View Full Version : Cooling tower as an economizer?
01-26-2008, 10:38 PM
I had a call on a large bank operation center the other day when it was 27 degrees for the high temp. The cooling tower was online and the chiller was offline and the tower was freezing up pretty bad. The tower pump and building pump both hit a heat exchanger in the mechanical room. Naturally there is a BAC system on the building that I have no control of. The water was definately dropping the building loop temp but I think the fan on the tower should cycle. Ironically the maintenance guy at the facility was calling me to tell me he thought the sump heater was not working. Why heat it if you are trying to cool with it? My question is--should the fan not cycle on tower sump temp to prevent ice build up or does it not matter?
01-26-2008, 10:56 PM
nevermind I had a question but im not understanding what you have for equipment...:eek:
01-26-2008, 10:59 PM
Is the cooling tower the condenser for the chiller or does it work parallel to the chiller??? :eek:
01-26-2008, 11:12 PM
Is the tower fan on a VFD? If so, the VFD may just roll down to its minimum HZ output once the signal from the BAS is no longer there.
I have mulled over the idea of tower economizers for my plant...the scenario you mention above is something I've thought about. Seems to me going in there should be a control sequence that stages the tower fan off when just the water falling through the tower can keep the building loop cold. Simultaneously, enough heat in the building loop should be available to the tower to be rejected in order for the tower not to freeze.
My own plant does not have an economizer, the chiller runs year round. In winter, in order to keep the towers from overcooling the chiller condensers, there is a bypass loop with a modulating actuator on a PID control sequence, controlled by entering condenser water temperature and ECWT setpoint. The towers have sump heaters...so far I have not experienced any tower freezing....there's enough heat being rejected by the loop to keep the water above 32 degrees. That would not be possible if it were not for both the bypass loop and the VFD on the towers.
01-26-2008, 11:13 PM
01-26-2008, 11:21 PM
I've seen this done, it works and then it can be a mess, depends on system.
It works best if there is a good cooling load to keep things warm and the fan can reverse to thaw things out. I've seen towers turn into nothing but a big icecube and the damage to the fill by the weight of the ice, caused by any so called "free cooling", offset any savings (those ones are nolonger used for free cooling). LOL
The sump heater is actually used to add heat to maintain setpoint temps and prevent sump from freezing on a off cycle.
01-26-2008, 11:29 PM
Yes it is the condenser for the chillers-2 RTHDs. No VFD on it just motor starters. The BAC shuts a valve to the condensers and routes the water to the exchanger. My concern is the weight of the ice on the tower fill and possible damage since it is plastic or pvc. The lead engineer says it saves them tons and works great. BTW there are flow switches everywhere so there is no possibility of the chiller indexing on. I don't see where it would hurt to put a stat in the sump and break the starter wire on the fan.
01-26-2008, 11:35 PM
PS-- I did fix the sump heater since it will not ever outrun the fan and it has been set to come on at 35 degrees. The sump was about 44 degrees but the fill was odt of course.
01-26-2008, 11:46 PM
It's late, what's odt?
01-26-2008, 11:52 PM
odt= out door temp
01-27-2008, 12:06 AM
By now, yer probably wondering how they let me loose on my own. Hehehe
01-27-2008, 11:00 AM
the fan for the tower should cycle off when the troth reaches 45F then with the return econo water coming back from the building will bring it on...set the troth heater stat...to 30-35F and use it to maintain the troth when the system is off(if it is one of those that mounts on the side on extends in to the troth it is not there to heat the troth in the cycle..it is there to keep the overall troth above freezing when the water sits).if you have a good heat load from the building the fan cycling on will maintain that troth with the fan and ambient air...ice buildup thru the fan and on the tower discharge is common.if you take the diameter of the fan discharge opening and mount a BUFFER plate 1'-2' off the cover screen this will retsrict the discharge air a push some back into the iced area and keep it ice free of the blades
01-27-2008, 11:35 AM
You have an issue with the fans, the tower fans should be cycling. Typically with a heat exchanger, I use 45-50dF as a setpoint on tower supply water to the heat exchanger.
