Chilled Water GPM per Ton 3

[Jfk1]

We have a humidity problem on a 220 Ton CW system. The Chillers were recently replaced and new circulating pumps were added. The circulating pumps have a rating each of 375 GPM and TDH of 110 Ft.
The return water has only a 6-7 F rise from the supply temperature
I understand there is a rule of thumb of 3 GPM per Ton of cooling and that Trane has used 2 GPM per Ton of cooling. Are short on GPM for our system?

 

[UtilityLouie]

Jfk1,

The GPM/ton rates you are talking about are "rule of thumb" numbers. Typically for chillers systems, 2 GPM/ton is usually the evaporator flow and 3 GPM/ton is usually the condenser flow. These rates came about because they correspond to the temperature change of the water in the system. The 2 GPM/ton evaporator flow rate is a system with a 12 degree delta T (i.e. 44 degree supply water and 56 degree returned to the chillers). The condenser flow may not be the problem here but you may need to check it.

With regards to your flow rate, you may or may not have enough flow depending on the design of your system. The above rules of thumb were derived (this is my own opinion) to make the design of systems for uniform. They also came about because some designers felt you got the best delta T for the best balance between first cost and operating cost. You can design for any delta T, there are just different cost considerations for doing so.

If you are having a humidity problem, it may not be with water flow, but with water temperature to the cooling units or the cooling unit itself. The temperature of the cooling coil determines how much dehumidification you get from your cooling coil.

If you have a psyc chart, you should plot out what you WANT to happen (air supply conditions, return air conditions, outside air conditions) based on your Sensible Heat Ratio. The chart will give you the temperature that your coil needs to be to get the air to the conditions you need. Compare these numbers with your unit's original design conditions (these can be gotten from your local rep with the serial number of the unit).

If these aren't the problem there are other items to look for:
- uncontrolled outside air into the system (have a testing and balancing contractor rebalance the air side of your system). More outside air adds a large amount of moisture to any system. They can also rebalance the water side of the system.
- the chiller may not be functioning properly. Have a technician look at it.
- dirty or plugged cooling coils. This would result in more bypass air and therefore less dehumidification.

These items are just off of the top of my head. Trane or Carrier have some psyc charts that they usually give away. These websites also have some good system design information which may help you further pin point your problem.

Hope this sends you down the correct path.

 

[ChasBean1]

Good post by Louie & I agree. The big questions are, what is the supply air temperature from the HVAC units feeding the space with the humidity problems, what are the positions of the chilled water flow control valves serving the coils of the AC units, and are they three or two way valves? If you're returning cold water, this tells me it could be a three-way valve system.

Sometimes high humidity comes about by building operators setting AC unit discharge temps too high. If the building is not equipped with reheat, operators might boost supply air setpoints following cold complaints. If the supply air setpoint gets pushed too high (62-65°F and beyond), the coils will not dehumidify as they are designed.

Look at the AC units that are supplying the areas of high humidity, their supply air setpoints, their control valve positions, etc. If you simply had a low chiller flow issue, odds are that your return temperature would be higher. A low delta-T indicates either excessive flow, lack of coil ability to extract the CHW's energy, or three-way valve operation. If you have access to this AC unit operating data, repost it so we can better help. And one last question, how many square feet of building is served by this 220 ton unit?

'Til then, good luck. -Chas

 

[bumpjones]

Jfk1
If you are getting a 7 degree temperature rise across your chill water system at a flowrate of 375 gpm you are only getting 110 tons of cooling from your '220' ton machine. The 2.4gpm evaporator flowrate and the 3gpm condenser flowrate per ton thumbrules come about due to the increase in heat energy sent to the condenser. The condenser must reject heat from two sources. One source is the heat taken in by the evaporator and the other is the heat input to the refrigerant due to the compression of the gas. That is why the condenser flow must be higher than the evaporator flow for all centrifugal chiller machines. I am assuming that you use your chiller for comfort cooling and if so you should control the chill water out of the chiller at 44F and get back chill water at a temperature of 54F. If you cool the air in the air handlers to 55F and then reheat it to 75F your space relative humidity will be less than 50% and well within the comfort range for human occupancy. If you do not have reheat capability the 55F air temperature will be to cold for the people in the space and they will raise the setpoint for the chill water up from the 44F and you loose your ability to control humidity. I have fought this battle before and we are currently reinstalling our reheat coils so we can control humidity in the offices. We are also installing new cooling towers. We are spending a ton of money on this project and we have engineered it one hundred ways from Sunday. If you want more help or need more information let me know.

Good luck
Bumpjones

 

[Chillermec]

You did not state what the supply water temp was. Is in in the "normal" range of 44-45 degrees? Also, is this a primary only, or, primary-secondary pumping system? I am suspecting it is a primary only system.
:The chilled water flow may actually be excessive. Check the original design conditions for water pressure drop across the chiller evaporator. If this is a primary only system the excessive flow rate will not allow the chilled water coils to "wring out" the humidity.
:Are the chilled water coils dirty? Either inside or out?
:If chilled water supply temp is not in the "normal" range it is a chiller performance issue.
This would be a good starting point.......

 

[Islander]

We have a humidity problem on a 220 Ton CW system. The Chillers were recently replaced and new circulating pumps were added. The circulating pumps have a rating each of 375 GPM and TDH of 110 Ft.
The return water has only a 6-7 F rise from the supply temperature


*****sounds like too much flow to chiller.
but what is the temp setpoint and actual entering and leaving temps perferrably with all cw coils open...
you should have the system balanced...they most always need adjusting once installed, and at any rate should help find a definitive answer to any problems.
Islander
Test and Balance