AHU LAT Variation 48-56 deg F 3

[Rhonda2]

With conventional (pressure dependent) control valves installed at large cooling coils, what leaving air temperature (LAT) variation should be expected relative to setpoint in operation with modern controls? Does it vary depending on the location of the coil in the system?

We are seeing numerous situations where LAT varies significantly due to differential pressure variations across control valves. This leads to variation in chilled water flow and subsequently to variation in LAT.

When LAT falls below setpoint, reheat masks the problem but causes additional ton-hours to be consumed. When the LAT rises above setpoint, fans work harder to sustain comfort conditions and humidity issues may arise. Many facilities have raised the LAT setpoint in operation because of inability to meet the original value.

I'd like to hear about some experiences in the field and what design options are being employed to address these concerns.

Thanks,

 

[imok2]

Rhonda2
You should be able to control within a throttling range of 3 to 4 degrees if the sensitivity is set correctly properly calibrated and the proper Cv on the valve. Or + - 2*F from set point, closer if you use DDC controls

 

[moonpe]

Your LAT temperature variation may have nothing more to do with your chilled water system controls than your air handling unit controls. I have found that if the control valves are not properly sized for a sufficiently large pressure drop (yes, large pressure drop) that the water will short circuit through the coils closest to the pumps which starves other coils. All of the coil control valve has to have enough of a pressure drop to minimize the effects of the piping pressure drops. In other words, if the control valve pressure drop is low, then the water will go to the path of least resistance which means to the coils closest to the pumps that have the lowest pressure drop. Bell & Gossett's Little Red Schoolhouse has some good manuals that discuss this if your interested in more information.

I have found coil flow variation from 75% to 140% of design flow with only a +/- 1 F change in the leaving coil temperature. You could be flowing a lot more water through some coils and not even know it.

In addition to the problems with your LAT, your chiller plant is probably suffering from a low temperature differential (i.e. design temp delta t is say 10 degrees but plant never gets better than a 6 to 7 degree differential). This is also probably costing a lot in energy. Hope this helps.

 

[ercanbaser]

As moonpe said, control valve selection and system balance are the main considerations.A god design and installation dont mean that your system can work perfect and control valves operates as they designed.Especially, In large system , using balancing valve to prevent flowing more water through coils closest to the pumps than others, is a good practice. For each coil ,do not forget to install automatic air vent for proper operation.In design level its also good idea to divide large coils to smaller parts is also a good idea.
For my opinion VFD pumps(secondary loop)+ two way control valve+balancing valve(dynamic type) is the most effective design alternative.

Regards,

 

[wilg]

We provide specific performance requirements for (viz.) fans, cooling coils, humidifiers etc.,
but control valve performance is being left to the controls vendor.

Normally control valve schedules are not included on projects coming back from Consultant offices.
The problem is that we don't design, calculate or specify CV values for control valves and we depend on the control vendor to provide the proper valve cv's, these values are part of documentation for IQ/OQ, validation and system commissioning and maintenance manual turnovers but are not seen on engineering offices drawings.
When determining the pressure drop through existing chilled water systems,pressure drop at the end point of use cannot be correctly factored in without
knowing the valve CV value. Field inspection of control valves may provide a model number and size but will not indicate the orifice used.

 

[Rhonda2]

Seems that many have focused on the control valves close to the central plant. Wilg brings up a good point about the valves at the end of the line.

In many systems, it appears to me that excess flow further down the line leads to an increases the differential pressure across valves closer to the plant. This can lead to problems with conventional control valves lifting off their seats or, at low flow, banging shut.

Consider one of the hydraulically more remote control valves in a system serving a cooling coil with design delta T of 12 deg F and 112 tons cooling load. Lets say the valve has a Cv of 100 and was selected with a pressure drop of 5 psid, this gives it a design flow of 224 gpm. If the load at this coil does not change but the load at a coil close to the plant goes down, this may cause an increase in the differential pressure from 5 to 10 psid (for example) further down the line.

With a 5 psid increase and no balancing valve installed, flow at the more remote valve becomes 316 gpm and delta T drops from 12 to 8.5 deg F. This aggravates the problems with high differential pressure across valves closer to the plant and pumps. In most systems, this will likely lead to return water blending (through the bypass or non-operating chiller) or the operation of additional central plant chillers and their auxiliaries as the evaporator flow limit is reached.

I think the key must be to ensure high delta T is achieved at each coil wherever it lies in the system (without raising the chilled water supply temperature). Overflowing coils close to the plant starves coils down the line. Overflowing coils down the line increases the differential pressure across valves close to the plant causing problems. Variation is the flows at fixed loads cause LAT variations at the coils. Looks like a viscious low delta T circle.

Are DDC controls and VFD's really capable of addressing these issues? What delta T performance should be expected at peak and part load? Shouldn't we strive for LAT variation of +/- 1% to minimize undercooling (affecting fan speed) and overcooling (causing reheat)?

Apologies for the excessively long post. I should resolve to not drink so much coffee this year.