Free-cooling at these low oa temps can cause issues. Depending on the load of the building and with the fans off, the water can still ice up as it flows thru the tower. The sump heater will add heat, but not designed to reheat the tower water. Sump heaters are designed to keep the sump from freezing while the tower is off.
I typically shut down free-cooling below 30dF oa and revert back to mechanical cooling. Depending on the setup you may be able to run lower. I have a building I can go down to 25dF oa and have had no issues. I understand the energy savings, but freezing up the tower to the point of breaking something can cause big bucks in repairs.
01-28-2008, 10:21 AM
Fan control is really important...I run a "free cooling" system until OA is 28-30 deg F. The actual temp is determined by building heat load. You must have enough heat from the building to keep the tower thawed out. VFD's on your fan would help a lot. It is very difficult to keep from overshooting with an on-off fan control and low ambient temps. I do have to shut off the fan to keep from turning the tower into an ice cube during the off-peak hours. during this time, just water spilling through the tower rejects enough heat to get the job done. In cold weather, humidity is not usually a problem, raising the building supply water temp during tower operation would help keep the sump further from the freezing mark, and although it would reduce latent capacity of the indoor units, you should be ok while the outdoor temps are low. I guess that would just depend on your particular building load.
Good Luck, HVAC_Controlnut
01-28-2008, 07:04 PM
Thanks for the replies! I like the idea of the VFD on the fans. Fortunately this customer is a :D "go ahead if it will help and order 2 extras for backup" :D type customer. The load on the building stays pretty high 24 - 7 as this is a service center. I do think I will put a hypothetical repair cost together of what damage could be done by the ice and try to convince them to switch back to the mechanical cooling at 34 or 35 degrees outdoor temp. If not we will just order back up fill, back up belts, back up bearings etc....
01-28-2008, 09:37 PM
Imersion heaters are usually off line if fan is operating.As stated by others you want to set tower water for 45F if being used for economizer cooling. Sump heaters should come on when fans off, set at no lower than 40F to keep sump from freezing when fan is off. Your building load should be enough though to keep sump warmer than 45F.
If you have propella blade, vfd is the best. You can also reverse the direction of the fan at certian intervals to help defrost the ice build up on the tower inlet and fill area.
01-28-2008, 11:39 PM
It costs a lot to save money.:D
01-29-2008, 06:32 PM
I have seen several of these set ups and yes you do need sump heater to run. Th e problem is usually in the controls or lack there of... Normal operation the tower fans cycle base on condenser water to the chiller or heat exchanger. Now in winter operation you need to add new point control to operate the fan based on sump water temperature. A simple solultion is to install a VFD on the tower fan and stand alone controller to sense the tower sump temperature or the return water temperature from the tower and modulate the tower fan. On colder days you may not need the tower fan? This depends on your cooling load. Good luck.
01-30-2008, 01:33 AM
The best set up for these is to use a basin , rather than the tower sump for water storage.
Have seen a lot of set ups over the years, have one currently of this type.
The chillers come once a day just to exercise them and related valves for 1/2 hour, otherwise the towers will keep up with cooling setpoint(in econo mode) to 55-60 , depending on wet bulb .
Very energy efficient.
02-03-2008, 09:24 PM
These systems require a dry climate to work well. They are basically using it like giant swamp cooler.
Dont see the payback for all that goes into this design for a Carolina climate.
duke of earl
02-04-2008, 06:09 PM
I prefer an enthalpy sensor/controller for economizer changeover cause on a cold rainy day, outside air temperature might get you in trouble. I have seen mechanical cooling/ economizer short cycle back and forth in these conditions which is very undesireable. Like Freddy-B said, a dry climate is more well suited for plates than humid climates like the Mid-Atlantic where I live.
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