Smart questions
Smart answers
Smart people
Join Eng-Tips Forums
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

Birchwood (Mechanical) (OP)
12 Jan 09 16:34
Our air handlers' chill water valves (under building controls) are all configured as bypass regulators. That is, when the controls send a 100% signal, the valves squeeze off the bypass, and 0% opens the bypass. This doesn't give us complete control. I estimate that when set to 0% valve position, we still get about a 15% flow through the coil. There is a balancing valve (manual op) in the bypass, but it doesn't seem to give us any less coil flow than 15%.

There are many times/situations where we really need 0% flow and I have to run around manualling isolating the coils to keep the chill water circuits from adding unwanted heating or cooling during the temperate months.

Is this typical design? If so, why and what can be done to obtain complete positive control through the controls system?
flyrfan (Mechanical)
13 Jan 09 9:17
If you're saying that the control valve (CV) does not fully  shutoff flow when commanded to full bypass then maybe the actuator does not have the close-off pressure against the pump head.  You should check what the close-off pressure is for the valve & actuator combination.

The balancing valve that is in the bypass is used to match the pressure drop of the coil so that the CV maintains it's control ability.  Without that balancing valve, there would be minimal resistance in the bypass, so a small change in valve position to the bypass would equate to a large change in flow in the bypass.  

Hope I explained that clearly.
Birchwood (Mechanical) (OP)
13 Jan 09 10:04
I'll have to "study on" your reply, but have the gist of it. I guess I will repeat the experiments with the balancing valve. It's been a year, so I don't remember how thorough I was.

Thanks!
ChasBean1 (Mechanical)
15 Jan 09 23:28
This is a performance problem. Chilled water should be shut off to the coil when commanded closed, whether it's a 2-way or 3-way configuration. The balancing valve is only there for a high flow limit. If the system is pneumatically controlled, the head end command doesn't match the actuator spring range and needs to be adjusted.  -CB
Birchwood (Mechanical) (OP)
16 Jan 09 14:05
I ran a quick test on one of the air handlers the other day. With at least a 15 degree F temp difference between air flowing through the handler and the chilled water, withe the bypass balancing valve manually open 100% and the chill water valve set to 0% by controls, I opened up the manual valves to the coil. Within 15 minutes, the air temp was witin a few degress of the water temp.

We have a two-way Bulemo electric motor controlled valve.

I really think they should have put three-ways in. That would allow the valve to completely close off one branch when it opens the other bransh, right?
flyrfan (Mechanical)
16 Jan 09 14:22
I am confused at your valve arrangement.  In your last post you are implying that you have a 2-way control valve, but you also say that there is a balancing valve in the coil bypass. Is that what you have?  If so it sounds like you have an incorrectly piped coil.  Can you attached a sketch of the piping arrangement.
Birchwood (Mechanical) (OP)
16 Jan 09 15:31
flyrfan (Mechanical)
16 Jan 09 16:47
OK.  You have a 3-way valve with a butterfly valve in the bypass, so that is fine.  

"I ran a quick test on one of the air handlers the other day. With at least a 15 degree F temp difference between air flowing through the handler and the chilled water, withe the bypass balancing valve manually open 100% and the chill water valve set to 0% by controls, I opened up the manual valves to the coil. Within 15 minutes, the air temp was witin a few degress of the water temp."

Are you saying that with the valve commanded to 100% bypass you are getting a small approach (difference btwn SA & Chilled Water Return), hence flow thru the coil?  If so, your valve is not working correctly.  Check to ensure that the close-off pressure rating is OK as I said earlier.  Also, check to ensure that the valve stem position is to the bypass when commanded 100% closed...maybe you have it backwards.
imok2 (Mechanical)
16 Jan 09 19:25
Sounds to me as if the controller has the wrong action on the 3 way valve. For example it may be reverse action instead of direct acting.
Birchwood (Mechanical) (OP)
18 Jan 09 13:53
No, the valves work in the right direction, they just don't shut off the coil circuit completely. In the summer, e.g., they regulate nicely between some cooling and a lot of cooling. Just need to be able to get them to shut off completely during most of the winter. 0% control signal equates to about 15% effective flow through coils. 100% contol signal  
imok2 (Mechanical)
18 Jan 09 16:16
If you have a diverting valve you would get some flow through the coil you can ck that by looking at the face and if it only has "B" and "AB" it's a diverting valve, if it has "A", "B", and "AB" it's a mixing valve.
 If it's a mixing valve and you are getting flow through the coil then Port "A" is not closing completely so you might want to adjust the linkage to the shaft and see if you can turn it to stop flow through the coil then retighten it
mauricestoker (Mechanical)
18 Jan 09 21:07
You might have a valve working perfectly well at 15% on shut-off, just not calibrated properly, or the wrong type valve. You'd have to look at the Belimo manufacture data and see if it is the right application. Is this a linear valve or equal percentage? If linear, then it might have a mismatch at low flow. If equal percentage, the zero flow might not have been properly set. I'd recommend getting the coil data and the valve data, overlaying the curves (as well as seeing if the Cv was right) and figuring if you are actually working with a linear response.
pretendfarmer (Mechanical)
21 Jan 09 21:57
If the valve is old (I'm guessing it is because very few new installations use 3 way control valves on large chilled water coils) you may consider replacing it with a 2 way pressure independent modulating control valve. These valves are rated for bubble tight shutoff at large pressure differentials. They also do not require a balance valve and can be used as a flow meter. But the biggest advantage is that pressure independent chilled water valves require much less chilled water flow to satisfy the load (they typically operate the coil at a Delta T of about 140% of design Delta T).

My advice would be to start replacing the 3 way valves with 2 way pressure independent control valves. your system will operate much more efficiently.    

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close