 

[wilg]

This is a typical KW/ton vs differential pressure
chart.

and condenser pump savings chart employing vfd's

 

[imok2]

Rhonda,
Ideally, a control system has a linear response over its entire operating range. The sensitivity of the control to a change in temperature is then constant throughout the entire control range. For example, a small increase in temperature provides a small increase in cooling. A nonlinear system has varying sensitivity. For example, a small increase in temperature can provide a large increase in cooling in one part of the operating range and a small increase in another part of the operating range. To achieve linear control, the combined system performance of the actuator, control valve, and load must be linear. If the system is linear, a linear control valve is appropriate. If the system is not linear, a nonlinear control valve, such as an equal percentage valve, is appropriate to balance the system so that resultant performance is linear. An equal percentage valve is used for proportional control in hot water applications and is useful in control applications where wide load variations can occur. Of course all of this would take place with a fairly constant pressure drop across the CW valves. However if your system doesn't have any Balanceing Valves then thats the first thing I would install. Are DDC controls and VFD's really capable of addressing these issues...Yes I direct your attention to this web site and read "Hartman's work"

http://www.automatedbuildings.com/

 

[Rhonda2]

With pumps turning on and off and changing speed while control valves are opening and closing, I'd contend that it is unusual to find fairly constant pressure drop across control valves. Small changes in the pressure across a 5 psid valve produces wide swings in the flow. Also, 50% of the flow should manage 80% of the cooling load and 200% of the flow manages only 115% of the cooling load.

It appears to me that these pressure and flow variations are a significant part of the reason that very few systems achieve better than design delta T performance at part load at both the plant and the coils. When balancing valves are installed, I find that the leaving air temperature setpoint is often raised or unachievable at high load, causing the fan to work at higher speed to manage the cooling load.

I have seen a lot of theory in Tom Hartman's articles but never actual data to demonstrate high delta T and good system performance at part load. Perhaps I have just missed it. If anyone can point me to such an article on one of Tom's projects, I'd appreciate it.

In the end, I'm trying to assess the benefits of using a pressure independent control valve with adjustable Cv.

 

[quark]

Rhonda has a good point in his 2nd Jan, post which justifies the use of balancing valves and in my view Imok's reply answered it clearly.

Most of the low side installations we did are far apart from the high side installation (as this is generally recommended in a Pharma Company). That is why I never had serious problems with short cycling so far. Moreover I prefer to go for reversed return headers if chilled water is supplied to multiple AHUs. (The average number for me is 30) This increases pumping and initial costs but I am safe with my job.

Yet, I feel the system is not totally foolproof and I always wecome your comments.

Regards,

 

[Rhonda2]

Quark,

All a balancing valve does is clip the flow if it exceeds the maximum set by the balancer. It does nothing to balance the system at part load conditions. This is why delta T in many systems tends to degrade at part load, either at the AHU coils or in the plant.

Pressure independent MODULATING control valves automatically and dynamically balance the flow at all load conditions and eliminate the need for reverse return piping. If the system is expanded there is no need to rebalance. This can be especially important in laboratories.

When high delta T is achieved, flow is significantly reduced. VFD's actually perform as intended. Minimizing flow and pressure drop saves pump energy. These control valves are not affected by variation in system pressure. Flow remains tuned to the load.

What gives you the sense that the system is not foolproof? There is no need to suffer with increased pump and first costs.

 

[lilliput1]

If you have controls add valve positioners they should become pressure independent.

 

[Rhonda2]

lilliput1,

Repositioning the valve in response to a pressure change will typically have a significant lag time. Is this what you mean?

Most conventional control valve actuators are designed to fully stroke in well over 1 minute.

With these same actuators on a pressure independent control valve, only the leaving air temperature controls valve position.

Isn't setting valve position with temperature alone better than both temperature and pressure due to the lag time involved with repositioning based on pressure?

 

[imok2]

Lilliput1 Whats your opionion on this information with regards to Rhonda's intention of using pressure independent valves?
:

http://www.flowcon.co.jp/fc/outline/out_04_e.html

 

[Rhonda2]

Imok2, Lilliput1. I'm aware of the company you mention but am not planning to use their product. My plan is to use the pressure independent control valve produced by Flow Control Industries (

http://www.flowcontrol.com).

Among other material, I have found to be an excellent resource.

You can find this guide by going into the DeltaPValve page and typing "system design manual" into their search engine. I initially ran across it under the title "unconventional design guide" on the automatedbuildings.com site.

 

[lilliput1]

A valve actuator with positive positioner (or positive-positioning relay) is designed to provide full main control air pressure to the actuator for any change in position. Positioners may be connected for direct or reverse action.

 

[lilliput1]

The prressure independent valve shown on the link above is an electronic valve with positioner type electric circuit. Any pneumatic valve can be made pressure independent with use of positive positioner.

 

[Rhonda2]

As I previously stated, there is a time lag associated with repositioning a control valve when it experiences a change in system pressure.

If a conventional valve with a Cv of 100 has 5 psi drop across it, the flow is 224. How much time does it take a pneumatic or electric actuator to return the flow to 224 if the differential pressure increases to 10 psi (316 gpm) with no change in load?

I see this as a key difference between conventional and pressure independent control valves. Pressure independent control valves respond almost instantaneously to keep the flow at the gpm necessary to best serve the load. The result is high delta T.

 

[lilliput1]

Test valves for leakage & if the actuator develop enough torque to close the valve against the service pressure. Dity coils would reduce heat transfer transfer & water temperature rise. Inadequate flow (because of restriction of rust in piping) would inrease water temperature rise. The mass of water & air have a thermal flywheel effect that would minimize control transients. Instead it seems problem is constant valve leakage/out of control.

 

[Rhonda2]

lilliput1. Absolutely true, the actuator must have sufficient strength to handle the service pressure and leakage at close off pressure must be nil. Many conventional control valves behave more like solenoid valves when insufficiently sized to handle the actual system pressures and pressure fluctuations.

Given that the majority of system in the industry fail to achieve design delta T at part load, I'll have to disagree with your flywheel assessment. If you look closely you will find plently of clean coils and even new systems with low delta T problems.

Compounding the issue, low delta T at locations far from the plant cause greater differential pressure close to the plant. If this is not anticipated, it is difficult to properly size actuators on valves to handle the system pressure.