Cooling Tower VFD energy savings
Cooling Tower VFD energy savings
(OP)
Can anyone suggest a resource or methodology about how to calculate the energy savings acheived by replacing a constant fan speed cooling tower with a VFD controlled cooling tower ?
THANKS - JK
THANKS - JK





RE: Cooling Tower VFD energy savings
What is your application?
RE: Cooling Tower VFD energy savings
JK
RE: Cooling Tower VFD energy savings
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
RE: Cooling Tower VFD energy savings
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.
http://myhome.hanafos.com/~criok/english/publication/thermal/thermallisteng.html
I strongly feel starting and stopping the fan within a temperature range is the better option which I follow.
Regards,
RE: Cooling Tower VFD energy savings
RE: Cooling Tower VFD energy savings
Regards,
Brian
RE: Cooling Tower VFD energy savings
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
RE: Cooling Tower VFD energy savings
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.
RE: Cooling Tower VFD energy savings
The expansion valve capacity will drastically drop with reduction in sub-cooling.
RE: Cooling Tower VFD energy savings
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.
RE: Cooling Tower VFD energy savings
RE: Cooling Tower VFD energy savings
Brian
RE: Cooling Tower VFD energy savings
Most chillers I have seen have tower fans cycled to maintain about 80 degree F. water.
RE: Cooling Tower VFD energy savings
RE: Cooling Tower VFD energy savings
RE: Cooling Tower VFD energy savings
RE: Cooling Tower VFD energy savings
RE: Cooling Tower VFD energy savings
Joe
RE: Cooling Tower VFD energy savings
RE: Cooling Tower VFD energy savings
The sacrifice, for those who look at the big pix, is CWS temperature rises. For every degree of CWS temperature increase, efficiency worsens by 1-2%.
At say 100 Tons applied, @ .6kW/T at full speed, if the tower full speed is 8kW, the total electrical demand is 68kW. If by lowering the speed to 80% speed causes an increase in CWS temperature of 5F, potentially the efficiency could rise to .66kW/Ton at 100 Tons. The total electrical demand then would be 66kW+4kW=70kW. In this case, because of the tower selection, it wouldn't make sense.
I've found that tower selection is critical for vfd fan consideration. If one doesn't plan for >15% reserve capacity in the tower, a vfd should not be considered.
Choose a 2 speed fan.
RE: Cooling Tower VFD energy savings
Boyceg