View Full Version : Large tonnage De-coupling

09-08-2004, 09:23 PM
Can any body explain the basic points for a large tonnage district / Zone A/C cooling plant's ( Centrifugal - water cooling system )De-coupling with VFD secondary pumps. And how important is the Supply and return Diff Pressure.( IN every site . i:e in primary and every where in the secondary load site) :confused:

09-08-2004, 10:35 PM
Ya Marc ;
you are very much clear to the point , Actually i have ask this to know " due to any / slightly changes in the main plant DP ( even for a 0.01 Bar ) more or less or for run-test any main secondary VFD pump in manual speed for a while it disturb many distrct buildings tertiary site flow and gives very hard time to settle the temperature.
one of my query is what should be the ratio or setting in betwen the main plant out put Diff-Press and tertiary site Diff-Press. (any rough caculation ) :confused:

10-08-2004, 06:24 AM
Do all the braches have a metering valve (don't know the correct name for a 'strangenregelaar') wherbey you turn in via a handwheel a certain DP before or after the AHU so that you can balance the whole system?

That's one I found via Google http://www.hga.honeywell.de/pdfs/v5032y-pd-ge0h0155ge25r0702.pdf

I doubt that 0.01 bar give such a problems. There must be other reasons. Are your PID algorithm settings on the VFD not too narrow setted?
Why not increase slightly main pressure on the system. It will increase of course slightly power consumption but if the actual settings give you such a headaches...?

One of the benefits of a hydronic circuit is that it can be controlled/balanced very well and that it forgives more faults then with a VRV or VRF system.

Try to monitor pressure on secundary circuits while you change on the primary side. You will notice - I think - much larger variations. You only can be sure if you measure, otherwise it's guessing.

As long as your pressure drops over the AHU remains the same, your flow will remain the same.

How is each AHU coil controlled and balanced for the moment?
When an AHU has reached setpoint, what actions are taken now?
Are there 2 or 3 ways valves fitted? Because you spoke of a VFD pump, I think they fitted 2-way valves.

We installed twice a Carrier Chiller of +/- 500 kW (production machine of Rieter) with +/- 30 branches on 2 extrusion machines (for some sort of very thin polyprop wire). All the cooling coils where independent from each other regulated. We had also a VFD controlled pump and we never had any problems with it.

10-08-2004, 08:17 PM
The link you posted looks like what we would call a balancing valve Peter. You can see the gauge ports each side of the orifice plate.

10-08-2004, 08:45 PM
If you don't install this balancing valves on a plant with different coils, you'll never be able to balance the system correct.

11-08-2004, 01:44 AM
Solutions that involve mixing return water with supply water undermine the thermodynamic efficiency of the system, destroy the capacity of the coils to meet their loads, and add further to low delta T problems. To solve the types of distribution problems that lead to low delta T, the design or retrofit needs to follow these rules:
1. Eliminate all possibility of direct mixing between chilled water supply and return: This means eliminating all decoupling lines and three way valves. In this era of networked DDC and variable speed control, pumping systems no longer need to be decoupled. Furthermore, modern chillers accommodate varying flows over substantial ranges without any loss of efficiency or operational stability. By selecting equipment wisely, it is not difficult to design "all-variable speed" chilled water generating and distribution systems without any mixing so that every bit of supply water must pass through a load before returning to the plant and supply chilled water at design temperature is available to all loads at all times.
2. Employ a direct coupled distribution system: This means that when multiple pumping circuits are employed they need to be connected directly in series rather then isolated with the use of decoupling lines. Primary/Secondary systems become "Primary/Booster" systems in which "all-variable speed" pumping stations are operated in series. Such systems are extremely effective and can save capital cost when compared to decoupled Primary/Secondary schemes because Primary/Booster configurations can incorporate built-in backup without the need for redundant equipment.
3. Focus delta T attention at each and every load: Once decoupling lines and three way valves have been eliminated, the only source of low delta T problems is overflow through individual loads. Overflow can occur because of improperly sized valves and varying pressure differentials across valves. It can also occur when the air side of cooling coils becomes clogged or other maintenance failures take place. A simple means for preventing overflow is to install a temperature sensor on the return water line at each load and to use this temperature as a limit for the control valve serving the load. When the return water temperature falls to approach the design leaving water temperature for the coil, the valve is limited from opening further. This step eliminates the problem of low delta T at the load and gives the designer a little more flexibility in sizing valves for each load. The simple logic that limits the valve operation can also be employed to notify the operator that a maintenance problem may be affecting the operation of the load.
For a full read go here :http://www.automatedbuildings.com/frame_resources.htm :D

11-08-2004, 06:10 AM
I don't think this is a 'mixing-system' because they use 2-way valves and VFD pumps.
Anayway, I didn't read bewteen the lines that it was a mixing system.

