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Cooling Tower Conceptual Design Issue

11/29/2010 2:34 PM

Need some advice.

We have an 800 ton condenser water system with redundant twin 800 cooling towers that serves as the heat rejection mechanism for our 800 ton chilled water system. The fans on the towers are VFD controlled and I can operate on just one tower, rolling over to the second if I experience a failure with the first tower. As temperatures get colder, the VFDs slow down the fan or fans to achieve the desired condenser water temperature.

When it gets real cold outside, the fans shut off completely and an automatic bypass valve opens to shunt or bypass the cooling towers, thereby allowing the condenser water to increase back to set point.

Over the past two years, we have experienced 4 failures of the bypass valve and on each occasion the temperature of the condenser water system has increased to the point where the chillers started surging and eventually had to shutdown. The failures of the valve have varied, but the net result has always been the same. On one occasion the servo motor burned out because of heavy drag on the valve. We remedy the drag issue and replaced the actuator (actually increase the torque on the actuator while we at.) On one of the other occasions, the actuator simply got stuck and we were able to cycle it and get it to start working. The other two occasions were due to a board failure. Once the board was replaced, the actuator and valve resumed normal operation.

I am questioning if this is the best design to accomplish the intended function of raising the condenser water temperature and would like to hear your comments and possible alternate design suggestions.

Other pieces of information that might be useful:

* The control valve is a 12" butterfly valve. The valve and actuator are made by Bray Controls (who have a very strong reputation.) We have at least 10 other Bray Control valves on this and other system and none of the them have given us any trouble.

* The valve and actuator are installed outdoors, underneath the cooling towers. The actuator can does have a small heating element inside which is supposed to help prevent condensation. I though about relocating the control valve indoors, but this is not feasible/possible due to the location of the piping and the condenser water pumps (pumps are located outdoors.) I would have to run a 12" line from inside the building to connect to the condenser water pump inlet header (about 75 feet.)

* The most recent failure happened when the temp had dropped to about 25 degrees outside.

* In each failure, the valve has failed open or partially open - which leads to the condenser water not being able to reach the tops of the towers.

I would appreciate any advice on component, configuration or any other helpful suggestions that you might have. Let me know if there are any other pieces of information that you need too.

Thanks.

Regards,

Paul

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#1

Re: Cooling Tower Conceptual Design Issue

11/29/2010 3:16 PM

If the primary difference between this valve and the other ten valves that have had no problems, is that it's outside and exposed to the elements, and moving the valve indoors is not feasible, why not build an insulated, weather tight, heated shed around that particular valve and actuator? Just build it a little home.

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#2
In reply to #1

Re: Cooling Tower Conceptual Design Issue

11/29/2010 3:35 PM

The thought has crossed my mind and will be considered. I am just curious if there is a different arrangement out there that is commonly used on cooling towers.

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#3

Re: Cooling Tower Conceptual Design Issue

11/29/2010 4:51 PM

Varying fan speeds and using a tower bypass is very typical in my experience. I've seen butterfly, globe, and plug valves used on the bypass line. From what I remember, there seems to be little difference in terms of reliability (I might lean a bit to plug valves).

That would lead me to agree with Kramarat. If you have success with a certain brand of actuator and the environment seems to be the factor, try to control that (via insulation and heat tracing, a small enclosure, etc).

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#4
In reply to #3

Re: Cooling Tower Conceptual Design Issue

11/29/2010 5:07 PM

In your experience, do you typically see a full line bypass installed? Or is the bypass line typically reduced in size (when compared to the header)?

I suspect one of the main criteria for selecting the bypass line and bypass valve is that teh pressure drop must be less than the head required to lift water to the top of the tower.

In your experience, do you see bypass valves attempting to modulate flow or just function in an on/off capacity?

Also, what type of plug valves have you used? As I mentioned, one of our failures was due to drag on the valve. The drag was due to corrosion on our piping which had built up to the point where it inhibited the valve from closing - eventually leading the servo motor to burn itself out. (We fixed the corrosion issue - firing our water treatment specialist and getting someone who knew what they were doing.) A plug valve would eliminate this failure mode from occuring in the future (assuming it was stainless body with stainless trim and internals.) Nice suggestion.

Sorry to pepper you with questions, but I have little experience with cooling tower design.

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#5
In reply to #4

Re: Cooling Tower Conceptual Design Issue

11/29/2010 6:07 PM

I should mention my experience with them is from an O&M standpoint. I haven't designed....

