PICV 2

[Manu68]

Hi Experts,

PICV as the brochure is a combination of flow control that works to deliver the required chill water flow irrespective of upstream pressure pressure in the pipe line. When that is the case I believe there is no need to have double regulating valve or other control valves. As the AHU load decreases PICV closes irrespective of the upstream pressure in the chilled water pipe line. Similarly when pressure is high in the up stream pipe but AHU load is low PICV closes. Could you please throw light from your experience.

 

[EnergyProfessional]

I use them wherever I can... they are great. They actually save money as you don't need balancing. Only need to verify flow to see if they installed the correct type. Also don't need to worry about CV value for authority.

I prefer the Honeywell, of very high quality while having less pressure drop than Belimo et al. Only 3 psi, which is less than regular valve and balancing valve.

You'll need a strainer and during flushing they need to take out the cartridge.


Very few application require pressure-dependent valves (i.e. bypass for min flow etc.). Other than that, PICV all the way.

 

[urgross]

We've replaced 36 pressure dependent ways with PICV's, one left to go. Have one secondary pump bypass and chilled water coil (then field testing for most challenging hydraulic run) before we can finally convert over from constant primary/constant secondary to full variable primary with distributed boosters. The boosters will be needed mostly to overcome the minimum operating pressure (6.0 psig for the PICV's we used) required.

Not only have they been maintenance free (other than removing and draining/airing out cartridges) but oping by next year to have the primary pumps VFD run off the percentage open for the PICV's. Can't do that with a pressure dependent. If you think about the air side vs. the hydronic side, would you install pressure dependent mixing boxes?

 

[EnergyProfessional]

urgross; wha tdo you mean by pressure dependent mixing boxes?

AFAIK for airside the only pressure-independent devices are VAV boxes with flow-station , and the CV American Aldes (or similar) devices.

 

[Manu68]

Thanks to all of you. I am planning to keep PICV in the downstream of all major and load variable units, eg for PAHU and AHU. And a manual balancing valve in the main CHW line before that branches to feed AHU and rooms with PAHU.

With refernce to HerrKaLeun's mention about VAV being an equivalent to PICV of a water line. Air being a compressible less density fluid, would that behave the same as that of PICV that effectively controls (flow rate) and manage Pressures.

 

[urgross]

HerrKaLeun,

When i was starting in HVAC, pressure dpendent boxes were the norm, and pressure independent boxes were considered a costly fad. Thirty years later it is difficult to even find pressure dependent VAV boxes. I beleive that the same arguments, and the same control optimization, that was argued on the air side 30 years ago will be pretty much the same as the arguments I've been hearing for PICV for 5 or 6 years.

 

[ChasBean1]

The cost for these devices is that they require a certain minimum differential pressure to work. Your central system might require 15 psid versus 10 psid for these devices to be satisfied. All the other benefits as outlined by the previous posts stand.

 

[SAK9]

To digress a bit has anyone done a comparison of individual PICV at AHU vs a combination of STAD-STAP valves at each branch serving a few AHUs.The STAD-STAP valve combination will keep the branch differential pressure constant .If the branch is not too long,all individual AHU two way valves will experience more or less identical constant differential pressure and the result may not be far off from what a PICV achieves.

Please read the attached brochure to understand STAD-STAP combination in detail.

 

[EnergyProfessional]

chasBean1: how do you come up with 5 psi additional dP? Teh Heoneywell PICV have 3 psi total pressure drop at design flow (less at less flow). but that ic counted against dP you have with regular balancing and control valve. I would claim most balancing valves need at least 2 or more psi to read anything of any accuracy (if you call them accurate to begin with). and a control valve adds some more. So the PICV may be about even. Plus most balancing is totally inaccurate, so you may have in practice less pump energy due to no excessive flow and a known pressuredrop.

in reality most balancers run the pump at high speed and squeeze the heck out of the balancing valves. sure more than 3 psi.

SAK9: in most cases i use a PICV or regular valve as bypass and control with flow-meter assuming a minimum boiler or chiller flow is what motivates you to have bypass (in variable primary). the exact method probably depends on why you want to bypass?

