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Cooling Tower VFD energy savings 1
- Thread starter Jonkatz
- Start date
- Thread starter
- #3
Jonkatz,
In my experience, the capacity of the cooling tower will fall of very sharply with reduction in air flow, so you might end up paying for a VFD that must run nearly full speed all the time. I haven't got my reference information to hand (I'm in the middle of an office move), but I'm sure someone will be able to confirm what I've just said (or enlighten me to the contrary!!).
Regards,
Brian
In my experience, the capacity of the cooling tower will fall of very sharply with reduction in air flow, so you might end up paying for a VFD that must run nearly full speed all the time. I haven't got my reference information to hand (I'm in the middle of an office move), but I'm sure someone will be able to confirm what I've just said (or enlighten me to the contrary!!).
Regards,
Brian
One of my electrical engineering colleague once told me that the best energy conserving device is a switch.
Every cooling tower is designed for a fixed L/G ratio and the characteristic is not always linear.
One point to note is as the chiller motor is of higher HP there will be more savings if you provide cooler water to the condenser. (M/S York did some study on it and unfortunately I lost those papers.Perhaps you may get it if you do a search)
Check this site for design details of cooling tower with some calculator downloads.
I strongly feel starting and stopping the fan within a temperature range is the better option which I follow.
Regards,
Every cooling tower is designed for a fixed L/G ratio and the characteristic is not always linear.
One point to note is as the chiller motor is of higher HP there will be more savings if you provide cooler water to the condenser. (M/S York did some study on it and unfortunately I lost those papers.Perhaps you may get it if you do a search)
Check this site for design details of cooling tower with some calculator downloads.
I strongly feel starting and stopping the fan within a temperature range is the better option which I follow.
Regards,
With multiple towers/fans, I have generally found that staging the fans with respect to common tower leaving water temperature is appropriate. As some have already suggested, more energy is to be saved by running the chillers at the highest possible coefficient of performance, which means keeping incoming condenser water cool. However, most chillers have a low limit in this respect as, if the condenser pressure gets too low, there can be operational problems (a typical figures for vapour compression machines is 16 degrees Celsius).
Regards,
Brian
Regards,
Brian
Jonkatz,
What are your objectives for condenser water control? If you intend to keep a constant setpoint year-round, and don’t mind a fairly wide temperature swing (to avoid short-cycling the motor under certain conditions), then on/off control of the motor is the cheapest, simplest option. The same is true if your chiller will not run during the colder months.
If your chilled water system operates year-round and you want to minimize energy consumption of the entire system, then you need to investigate variable condenser water setpoint control (AKA condenser water reset). The basic premise is that the kw/ton on the chiller drops as you lower the condenser water temperature; in low wet bulb weather, when you can achieve low condenser water temps "cheaply" (ie, with very low air flow), you can achieve significant energy savings by dropping the setpoint. In other words, the additional fan energy consumed when the setpoint is lowered from 80F to 65F in the winter is usually very small compared to the energy saved in the chiller. You’ll have to check out the energy consumption profile of your chiller at different condenser water temperatures to see if condenser water reset control would be a good choice. If it is, then this can be accomplished with multi-speed tower fans, discharge dampers, or VFD fan control. VFDs will do the job best; as far as which scheme is most cost effective, you will need to look at the details of your particular application.
Other than for condenser water reset, a VFD for your tower fan would probably not be justified.
Hope this helps!
---KenRad
What are your objectives for condenser water control? If you intend to keep a constant setpoint year-round, and don’t mind a fairly wide temperature swing (to avoid short-cycling the motor under certain conditions), then on/off control of the motor is the cheapest, simplest option. The same is true if your chiller will not run during the colder months.
If your chilled water system operates year-round and you want to minimize energy consumption of the entire system, then you need to investigate variable condenser water setpoint control (AKA condenser water reset). The basic premise is that the kw/ton on the chiller drops as you lower the condenser water temperature; in low wet bulb weather, when you can achieve low condenser water temps "cheaply" (ie, with very low air flow), you can achieve significant energy savings by dropping the setpoint. In other words, the additional fan energy consumed when the setpoint is lowered from 80F to 65F in the winter is usually very small compared to the energy saved in the chiller. You’ll have to check out the energy consumption profile of your chiller at different condenser water temperatures to see if condenser water reset control would be a good choice. If it is, then this can be accomplished with multi-speed tower fans, discharge dampers, or VFD fan control. VFDs will do the job best; as far as which scheme is most cost effective, you will need to look at the details of your particular application.