11-08-2004, 08:11 PM
I don't think this is a 'mixing-system' because they use 2-way valves and VFD pumps.
Anayway, I didn't read bewteen the lines that it was a mixing system.

Hi ,,
Thanks Shogan for yours Important respond.
and Peter here is a basic diagram for the primary side .
and in the seconday side its also almost same with every load/ building mean with booster-tertiary VFD pumps along with de-couple line with mean feature of 2 huge capacity 2 way motorize valve called 1/3, 2/3 valves....

12-08-2004, 12:27 AM

First set primary pumps on hand for full supply water pressure and chillers set on hand to maintain 60 *F water temp, better done with 2 men

When I look a your design and reading your remarks I will make some assumptions:
1. That you have 2 DP controls, one for the primary side and one for the secondary side so if thatís the case this is how I would approach the solution to your problem.
First I would determine what the DP is on the secondary loop with all chill water valves at full flow, then determine what the loop flow is at no flow for the 3 coils and last coil on full bypass. This should give you the full range of DP you are working at. Now slowly open #1 and see what DP reads at full flow and the same for the other coils until you get back to full flow. With these parameters you can determine the best sensitivity setting on the DP controller. Remember, the slower the reset the longer the time required to change the coil temperatures

Put system back on automatic and watch the action of the primary DP controller to make sure it correctly tracks the secondary controller and drops the required primary pumps and chillers off the line. In my estimation there is no standard setting for any control other then finding out what is best for a particular installation. This is just one mans thinking, others may have a better solution

13-08-2004, 01:03 AM
Thanks Shogun
You are right .
For every building there is its own VFD controler secndary pumps ( Each rated flow is 3000 gpm )work on the the same Building DP along with own Decoupling line and 2 non's 2way valves in the return line.to maintain the DP and Decoupling.

Any way currently the primary side DP is less then the secondary side's DP's and thats the only problam to get a exact and easy solution for Each buiding DP as its varies within no time and a having a big diffrence in Day and night time mean occupied and unoccupied mode.

Thanks again for yours such nice idea and tips.

13-08-2004, 06:36 AM
For every building there is its own VFD controller secndary pumps ( Each rated flow is 3000 gpm )work on the the same Building DP along with own Decoupling line and 2 non's 2way valves in the return line.to maintain the DP and Decoupling.


I think that if you don't fit balancing valves in the decoupling lines of each building, then as soon the 2 way valves are closing, the DP can take any value.

If in 1 buidling all the valves are closed, then the decoupling line is complete open without any pressure drop.
Altough,.. you have VFD controlled pumps who should 'see' this change and adapt to each stuation.

Also, if the suction pressure of the pump varies due to changes in the primary circuit, then flow will also vary.

Are the VFD pumps controlled by a DP pressure signal on their specific circuit?

Is this a complete new system with problems since the beginning or did the problems started after a long running time without problems?

I think that as far as I have experience with this that if the VFD pumps are correct controlled by a correct signal, then it should work.
With the VFD controlled pumps, you must be able to maintain proper DP's /flows, so I think that the algorithm of the PID for the pumps is something wrong setted.
Do they have a minium speed swhich is perhaps too high for minimum load.

13-08-2004, 06:00 PM
Ya Peter
its a first term with 1/3 load for the entire plant ,

And in the secondary side if there is a low suction flow then the Secondary pumps will run @ full speed and try to maintain the supply pressure, and DP and each seconday side/building has all AHU and FCU's with 2 way valve's but in the end or 2/3 supply pipe line for each secondary side there is a DP switch and in the Last AHU there is a 3 way valve . So in case if there is even no load there will be a minimum circulation of water in the secondary side and the Sec-VFD pumps keep on running.and these pumps are running totaly on base of same building DP and its linked-up to return line 1/3 , 2/3 valve after the decoupling.

13-08-2004, 07:42 PM
So Anis, Are you making any progress on a solution? If not, what are your intentions in a resolution to your problem :)

14-08-2004, 03:28 AM
ya :)
Our aim was to keep maintain/Establish & similar the building secondary return temp and main return temp after De-coupling .So chiller get's the full load.
( i:e at a certain DP ratio for Primary as well as secondary system flow, you can maintain the Definite return )
Now the observation is totaly base on tril and .....///! but one thing is definate ( Its my opinion ) in this type of situation always 1st reduce the DP in the secondary side and then set it accordingly.so there would be a less chances of bypass from De-coupling.and you can set DP by monitoring temperature, Flow and pressure. :)

Thanks Shogun and peter for many tips.....and all :)

14-08-2004, 06:29 PM

Have perhaps a read here and more specific about the hydraulic isolator


"The hydraulic isolator must be full line size, with no restrictions or valves. The idea is to maintain a very small pressure drop between point A and B. Then the flow rates and pressure changes in the chiller loop will have no effect on teh flow rates and pressure changes in the distribution loop, and vice versa."
Reference: HVAC systems & Design Handbook,page 142 from Haines.