 

[Manu68]

By function I don't believe VAV is completely equivalent to a Chill water PICV. The duct length connected to the VAV and the leak permeability would cause an issue unlike PICV.

 

[EnergyProfessional]

Well, yeah, a valve is different than a damper. but from a controls point they are similar in that you can set a maximum flow rate and you can modulate below that rate.

Obviously the one requiring active electronic control and a meter make it much different.

a regular valve is similar to a regular damper (= pressure dependent VAV "box")

 

[ChasBean1]

HerrKaLeun, I did some mad math… re-read my post. I didn’t define the 5 extra psid… In general, pressure controlled devices require a higher DP to work (be it Phoenix, delta-P valve, etc.).

 

[EnergyProfessional]

Chasbean: per Honeywell spec it is 3 psi for the entire vale (in case of smaller ones). Compare that to pressure drop of a regular control valve and a balancing valve (you need at least 2-3 psi to measure with any sort of accuracy). I'f say in most cases the PICV has less, or at least not more dP.

I know Belimo etc. have higher dP, one more reason to use Honeywell.

 

[ChasBean1]

HerrKaLeun, I’m not trying to spar with you and I’m not in sales. I’m just saying that there is a benefit and a cost to pressure regulating devices. In any loop with greater than a few loads there are varying pressures at all terminal points.

You can find that your entire loop is satisfied from a flow perspective without pressure independent valves at some value, say 10 psid. Now, if we have a system full of pressure regulating devices, that 10 psid might become 15 psid because under a dynamic condition, some remote loads might require that higher value to keep the 3 psid you mention.

My analogy was to Phoenix Control air valves, which are great for a number of reasons but it’s known that the central air delivery systems require a slightly higher pressure (and more fan energy) than a system full of Price/Nailor/Enviro-Tec VAV boxes.

That said, the 3 psid minimum value seems pretty good…

 

[EnergyProfessional]

ChasBean1: I looked at that Phoenix Control valve. But this seems to be for Constant Volume only? Or do they have an option to modulate flow? We have many VAV boxes and they calibrate out of whack... so a solution not requiring calibration my be good. But from what I saw those valves are for a specific flow and not VAV, right?

VAV boxes are a real hassle for us, pressure sensors fail, drift, need recalibration. I have one that has almost 5 times the flow it "measures". So if there was something like the Control valve, even at higher dP, I take it if it is accurate and hasslefree. Also seems easier to replace with the drawband etc. Can even be retrofitted.

anyway, I don't fully follow your logic of the additional 5 psi. but using PICV really depends if you want a defined flow, or not. In some cases you may want to have a pressure-dependent valve (i.e. bypass valve). so it really depends. I've been using PICV for a while and really happy with it. Even if dP would be a bit higher, you would make up those losses with never exceeding flow. and if you ever talk to a balancer, you know the first thing your design needs to do is get rid of things that need balancing

 

[ChasBean1]

Phoenix Controls are not constant volume only – they have many options.

This could go in circles for a while. Back to 101, a fan serving a distribution system with volume dampers might require a half-inch static, max. You produce the pressure, leave the most remote devices open fully, then tweak back the others.

If each device becomes an EnviroTec VAV box, the half-inch becomes maybe 1.0 inches, because under dynamic conditions, boxes might need that pressure to deliver the flow that they require.

If each device becomes a Phoenix Control Valve, the common duct static might become 1.5 inches because each device needs 0.6” pressure across it to perform as needed.

The same goes with water. If you want guaranteed flow control at terminal devices, it will cost you with pump power and system pressure.

 

[DRWeig]

Kudos to ChasBean1, you saved me all that typing.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[EnergyProfessional]

I came across these Accutrol air valves that seem to work more like an actively controlled VAV box and with similar pressure drop (0.1"- 0.2"). Unlike the Phoenix valves, they actively measure the flow and control a damper.

sorry for hijacking this thread to go from valves to VAV boxes

 

[ChasBean1]

DrWeig, thanks

 

[Timothi]

How does the PICV adjust the flow when AHU load is less?