In your experience, do you typically see a full line bypass installed? Or is the bypass line typically reduced in size (when compared to the header)?

I suspect one of the main criteria for selecting the bypass line and bypass valve is that teh pressure drop must be less than the head required to lift water to the top of the tower.

It has always been reduced size. I assume for the reason you mention (to generate a bit more headloss so the pumps are still happy).

In your experience, do you see bypass valves attempting to modulate flow or just function in an on/off capacity?

All the ones I have operated were modulating.

Also, what type of plug valves have you used?

I'm going from memory here... that was the one we didn't have issues with and had almost no maintenance... the only thing I remember about it was that it was an "eccentric" type. I don't remember the brand...sorry.

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#6

Re: Cooling Tower Conceptual Design Issue

11/29/2010 11:46 PM

How frequently do you clean your CTs?. When large quality of air moves through a small space, these towers wash the air and soon the bottom of the basin gets covered with dirt and sledge. Therefore these systems need more frequent flushing and cleaning. Also cheek the formation of ALGAE. Algae blocks the circulation of water by blocking filters and nozzles.

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#7
In reply to #6

Re: Cooling Tower Conceptual Design Issue

11/30/2010 8:39 AM

We clean 1 to 2 times per year.

Our most recent clean was 2 months ago.

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#8

Re: Cooling Tower Conceptual Design Issue

11/30/2010 8:51 AM

They are all good suggestions. Based upon my experience Kramarat has the most logical solution.

I tend to be brand loyal, if I have a good history with a product. Then something happens and all of a sudden that particular product starts giving problems that you never used to have. The design may be the same but the supplier of the components may have changed. The usual suspect is China. Go out and look at high end PC Mother Boards. The one thing they stress in bold on the box is, "100% Japanese made solid capacitors". That says it all.

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#9
In reply to #8

Re: Cooling Tower Conceptual Design Issue

11/30/2010 9:41 AM

UPDATE:

We also have two additional Bray control valves installed outside that divert water flow to the towers. These are on/off control, whereas the bypass valve is variable control. We have not experienced a failure on these valves.

I thought I might add this as the concensus so far is toward a heated enclosure.

The other two valves have control boards and servo motors like the bypass valve.

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#10

Re: Cooling Tower Conceptual Design Issue

11/30/2010 10:07 AM

Excersize the valve(s) once or twice per week. Everything else seems typical.

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#11

Re: Cooling Tower Conceptual Design Issue

11/30/2010 10:26 AM

has the heating element been checked if functioning properly?

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#12

Re: Cooling Tower Conceptual Design Issue

11/30/2010 10:30 AM

I tend to agree with kramarat and Chaotic. Heat trace the control valve and actuator, and cover both with insulation. Use a temp regulating heat trace to maintain 80 or 90 deg F.

If the actuator is pneumatic, the other possibility is the air line has water that is freezing and pocking the control valve. In this case the entire air line needs to be heat traced.

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#13

Re: Cooling Tower Conceptual Design Issue

11/30/2010 10:50 AM

You mention that your valves by-pass to maintain the condenser water set point.

Is there any reason that the chillers cannot see water cooler than, say 85F? Generally, there will be an increased efficiency (lower kW per ton) with cooler water.

Additionally, as the entering water temperature falls (because of lower entering water to the chiller and possibly lower load on the chiller) the ability of the tower (without fan assist) to drop the water temperature falls as well.

I would recommend that you check with your chiller supplier and see how low the condenser water can go without causing any problems. If there is no "reasonable" lower limit, forget the bypass valves and just feed water through the towers, getting whatever temp output is available.

Additionally, as the ambient temperature falls, the ability of the ambient air to accept reject heat also falls because the moisture-carrying capacity of the air falls and cooling towers work by evaporating water into the air stream. If you cannot evaporate water into the air, you cannot reject heat to cool the incoming water. The incoming water "warms" the air initially, before the evaporation cools it down so cooler incoming water will also reduce tower "no-fan" capacity. If necessary, you can still fully by-pass one of the towers.

As another issue- you should operate BOTH of your fan-powered towers whenever you are running the chillers. The VFD will slow the fans down as needed to meet the load, but- even at full load, rather than running one likely 60 HP fan motor, you will only need two motors running at 50% speed (fan speed equals water flow percentage) so each motor will only be running at about 8 HP (HP = full HP times speed ration cubed, so 1/8th of full HP) or 16 HP to do 60HP worth of work.

