run on water pump first (pump motor is 3kW)
3. when the temperature of the outside air drops, continue to use water to cool the coils
4. is there a "critical temperature" where continuing to use water to cool the coils can damage the water piping?
5. what is the reason for the setting that the fan turning on at 8-bar at dry mode, and 10 bar at wet mode? should I set the fan to turn on at a higher pressure? (compressor kW vs fan kW).

What I have done is now to set the setting for wet/dry operations at -28 deg C. At wet operations mode (summer), the pump is taking the lead to spray water in the evaporative condenser. I see discharge pressure at about 9.0-10.5 bars on one compressor full load (depending on outside air temperature).

When the discharge pressure goes above 10 bars, the fans turns on, and we have combined cooling from water/fan. Then I see discharge pressure at around 8.5 bars on one compressor full load, and 10-11 bars on two compressors full load (depending on the outside air temperature).

I have checked the nozzels, and so far no damages. Also check the drift eliminators, also no freezing damage. Only some ice/flakes formation at the edge. The coils also not frozen - hot discharge gas always melts the ice formed, if any.

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Originally Posted by stepperider

When the discharge pressure goes above 10 bars, the fans turns on, and we have combined cooling from water/fan. Then I see discharge pressure at around 8.5 bars on one compressor full load, and 10-11 bars on two compressors full load (depending on the outside air temperature).

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One other item you need to be careful of is; at what discharge pressure will the pressure be low enough to cause problems with feeding liquid refrigerant. If the liquid feed pressure drops too low, the pressure will not be high enough to push liquid through the expansion valves (hand or thermostatic).

You need to find out what this pressure is and balance it with the desire to save energy.

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Originally Posted by stepperider

Dear all,

Thanks for your input.

Segei: We have an (ONE) evaporative condenser, BAC type VXC-125 - Centrifugal fan, forced draft. The ultimate goal is of course energy conservation (from running compressor at lower discharge pressures) and also to know the correct way to operate the evaporative condenser in extreme winter conditions. (I am from the tropics). We are a brewery in Ulaanbaatar, Mongolia.

Andy/Iceman: Our coils are hot dipped galvanised, so no problems.

The control of the condenser in dry (fan only) or wet (pump + fan) mode is based on the the outside air temperature. We can set the temperature where it changes automatically.

Actually, I am trying to find the right set point for dry/wet operations during extreme winter conditions (lowest -40 deg C), at the lowest discharge pressure for our two compressors at full load, without damaging the evaporative condenser water piping/nozzels. Goal is beating the previous month's electricity consumption by taking advantage of natural environment's cooling.

What I learned from you all so far, and still questioning myself and experimenting:
1. cooling the coils using cold air uses a lot of power (fan motor is 12.5kW).
2. always run on water pump first (pump motor is 3kW)
3. when the temperature of the outside air drops, continue to use water to cool the coils
4. is there a "critical temperature" where continuing to use water to cool the coils can damage the water piping?
5. what is the reason for the setting that the fan turning on at 8-bar at dry mode, and 10 bar at wet mode? should I set the fan to turn on at a higher pressure? (compressor kW vs fan kW).

What I have done is now to set the setting for wet/dry operations at -28 deg C. At wet operations mode (summer), the pump is taking the lead to spray water in the evaporative condenser. I see discharge pressure at about 9.0-10.5 bars on one compressor full load (depending on outside air temperature).

When the discharge pressure goes above 10 bars, the fans turns on, and we have combined cooling from water/fan. Then I see discharge pressure at around 8.5 bars on one compressor full load, and 10-11 bars on two compressors full load (depending on the outside air temperature).

I have checked the nozzels, and so far no damages. Also check the drift eliminators, also no freezing damage. Only some ice/flakes formation at the edge. The coils also not frozen - hot discharge gas always melts the ice formed, if any.

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To save energy, refrigeration plant should operate at optimum discharge pressure. At this pressure total power use(compressors+condensers) is minimum. In winter time optimum discharge pressure is lower than 5 bars. It means that the lower discharge pressure will give you better efficiency(energy savings). Try gradually step by step to lower discharge pressure to 5 bars. Performance of metering devices(TXVs, expansion valves...) can be the barrier to low discharge pressure. However, every barrier has a solution. For your refrigeration plant wet operation(pump first) is always better than dry operation. Water pump should run nonstop and fan should cycle. Just check eliminators. Water pipes and nozzles won't freezer because water temperature will be higher than 0C(10C-20C).

I run my BAC evaporative condensers with a water pump thermosat. when the air temperature falls below 25 f the water pumps lock out until the air temp rises above 28 f so far I have not had any ice related issues.

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Originally Posted by chilldis

I run my BAC evaporative condensers with a water pump thermosat. when the air temperature falls below 25 f the water pumps lock out until the air temp rises above 28 f so far I have not had any ice related issues.

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One issue is the ice formation, another issue is efficiency. Usually, it is more efficient to run water pump as long as possible

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Originally Posted by chilldis

... when the air temperature falls below 25 f the water pumps lock out until the air temp rises above 28 f so far I have not had any ice related issues.

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I believe it is better to have the pumps ON and the fans OFF in the temperatures described above. These are not extreme temperatures by any means.

I would not cycle the pumps for head pressure control especially in these moderate or warmer temperatures as you risk increased scale formation on the condenser coil.

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Originally Posted by US Iceman

I believe it is better to have the pumps ON and the fans OFF in the temperatures described above. These are not extreme temperatures by any means.

I would not cycle the pumps for head pressure control especially in these moderate or warmer temperatures as you risk increased scale formation on the condenser coil.

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Sometimes, axial fan blades can be frozen at sub zero temperatures.

Comming from a very cold climate I can tell you that in the dead of winter with temperatures hovering at -30C during the weekend when there is very little demand on the system the fans actually shut off and condenser pressure sometimes drops to 70 psig. We use fans first, pumps second with the pumps not comming on until we get over 110 psig and the fans shut off at 80 psig and don't come back on till we have 90 psig. The fans are on VFDs. In real cold climates a small water leak in your condenser becomes a giant ball of ice on your roof pdq.