Three way vs two way vales

[MechGuy22]

Does anyone have some good literature explaining the benefits of a two way vs a three way valve system for a chilled water application or can someone give me some bullet points?


What would be your thoughts on converting a existing three way valve system to a two way with a vfd running the pumps? Feasible?

 

[imok2]

A three way control valve is used for constant flow systems while a two way control valve is used for a variable flow system. A two way control valve is more sensitive to high differential pressure and it is harder to close off against line pressure compared to a three way control valve of the same size. This is because a two way valve creates a dead head condition while a three way valve merely shifts pressure from one place to another Therefore, the actuater for a three way valve does not have to be as powerful for the same size valve. For chilled or hot water the equal percentage valve is recommended and is most effective operating at 50% to 100%

 

[KiwiMace]

The retrofit of a 3-way Constant to 2-way Variable is often feasible and is often carried out. Conversion of old hotels is one common example.

Whether your system is feasible depends on a lot of size and operation factors which is for you to determine. There is usually a lot of work in getting at and changing all those valves.

 

[mauricestoker]

If you look at any of the major manufacturer's (Belimo, Bell & Gossett, Flow Control, Griswold, etc) you should be able to get all the sales and engineering data that supports going to a PICV.

I'm in the process of doing that now for the campus chilled water distribution system.

 

[MechEngNCPE]

Yes, this is a very common upgrade to chilled water systems. It saves energy and reduces the possiblilty for low delta T on the chiller.

 

[cry22]

Keep in mind, that you will still need SOME flow on 3-way valves.

Typically, you let 5% to 10% of the total flow in each zone (floor for eg) to be on a 3-way configuration to relieve the pressure form the system (use end-of the run oils on 3-way valves). Or, install a 3/4" by-pass at teh end of the run of each zone

Because your when the load goes below the lowest VFD speed, your pump will dead head as your valves are closing but your pump slow any further (typically pumps do not go below 40% of maximum speed) as you will need a place to relieve the pressure in the piping system.

 

[Drazen]

cry is right, there is some silent conspiracy among VFD drive/pump manufacturers on how to solve the issue of having flow lower than pump minimum.

i regularly use bypass pressure control valves.

 

[zuccus]

It's not just the pump, but also a minimum flow on a chiller or heat exchanger is neccessary also.

 

[mauricestoker]

I'm not so sure it's a conspiracy, more a matter of doing your Appendix G and sizing the chiller, then the pumps. The pump for each chiller needs to operate below the minimum flow for the chiller, and I would certainly recommend including at least 5% spare at TAB setting. Never starve a chiller. Max flow is set by the Onicon, or whatever flow you use.

 

[Drazen]

when chiller min. flow is imposed, than it is not an issue, mostly, as min. pump flow is selected according to that requirements, but i had number of real life applications when different hydronic solutions were used, so there were no requirements imposed on pumps, or pump low flow is much higher than chiller low flow requirement and match cannot be applied.

however, you gave me idea that some manufecters reps look at "Appendix G" or similar as holy text so they turn their heads on the other side when faced with problems not covered by papers . that is not so uncommon...

 

[mauricestoker]

Having the dP's properly set with the chillers is the most common issue I've had. They don't come factory set, or at not any useful way. Just finished with testing three this morning. A single 1200 gpm condenser pump was fooling three chillers (2-940's, 1-400) into proof of flow, then just waiting to go out on high condenser. Hard to believe custom manufacturer wouldn't include setting dP corresponding to manufacturer's flow minimum, or slightly above, but that's been my experience for the last five chillers (all from the same manufacturer).

 

[mauricestoker]

Certified factory start-up rep was in this morning. As per usual, he emphatically stated that the dP's were set for flows given. Showed him the sequence for adding on pumps and chillers.

"Oops, we must not have set that one in the factory".

Number 5 in a row. Great chillers, just hard to get past finger pointing between controls, TAB and mechanical.

 

[MechEngNCPE]

Let me guess... Trane.

 

[mauricestoker]

I guess that's just too obvious.

 

[MechEngNCPE]

haha, yep.

 

[TenMan]

When designing a chilled or hot water loop, its usually better to have a primary pump to circulate water through the equipment in mechanical room, then a secondary pump to send water throughout the building or other source, needing chilled or hot water. By designing a system like this, there is less chance of reduced water flows through chiller evaporators and boiler tubes. I've seen them installed both ways and seen the many problems when its a one loop system. I am retired from the industry leader in Chillers and HVAC equipment. Hope this helps.