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#14
In reply to #13

Re: Cooling Tower Conceptual Design Issue

11/30/2010 11:12 AM

Good points regarding a lower operating temperature and increased efficiency with said lower cooling water temps. There is quite often very drastic efficiency improvements with lower sink temperatures (assuming the equipment is equipped to handle it).

I would politely disagree with a lower performance during cooler weather. I don't think you can generalize performance to a season. In the geographical areas I worked the cooler, denser, drier autumn/winter (compared to the spring/summer) air provided an enormous increase in the capacity of the towers... hence the need for the tower bypass line. Our difference in opinion on this would suggest the capacities are simply affected by the air qualities (temperature and humidity) and how these vary throughout the year.

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#15
In reply to #14

Re: Cooling Tower Conceptual Design Issue

11/30/2010 11:08 PM

The other caution is if freezing temperatures occur, the cooling tower could become a large ice cube. This will cause structural damage within the cooling tower as well as interupt cold water flow.

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#17
In reply to #14

Re: Cooling Tower Conceptual Design Issue

12/01/2010 12:29 PM

You are correct for those "special" conditions with both cool AND (especially) low humidity air.

My comment was based on the ability of air in much of the US where cooler usually means moderate humidity (40% to 70%) and the limitation where the air can only accept a relatively small amount of water before becoming nearly saturated (90% or higher humidity) compared to "design" conditions where 95FDB and 78FWB is also about 50% RH but the 95F air is able to absorb substantial amounts of water before it gets near saturated.

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#16

Re: Cooling Tower Conceptual Design Issue

12/01/2010 12:06 PM

It sounds like your bypass valve directs tower inlet water directly into the suction line. This imposes severe duty to the valve. Think of it... Extremely high pressure on one side and extremely low pressure on the other. Erosion and wire drawing should cause early valve failure. Flow through the pump should increase dramatically as the valve is cracked open. I'm suprised the pump doesn't overload.

A cooling tower bypass should not be of the modulating veriety and should always direct water into the cold water basin. I found http://spxcooling.com/pdf/h-0038.pdf to confirm this recommendation.

Good luck.

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#18
In reply to #16

Re: Cooling Tower Conceptual Design Issue

12/01/2010 1:20 PM

I was interested in reading the link you posted, but it seems to be broken and I can't locate said file on their website. Would you mind reposting it?

The towers I operated all operated in a similar fashion. The return water line which directed to flow through the tower had a bypass line with a modulating valve. Once the fans were off and the basin temperature continued to decrease, the bypass valve was set to automatic and throttled open to divert water from the tower riser directly to the basin. The bypass was under automatic control using the basin temp as the set point reference. (Hence my interest in reading their recommendation since one of them was Marley brand.)

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#21
In reply to #16

Re: Cooling Tower Conceptual Design Issue

12/01/2010 8:53 PM

Hey Don,

That is exactly how our set up is configured. The bypass valve flows directly into the suction head for the pumps.

Now, except for one occation, it doesn't appear that the valve is failing - rather it is teh actuaor that is failing. The last occation it was the control board that failed. Specifically, the board de-potted. We could press down on the board and it would start to opperate.

Would what you are saying still hold true for the board?

Thanks by the way.

Regards,

Paul

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#19

Re: Cooling Tower Conceptual Design Issue

12/01/2010 1:37 PM

Sorry, change the number '8' to capitol "B"

Alternately, Google "Cooling Tower Bypass" and you will find SPX's paper on Winter Operation of Cooling Towers among the first four or five entries.

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#20
In reply to #19

Re: Cooling Tower Conceptual Design Issue

12/01/2010 1:56 PM

Thanks I found it and you're making a good point. We do need to clarify your statement a little though. We need to take the information in the paper in context, it is for operation during freezing weather. In this case, I would fully agree that trickle flows through the tower media with a high freezing potential only leads to destructive blocks of ice.

Our modulation was to maintain our cooling water temp at ~55F. During the cool late autumn and early spring days it was common to have it modulating. But as the temperature dropped into the freezing range it didn't take long for it to modulate fully open.

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#22
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Re: Cooling Tower Conceptual Design Issue

12/02/2010 11:17 AM

To Chaotic Intellect

Don't forget that in addition to freeze protection, towers like a fixed flow in order to maintain even water distribution over the wet deck. This helps fight the rapid accumulation of scale that forms on surfaces that are alternately wet and dry.

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