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Clymber (Mechanical) (OP)
6 Mar 03 9:15
I need to estimate the condenser water flow through a pair of TRANE Centravac water chillers (one 750-ton unit & one 1150-ton unit).  I have data for % compressor load on both units and incoming/exiting cooling water temperatures, but no flow data.  Can the flows across both chiller condensers be estimated from the information available?

Thanks in advance for your assistance.
Brad
DouginMB (Mechanical)
6 Mar 03 15:54
It sounds plausible. Is there a reason you can't take this to your Trane rep?
TBP (Mechanical)
6 Mar 03 20:03
If you have pressure gauges on the inlet and outlet condenser water lines, there should be tables in the manuals for your machines that let you look up your model, and see what the flow is, based on the pressure drop observed, for a new, clean unit. If you don't have these tables, Trane should.
wj33 (Mechanical)
9 Mar 03 21:30
For any chiller with a compressor, you have to know what temperature range the chilled water and the condenser water are supposed to be.  Typically many chillers (not absorption) will run 45 - 55 deg F range on the chilled water which results in 2.4 gpm/ton.  The condenser water typically would run 95 - 85 deg F range which would be 3 gpm/ton.
Clymber (Mechanical) (OP)
10 Mar 03 11:05
Thanks everybody for your help.

The chilled water outlet temperature setpoint is 42°F.
Normal condenser water inlet tempeture setpoint is 78°F.

I do have delta-T's for both of those flows.  Also, I have contacted the local TRANE rep for a copy of the differential pressure-to-flow conversion chart.

Here's a follow-up question.  On some TRANE literature, I found this equation:
Tons = (GPM X delta-T X specific heat* X specific gravity*)/24
* = for fluids other than water.

QUESTION:  Does this apply to the condenser flow as well as the chilled water flow?  The TRANE tech didn't have an answer.

Thanks for all the help.
Brad
TBP (Mechanical)
10 Mar 03 12:18
You can flip your data in BTU/hr by using:

GPM X 500 X delta-T (the '500' is just rounded from 8.33 lbs per gallon X 60 min per hour = 499.8

This gives you an accurate BTU per hour estimate for pretty much any situation involving water flow and temp change.

The '24' is 12,000 / 500.

Ton hours = USGPM X delta-T / 24

I do more heating than cooling, so I remember the first short-cut formula that takes water into BTU/hr. Then I can easily figure anything I need to, whether it's heating or cooling.
wj33 (Mechanical)
10 Mar 03 13:48
The Trane formula which can be found in most mfgs information is correct.  Unless you are running with a closed circuit cooler where you might be using a glycol/water mix, you would always use water on the condenser circuit, so the gpm x 500 x delta T is what you use.  
pardal (Automotive)
12 Mar 03 17:43
The flow at the chiller condenser it is determined from the flow that the Water Cooling Tower need acoording the Incoming , outcoming water temperature from the tower and the Wet Bulb Temperature of the air.

A cooling tower in the Arizona Desert 40% RH   ,will need about
the half flow for Florida or other place with 95% RH.

So first get the tower flow

As an example : for a 200 TR chiller the condenser allowed flow could be from 300 GPM to 1200 GPM for the same chiller



Pardal

wj33 (Mechanical)
12 Mar 03 18:00
Cooling towers are selected using Wet Bulb temperature not RH. together with approach temperature = Temp of water leaving tower minus WB temp.
Flow rate is dicted by chiller not the cooling tower.  The tower is sized to reject the heat from the condenser water.  
pardal (Automotive)
13 Mar 03 7:07

Hi wj33
I must dissagree whit you :

On my  "wood" desktop , I have four diferent proposal from 4 water tower vendors.
All they answer to my request to provide a tower to my chiller.
All them quote towers of diferents flow , because each tower dising as it's own heat tranfers coefficient.






Pardal

wj33 (Mechanical)
13 Mar 03 7:54
A cooling tower's capacity must match the flow rate from the chiller and the heat ( flow rate x temp rise in chiller) and reject this heat to the atmosphere.  You must tell tower vendors what your flow rate is.  This is determined by the limitations of the chiller, pressure drop etc.  If a vendor is giving you different flow rates then they may not be offering you correctly applied capacity for your needs.  Get an ASHRAE manual and look up the section on cooling towers.
pardal (Automotive)
13 Mar 03 8:57
no coments

Pardal

ChasBean1 (Mechanical)
13 Mar 03 23:09
Bleacher, using pressure differential and flow charts, don't forget to consider gauge error and height difference. Eliminate gauge error by using the same gauge for both readings. If the condenser water outlet pipe pressure gauge is, for example, 3 feet higher than the inlet gauge, a DP reading of 14 psig should really be 12.7 psig. This would mean that the flow you thought was 1,000 gpm is really about 950 gpm (~5% difference). Just a thought to add to the mix... -CB
Clymber (Mechanical) (OP)
14 Mar 03 7:11
ChasBean1,
Thanks for the info... I hadn't thought about the height difference.  Upon checking the condenser, inlet & outlet P gauges are at the same height (but different instruments).

*Also, does anyone have the Pressure/Flow tables?  My local TRANE rep. hasn't come through for me.

