AHU LAT Variation 48-56 deg F
AHU LAT Variation 48-56 deg F
(OP)
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,
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,





RE: AHU LAT Variation 48-56 deg F
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
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
For my opinion VFD pumps(secondary loop)+ two way control valve+balancing valve(dynamic type) is the most effective design alternative.
Regards,
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
chart.
and condenser pump savings chart employing vfd's
RE: AHU LAT Variation 48-56 deg F
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/
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
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,
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
RE: AHU LAT Variation 48-56 deg F
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?
RE: AHU LAT Variation 48-56 deg F
:http://www.flowcon.co.jp/fc/outline/out_04_e.html
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
RE: AHU LAT Variation 48-56 deg F
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
RE: AHU LAT Variation 48-56 deg F
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.
RE: AHU LAT Variation 48-56 deg F
RE: AHU LAT Variation 48-56 deg F
Furthermore, design delta T at part load is less than optimum. 19 deg delta T should be achieved on a 12 deg F coil for most load conditions (80% or less). The coil sets the delta T at design flow but the control valve sets the delta T at all other load conditions.
Flow Control Industries recently provided data from multiple installations showing me how a simple control valve retrofit alone substantially raised the delta T.
RE: AHU LAT Variation 48-56 deg F
"As I previously stated, there is a time lag associated with repositioning a control valve when it experiences a change in system pressure".
It has been my experience as a service mechanic that Positive positioning relays on valves and pneumatic motors are very powerful and there is no lag time to speak of, like lilliput says they get full main line pressure and are very sensitive to change without hunting. in my estimation Pneumatics are the best controls for end devices I love Hybred systems