Three way valve to be used in a well designed 2 pipe net chiller

[maythem]

Hello every body
i working now on a shopping center of 28,000 sqm with a 1100 ref.ton air cooled chiller (Daiken Mcquay) and the chilled water network design to partial load from 14 in in dia. from the pump side down to 3/4 in end in the FCU , the problem is the designer wants to put a three way valve with full modulator to save energy when divergence is good and to make the pressure equal in all net pipes but thats expensive because we have over 535 PCS of FCU so please advice !!!

[install monkey]

can u not fit inverter driven circulating pumps to maintain a set flow pressure,so when ur fan coils modulate down at setpoint then ur pumps will slow down thus saving energy

[Brian_UK]

^^ Agree with Install Monkey ^^

[Magoo]

Hello maythem
The 3 way bypass at each of the 500 + FCUs is fine but will be noisy, better to have pressure modulation bypass valve at end of supply loop, from supply to return and maintain supply pressure and loop flow rates for HX on chillers You should also have stat flow regulators at each FCU to give design flow rate to each FCU. water pump speed regulation will upset loop flow rates in heat exchangers of chillers risking flow and temp fault stoppages.

[Magoo]

Hi again Maythem.
If system constantly operates at between max capacity and mid range you could have a bypass regulator in the plant room pump set area, if system loads vary below that you could have chiller short cycling problems, better to have the loop circuit water volume as a buffer effect. Pump speed regulation can compound issues as the impellor speed in pump is governed by tip speed, and any speed variation is not always linear and drops off flow charts real quick, so the problem with flow rates and chillers

[Magoo]

Gone never to be heard of again.

[Scramjetman]

Maythem, there is some trickiness with this. For chillers to function correctly and maintain control stability, the flow needs to be carefully managed. To get that power transmitted to the AHU's is also critical otherwise all the money spent on the system is wasted.

There is a very smart man in the US named Gil Avery who has written a lot on designing and managing chilled water systems. Here's a link to one of his articles which challenges some deeply entrenched ideas in the industry on efficiency and control in chilled water systems.

http://www.kele.com/templates/content.aspx?id=4942
(http://www.kele.com/templates/content.aspx?id=4942)
What you will notice is that there is no simple quick fix solution. Large systems like yours need careful engineering and equipment selection. And design engineers can be just like some of us tradies. You get some good ones and you can get some not so good ones. Whoever makes the decision to make changes needs to intimately understand the system and the dynamics.

That being said, 3 way systems were popular in years gone by and most systems are now being designed as two way systems with a single bypass network near the chiller. For a detailed discussion on the energy implications check out this link.

http://www.engr.psu.edu/iec/publications/papers/energy_use_characteristics.pdf

That should give you a bit of light reading at bedtime. :D

Having the system deliver the power correctly to the AHU's across the wide range of load conditions and maintain stable control is the trick. Whether using 3 way or 2 way valves - there are penalties to each type of system, the dynamics need to be fully understood or there will be tears, I hope this gives you something to get your teeth into and to work with.