Thanks.
TBP (Mechanical)
14 Mar 03 18:58
Bleacher - you'll need the tables for your particular make and model.
canox23 (Mechanical)
15 Mar 03 21:47
Go to the Trane web site and look for Application Engineering Manual Multiple-Chillers-System Disign and Control. Is Good

Literature Order Number = SYS-APM001-EN(October 2000)
File Number = PL-000-SYS-APM001-EN
Supersedes = CON-AM-21 (MAY 1987)
Stocking Location = La Crosse
www.trane.com
comfort@trane.com
VEEKRISH (Mechanical)
17 Mar 03 6:06
Bleacher, your best bet (after reading all the posts) is to actually measure the drop across the condenser(and as suggetsed by Chasebean, and rightly too)by using the same instrument. This hould be then compared to the chart from trane. I can't believe trane cannot help out with a chart. what a shame. This is the method I always fall back on, and it has always proved right. Ofcourse, I've only worked with carrier chillers.
jimmyjam (Mechanical)
12 Jul 03 20:23
Hey Bleacher,I am a Chiller Mechanic@ Trane,
You only need to do two procedures for your question:
1)Using a differential H2O pressure gauge purchased at any
instrumentation supply company,find out what the pressure differential of the supply & return is by attaching the ports to the the In & Out piping going to either the condenser or evaporator barrels.
2)(Simply obtain "COPIES" of both chillers' flow charts at your local Trane Parts Store...Make sure you have model & serial numbers of both chillers!)Multiply your pressure differential reading by 2.3 and the left side of your charts
will show you actual feet pressure and you will now be able to see the values for the gallons per minuite /Min-Max Ft. Pressure allowable by Trane standards on these charts.
Clymber (Mechanical) (OP)
14 Jul 03 8:34
Jimmyjam,
Thanks for the info!  That's exactly what I was looking for.
ChillerPro (Mechanical)
15 Jul 03 23:46
Bleacher,

I have just joined the group and read your request regarding flow rates for evaporators and condensers. I have been in the chiller service and repair business for 20 plus years now. The questions that you have asked are common and easy to answer. I to have worked for Trane as a chiller tech for 10 years. I now evaluate and survey large chilled water plants and systems for performance and non-performance. Being in this position gives me the opporunity to see hundreds of facilties and at the same time see so many chillers that are running way outside the design conditions they were selected at. Many machines in the field today do not run at design conditions and for many reasons some good and some not so good.

I say all that to say this. If you know the tonnage you need for the evaporator then you have many choices as to what the flow rate and delta t could be. These two items are not selected based on what a chiller needs but are based on what the system needs mainly the coils that are being used. If the coils are selected for a wide delta t then the flow can be reduced or just the opposite. You also have to consider the pressure drop through any machine carefully because this will impact the pump horsepower needed to move the water. The formula that was mentioned by some of the other folks is the formula I use daily when calculating tonnage for systems that use water only. Tons=GPM*Delta T/24. If you know that one of the machines is in fact a 750 ton machine then you have to know what the temperature spread is going to be before you will know what the flow will be. Also bear in mind that if your are trying to determine the tower flow rate and the delta t on the condenser side there will be somewhere between 16 to 18 percent more heat being rejected at the tower! You have the heat of compression plus motor heat if it is semi-hermetic machine. There is another limiting factor to what the flow rate should be through a machine and that is the velocity of the water through the tubes. Typically the maximum is 11 feet per second and the minimum is around 3 feet per second.
This are given in the design info of most chiller manufactures.

Hope this helps.

Chiller Pro

  
lilliput1 (Mechanical)
16 Jul 03 13:31
The total heat rejection to the cooling tower is the sum of the cooling capacity of the chiller + the compressor input (assuming hermetic compressor so compressor input all goes to refrigerant, not to ambient). So say the 750 ton unit compressor input is .7 kw/ton, since 1 kw is 2545 btu/hr and 1 ton is 12000 btu/hr, heat rejection of the 750 ton chiller is 2545 x.6 + 12000 = 13527 btu/hr per ton of cooling. Now typical condenser water temperature is 85°F entering and 95°F leaving or 10°F temperature rise. Solving for cooling tower gpm per ton:
Gpm = (btu/hr)/(500 x t rise) = 13527 / (500x10) = 2.71 gpm per ton. The 750 ton chiller therfore will require 2.71 x 750 = 2033 gpm at 10 degrees TD. The gpm for the 1150 ton chiller can similarly be calculated. Actually it is usually possible to select electric chiller & cooling tower combinations that would work in the 2 to 3 gpm (note absorption chillers would require about 4.6 gpm/ton)range. But the minimum entering water temperature is always 85°F based on typically 78°F wb, (occassionally 80°F wb but selection will not be CTI certified). Typical is about 3 gpm per ton but we do design for the minimum, 2 gpm per ton if we want to save on cooling tower space (existing building limits available space) and pumping energy.
with (Mechanical)
19 Jul 03 11:18
The cooling water pump characteristic should be helpful;
Clymber (Mechanical) (OP)
21 Jul 03 9:13
Guys, thanks for all of the useful information.  At this point, I do know the flow across the cooling towers.  My goal was to estimate cooling water flow across the chillers in order to determine the %chiller load vs. total loading on the cooling towers.

Brad

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