Other than for condenser water reset, a VFD for your tower fan would probably not be justified.
Hope this helps!
---KenRad
What is the motor hp of your cooling tower fan?
for a typical 125 tons it would 5 hp motor.(one chiller - one tower)
I would use the VFD to soft start the motor.
You could track the LWT with the drive if you want controls on the tower.
Be sure to get a manual bypass on the VFD so if it fails you can switch to manual control.
for a typical 125 tons it would 5 hp motor.(one chiller - one tower)
I would use the VFD to soft start the motor.
You could track the LWT with the drive if you want controls on the tower.
Be sure to get a manual bypass on the VFD so if it fails you can switch to manual control.
I would suggest to do some research on the wet bulb temperature profile on a whole day/annual basis.Based on the variation wet in bulb temp work out a second operating point for the tower.Check if you can achieve the other duty point with a two speed fan motor.This is better than on off control.
A better approach still is to go in for a multiple cell tower installation.Foe eg a 300 ton tower can consist of 3 100 ton cells.Stage them based on the condenser water temperature you require.This will take care of chiller operation at part loads(assuming the chiller unloads downto 30% of its maximum capacity).All said and done,for best returns in an energy analyis on chilled water plants the focus should be on reducing the chiller motor current either by lowering the condenser water temperature or raising the chilled water temperature.
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- #12
I'm currently using VFD fans (six fans 4 motors) on a 1500 ton R717 unit. We control fan speed off compressor discharge pressure. Original reason for going to VFDs was that the cycling of fans in winter months left only one side of tower running. This created a recycling of air flow through the offline side of the tower.
If the chiller runs a fair amount of time in low ambient conditions which is common than vfd would make sense. If it's strictly for comfort air conditioning and fresh air is used between seasons than a 2 speed tower fan would do.
Most chillers I have seen have tower fans cycled to maintain about 80 degree F. water.
Most chillers I have seen have tower fans cycled to maintain about 80 degree F. water.
Contact Fincor, they can give you the calc for a standard VFD energy curve. In response to some of the input you have been receiving, the VFD ramps according to capacity which is continuosly changing and therfore is the only way to achieve a float pressure within 1-2 pounds (Iwith R-22 you would see significant energy savings keeoping discharge at 140 psig). Also, if you can avoid starting and stopping or high to low ramping you will save your bearings and belts and in turn have fewer repair costs.
buildingmanagement
Mechanical
Instead of using VSD for fans, try using it in the condenser pumps however it is only applicable to reciprocating type. Im enjoying a 12,000kwh per month worth of savings because of this project. Reducing the air flow has the same effect when you control the flow of water.
Changing your cooling towe belts to more energy efficient cog belts or synchronous belts will increase the efficiency of the motor by preventing slippage. These belts are also easier to flex so they use less energy flexing. The normal efficiency of a belt is 92%. The classical abc cogged belt runs at 95-96% and the synchronous belt runs at 98%. Tie this in with a VFD, and you could see savings in the 10 to 30% bracket. The result is from requiring less power to do the same work. See dept. of technology and search v-belts, or go cptbelts.com click energy efficiency.
I know that Marley suggests VFD for cooling towers but can only justify on the basis of savings in mechanical life of gear reducer assembly. Otherwise...I would investigate what the lowest temperature that the chiller can stand on the condenser water side and set an on/off thermostat to cycle the tower fan to give that entering condenser water temperature.
Joe
Joe
You have a lot of great responses here to look through. Wow. In my limited experience I would recommend looking for energy savings on the large equipment as well. The warmer condensed water supply into your chiller, i.e. CT fan on a lower frequency, will just cause the chiller to increase energy usage when an increased load occurs. I think extra energy here will be more of a concern than the power savings from a VFD on the CT fan. Depending on what climate you are in, a two speed fan would probably do the trick and cost the least. However, I really think "KenRad" and his year round use concept are probably correct. In that case, I would just worry about the costs associated with implementing both the research and equipment against the actual energy savings over the life of this chiller plant.
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