Multile Cooling Towers/Chillers

[iken]

I am currently involved in the installation of the above, and have an major issue with the combined operation of these. Any advice on the details below would be greatly appreciated.

On site we have installed 3 new cooling towers, and 2 new chillers and one exisiting chiller. At present both suppliers are pointing fingers at one another, which leaves me in the middle trying to resolve why they do not work together.

The design conditions are 35.1 deg and 29.4 deg for the condensing water and 11.6 and 5.6 deg for the chilled. At present we are only running with a 2 - 3 deg split on the condensing water side (where the problem lies). The two new chillers are operating with their safety locks on and only at 66% and 80% capacity (one screw and one centfif. chiller). The main chiller is the centrif. which will go into overload prtection (high oil pressure - cond. water temp) when this approches 35 deg - never reaching, at patial load.

This leads me to believe the cooling towers aren't given the opportunity to work fully - am I correct?

I also understand the fundimentals of heat exchangers, and LMTD, but have been advised cooling towers do not operate in the same way - is this correct - if so what is their basis of heat rejection?

Why would the chiller cut out near design conditions - assuming this chiller is the correct one of course?

If any one has any ideas on what I should look for, or answers to the above I would be very greatfull.

 

[Cooky]

iken,

Can you explain what you mean by 'safety locks on'. Similarly, when you say '2 - 3 deg split' are you referring to the flow and return temperatures?. If so, can you tell us the actual flow and return temperatures.

Have the water circuits been commissioned?, if they have, you need to give us the flow rates and pressure drops.

With this information, I'm sure we can help find an answer to your problem.

Cooky

 

[lilliput1]

The main chiller condenser water flow may be less than the required design flow. What are the chiller capacities, kw/ton, design condenser water flows, actual condenser water flows. Are there any control valves on the condenser water? What are the duties of the condenser water pumps?

 

[iken]

Thanks for resply, here's answers / further details you require.

Safety Locks - These are limits (max) at which the chillers will operate. I believe this to be load based, hence the 66% & 80% values given to me.

The 2 - 3 deg C split is the condensor water split. The actual values range from

CondFT (Flow) CondRT (Return)
28.13 25.85
29.16 26.84
26.93 24.85
29.95 27.16
31.27 28.01
25.00 22.41

Above are values taken from logged temp values, all in deg C

The condensor water circuit has been commissioned, along with cooling towers, (spray pattern, nozzle head pressure, and water height in tower etc).

The following is the design conditions and ratings for the plant.

Cooling Towers (3 identical)(Values given for each tower)

1695 kW Heat Transfer
74 l/s Water Flow
35.0 deg C Water Inlet
29.5 deg C Water Outlet
22.4 deg C Wet Buld Temp

Chillers
Centrif. Screw
Capacity 2200 kW 1265 kW
Leav. Temp 35.1 deg C 35.0 deg C
Enter Temp 29.4 deg C 29.4 deg C
Flow Rate 110.0 l/s 64.0 l/s

There are no control valves on the condensor water side. We have installed T & A IR valves on the condensor water lines. I Have set these up to give the required flow through each chiller (within + or - 3 l/s).

I can only give you the duty for the centrif. pump, as this was installed by us, and the pump on the screw chiller is old.

By the way, this job is an upgrade of exisitng plant. Two chillers out (these were smae size as each other) with two new ones. Some modification to pipework. Condesor side 100% complete. Chilled 75% complete, but we have been advised that this will not effect the cooling towers performance (or chillers).

Centrif. pump duty 110 l/s @ 202 kPa

 

[absrbrtek]

What model chillers do you have? What is the condenser water approach/LMTD? The liquid line temperature? The condenser refrigerant pressure? Water leaving the condenser temp?

Was this system properly flushed out before putting it online?

 

[Stormxtc]

iken,

Don't quote me on this just yet but it sounds like you may have differential pressure between evap. and cond. sides of the chillers.

Some chillers do not have an oil pump and therefore rely on a pressure differential between the chiller evap. and chiller cond.

Usually, a 280 kPa pressure differential is required, if the pressure diff. falls below this than the chillers will cut out (usually after 2 minutes of low pressure).

You may also want to check your minimum condenser entering temperature for your chillers because below a certain temp (usually 18degC) the chiller has trouble operating.

To solve the pressure problem you may have to install a by-pass valve on the cooling towers or a throttling valve on the cond. water leaving. Either valve option is than controlled by the temperature difference between the leaving chilled water and condenser water temperature.

If you have a BMS system than this can also control the valves.

There is also a stand alone system to control the valves but you need to research it.

Hope this is of help, and I hope I understood your problem correctly.

Good Luck,

Stormxtc

 

[lilliput1]

The chiller load + the compressor input is equivalent to the heat rejected to the condenser water. Thus if the 626 ton centrifugal chiller operates at 80% and assuming its compressor input is 0.65 Kw/ton, the heat going to the condenser water should be .8 x 12000 x 626 + .65 x 3413 x .8 x 626 = 7120600 Btu/hr
If the condenser water flow to this chiller is 1744 gpm, the conmdenser water temperature rise should be 7120600/(500x1744) = 8.2°F.
If not & if the kw/ton input is greater than design, the condenser most probably is fouled.
The condenser water heat is rejected to the athmosphere by evaporation.
What is the cooling capacity & condenser water requirement of the existing chiller? How many condenser water pumps are there. You only gave 1744 gpm for the pump but the (2) new chillers require total 2759 gpm.

 

[iken]

All three Chillers have their own condensor water pump.

The new screw chiller has design flow (measured at the IR valve), however the actual duty point of the pump is unknown, as for the pump on the exisitng chiller.

The existing chiller pump has been assumed to supply the remianing water flow to acheive total design flow for all 3 cooling towers (48 l/s and approx 1000kW of cooling).

It may also be worth noting that the original design spec called for 2 identical chillers to replace 2 existing, size for size. The supplier offered an alternative of the 2 above with different capacities. I am not 100% privy to the full info, as we did not supply the chillers, we only installed them (supplied by client).

 

[jherbert]

hi,
Is this the situation:

total chiller heat rejection
2200kw + 1265kw + 1000kw = 4465kw *1.25 = 5581 kw
therefore required cond. water = 240 L/sec

available HR towers:
3 * 1695kw = 5085 kw (500 kw short)
3 * 74 L/sec = 222 L/sec (20 L/sec short)

What was the ext. wb, when recording
cond. water temp.?

If everthing is *NEW* fouling would be minimum,
assuming WT, filters, etc. are all live.

-how does the chilled water side perform,
maintaining delta T?
-any free cooling CT mode installed?
-CT bypass closed?
-CT type?
-chiller kw/ton?
-flow switches have been checked?
-what is diff. pressure across condensers?
-online schematic drawing?
-how could you balance multi-CT system with
different sized pumps?

sorry, more questions than answers

cheers,