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ESP on large air handlers?

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#24 ·
then you better start stocking up on compressors or

install some suction line accumulators with hot gas heat exchangers:gah:

jpsmith1cm said:
No mistake. VFD is controlled through a very basic BMS system. Logic is as I explained a few posts ago. Supply temp goes down, fan speed goes up.
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#26 ·
Ok, lets make sure we have no mis-understanding. After the air leaves the ahu, there is no modulating air dampers that can reduce or increase the air-flow. The vfd is the only variable.
 
#28 ·
Hey jp, could it be a combo of heating coil stopped up and return duct undersized. Do you think this could be causing low superheat, in turn scrambling comps. When you put the drive in bypass did it change superheat? I do agree with the others that putting a drive on a dx isn't a great idea, because you have @#$%^@# bldg. controls telling the drive how fast to run, it does'nt know comps. are slugging, and that #$@%^@# control tech. can't change comps. on a key board. Ahh I feel much better now.
 
#29 ·
i had a very similar building.

cv system, dual duct, with dx recips (200 tons total)

now it is a chilled water system, dual duct, with vfds on the fans...most of the time the fan is 60 hz. at night and during cooler seasons the vfd slows down...it is not changed due to leaving air temps.

it sounds like you may have a 'concept' problem. a situation where you are not sure who controls what or why...very similar to how i found the above building. i found compressor staging based off of outdoor air temp! who cares about outdoor temp and how does that relate to the building load anyway! it does relate, but so does humidity, indoor loads, solar loads, etc. changed it to controlling SAT and everything started to slow down.

fan should be controlled to duct pressure or 'load' but not determined by leaving air temp.

so if the SAT drops, then you raise vfd speed. if the SAT rises, you slow it down...correct? does the fan work this way in heating mode as well?

it appears that you have 2 different things controlling SAT...fan and compressors.

lock the fan on a single speed and leave it (forever if need be) until you can control it properly. this will slow down the loading and unloading of the compressors. it will also simplify your situation.

you need to stop the compressor bleeding so look into that and don't mess with the fan right now. if you have high delta t then maybe you have dirty coils or some other restriction like fire dampers and such. but at least it will not be fan speed on top of everything.

i love buildings like this! i like to tell the newbies that every building has issues. you need to work on the biggest ones first and then work on the next biggest ones, etc. it sounds as if the compressors may be the biggest problem...is that correct?
 
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Discussion starter · #31 ·
jayguy said:
i had a very similar building.

cv system, dual duct, with dx recips (200 tons total)

now it is a chilled water system, dual duct, with vfds on the fans...most of the time the fan is 60 hz. at night and during cooler seasons the vfd slows down...it is not changed due to leaving air temps.

it sounds like you may have a 'concept' problem. a situation where you are not sure who controls what or why...very similar to how i found the above building. i found compressor staging based off of outdoor air temp! who cares about outdoor temp and how does that relate to the building load anyway! it does relate, but so does humidity, indoor loads, solar loads, etc. changed it to controlling SAT and everything started to slow down.

fan should be controlled to duct pressure or 'load' but not determined by leaving air temp.

so if the SAT drops, then you raise vfd speed. if the SAT rises, you slow it down...correct? does the fan work this way in heating mode as well?

it appears that you have 2 different things controlling SAT...fan and compressors.

lock the fan on a single speed and leave it (forever if need be) until you can control it properly. this will slow down the loading and unloading of the compressors. it will also simplify your situation.

you need to stop the compressor bleeding so look into that and don't mess with the fan right now. if you have high delta t then maybe you have dirty coils or some other restriction like fire dampers and such. but at least it will not be fan speed on top of everything.

i love buildings like this! i like to tell the newbies that every building has issues. you need to work on the biggest ones first and then work on the next biggest ones, etc. it sounds as if the compressors may be the biggest problem...is that correct?
Click to expand...
I agree that this is a concept problem.

The id10t engineers that live in the central office like to put things into the stores to save energy, like a VFD, but not do it correctly, like this situation.

Like I said, I am trying to stop killing compressors. I think what I will do is set up the VFD to run 100% in heat/cool/DH and it can drop down to 50% during off cycles. That is probably the best I'm going to get as far as control goes. BMS doesn't even know what to do with a static pressure sensor.

Yes, unit has high delta T. With entering air of 73 and 74 degrees, I've seen leaving air in the low 50s and high 40s. I'd call that high.

As far as undersized returns, I think that plugged coils are the more likely restriction. This is a 20+ year old AHU and I don't believe that the coils were cleaned for the first 15 years.
 
