View Full Version : Cooling Towers or VFD???

01-03-2016, 03:31 PM
Hi to everyone!

I am in need of technical assistance regarding an energy efficiency project I am conducting on an Air Conditioning system for a Shopping Mall.

This air conditioning system uses a chiller for supplying cool water to air handlers. This refrigerating cycle has COOLING TOWERS to lower water temperature used on the chiller condenser. The cooling towers (obviously); work on a closed loop with two manually operated 40 HP pumps circulating water.

Based on this, I proposed to install a VFD on the pumps and make a control logic using the returning water temperature value. This would allow important savings on the pumps energy consumption.

I was about to close the deal, when the Shopping Mall owner asked me if itís better to lower as much as you can water temperature (keeping pumps at a maximum capacity all the time), or to lower pump water flow.

I understand that by subcooling the refrigerant, the chiller will have a better COP.
So when i said that he convinced himself that we should install more cooling towers to achieve lower water temperature.

I know that this water cooling system is limited by wet bulb temperature, and adding more cooling towers is probably not a good solution. But he has a point, i really donít know if lowering the water flow is better than trying to achieve the minimum possible temperature on the Cooling Towers all the time.

Any thoughts regarding this problem would be greatly appreciated


The Viking
01-03-2016, 06:47 PM
First of all, it is never a good idea to lower the flow of water through a chiller's condenser. It has been designed for a certain flow rate and if this is not met the chiller will fail.

Then we come to the question about water temperatures, this is a complicated question with a lot of variables. The only way to say with confidence if a lower water temperature would be beneficial or not is to get a system design engineer who is experienced in this type of calculations involved.



Glenn Moore
01-03-2016, 10:56 PM
Hi mcuriel
without full details of the cooling tower water circuits , pumps ,control valves & condensers connected to the system it is difficult to advise, but you are on the right track by fitting VFD's.
The water cooled condenser on the chiller requires a variable flow of water to be able to control the chillers discharge pressure as design. The cooling tower water temperature will change as the ambient temperature changes, by modulating the condenser water flow the chillers discharge can be stabilised so that design system status can be achieved. If you allow the water flow to be constant , as the temperature of the water rises and falls so will the chillers discharge pressure. This will cause the chillers expansion valves to hunt as the pressure drop across the valve changes. Colder condenser water will give more liquid subcooling but again this could disturb the chillers control and probably cause more problems
By fitting VFD's to the condenser water pumps, controlled via a pressure transducer on the chillers discharge line this will modulate the pumps speed and keep the chiller discharge pressure stable, and a further VFD on the cooling tower fans will then control the cooling tower pond temperature at a temperature that satisfies all other systems connected to the cooling towers.
If there are 3 port mixing valves or 2 port water control valves controlling the condensers these could be removed or wound out of circuit.
Remember a VFD on Pumps and Fans works on the cubed rule ie the pump or fan running at half speed will use only 12.5% of Full load amps . Just on the 2 x 40HP pumps using a VFD would save a huge amount of energy.

03-03-2016, 12:24 AM
You should evaluate system performance. It means energy use of the compressors + condensers. Wet bulb approach should be controlled by sophisticated PLC. This approach is temperature difference between condensing pressure and wet bulb temperature of ambient air. Condensing pressure should float based on ambient conditions. Several barriers should be overcome to operate at low condensing pressure. These barriers are expansion valves, screw compressor oil carryover, liquid supply ... Every barrier has a solution.

03-03-2016, 11:54 PM
However, sometimes complicated things have simple solutions.
You should do 2 steps to save energy. First, lower condensing pressure until one of the barrier will prevent further lowering. Probably, it will be expansion valve. Second step, start slow down VFDs of condenser water pump and condenser fan.

04-03-2016, 12:54 AM
Do not adjust pump speeds, with centrifugal pumps they need the speed to pump, flow rates are not linear with speed changes.
Run the cooling towers as low as practical and lower SDT to a point and gain energy savings from lower compression ratios. The chiller manufacturers with state a minimum SDT lower limit to maintain chiller performance.
Lower SDT will increase chiller performance so at low load conditions can create issues with short cycling.

05-03-2016, 02:08 PM
Definitely, it should be certain limit to reduce pump speed. You can determine this limit by testing your plant. However, lowering pump speed to 90% will reduce pump head pressure to 81%(most likely plant will work fine at this head) and pump energy use will be reduce to 73%. It means you will save 10 HP per each pump by reducing speed to 90%.

23-03-2016, 04:18 AM
I would install a VFD to control loop temp, on the tower fan.
I would install a VFD on the Pump to deliver the exact GPM based on chiller condensor Pressure drop. A good Balancer willl plot GPM at 60HZ, do some math for required HZ to hit your required GPM.
if no drives exisit the job will pay for its self in no time