#32 · (Edited)
Jumping in here late jp, what you measured was the total static that the fan has to develop. So before you had the digital you were smart enough to get the magnahelic and to measure the fan inlet pressure.

The trap stem has to be deeper than this fan inlet "suction". The stem depth basically being from the bottom of the drain pan down to where the water comes out of the trap. As filters get dirty the negative static at the fan inlet will increase.

Example a 2 inch pleated filter may only be say 0.1" pressure drop when clean, but by the time it is loaded up it will be an inch.

Cartridge final filters could also increase in pressure drop by at least an inch by the time they need to be changed, so when allowing for filters that trap stem will need a couple extra inches to allow for filters loading up.

Air balancers do a thing called static pressure profiling.

It is basically get the pressure drop between return air connection and after filters. Then after filters and after coil(s), then fan inlet and fan outlet.

It gives you the ESP of the duct work, all the internal pressure drops of the internal components, plus the total static developed by the fan.

Is 4.1" a normal total static in your situation ? Perhaps, or maybe it is excessive, you have a plugged coil.
 
#34 ·
#35 · (Edited)
jpsmith1cm said:
120 ton split system. 2 60HP copeland recips.

Has been a problem unit for YEARS.

Poor cooling/dehumidification performance, oil trips, lost a compressor last year (scrambled).

About 6 or 7 years ago, we had a repeat problem with the unit drain backing up. Nobody could figure it out until I went and bought a mag gauge and showed them that the negative static in the unit was bad enough to prevent water from draining. We spent 2 days cleaning coils, got some improvement, but still not where I think it should be.

The lost compressor last year is the reason that I'm spending so much time babying this unit. 60hp recips ain't cheap.
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120 ton, 2 @ 60 ton semis, you got 4 liquid line solenoids, so basically 4 stages of cooling?

If so then I am not quite following what they are using the VFD for, and especially why they are doing it off of static if there are no VAV boxes. Or did I skip too many of your posts back and forth with mr bigtime and you said there were boxes?

To save energy by trying to avoid running a 50 HP motor loaded all the time, maybe come up with a way to make the system into a large single zone VAV. Ramp the blower speed up according to return air temperature and stage the liquid line solenoids to control the discharge air temperature.

Off of static, I would think you would have to measure the static when you have each of the four stages running and would therefore turn it into a 50 hp four speed motor, or at least turn it into a big two speed motor. Run at half speed when only one compressor runs, bypass to 60 Hz when both compressors run.
 
#36 ·
I'm with Jayguy - Get the fan speed under control. There aren't any good reasons to have a vfd on a constant volume fan, so make it a glorified soft starter. Run in at 60 hz, and troubleshoot the airflow as if it were a constant volume fan. FLA is 100 something, and you're at 50-60% of FLA? You're not moving enough air. If you have an open return plenum, get a vane anemometer and calculate the airflow across the air filters. You will have better luck with that than trying to track down performance curves and original design/specs/balancing reports. Rule of thumb, you'll need 400-450 cfm/ton. At best, you probably have 8 cylinders and two unloaders per compressor, right? The farthest you could unload would be one compressor at 50%. With the potentially blocked coils, airflow is already compromised and vfd ramping down is compounding that.
 
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#37 ·
bigtime said:
Ok, Im not an expert on 120 ton split systems, and Im just trying to reason through this. The vfd is controled by the sa temp. So what happens when the sa temp gets to low. The vfd slows the fan down, and does the cu start to unload?
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I believe it's when the sup. temp. is high the fan slows down and when the sup. temp is lower <55* the fan speeds up to 100%.The VFD prevnts coil ice-up's,floodback,high superheat and oil return problems, controls humidity and temperature of condition air(return and outdoor air).
 
#38 ·
#39 ·
I guess I should of read your original post twice, you all ready profiled the air handler. That heating coil at 1.8 inches is definitely plugged like you were saying. I would expect this dry coil to have less pressure drop than the wet evap coil.

0.4" on the cooling coil could be a little low, depends on the amount of rows and fins per inch, probably low on account of restricted air flow.
 
#40 · (Edited)
Could there be a return fan or hepa filter bank somewhere you dont know about,or prhaps a sound attenuator that has become a filter over the years.Also take a close look at your txv bulbs and ext. equalizer lines and make sure there landed correctly,when trying to resolve comp.failure issues like this i try to 1st focus on what the system does when it shuts down/starts up and last when it is running.Usually it needs a suction accumulator but no one ever installs them.
 
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