reverse engineering help

[hvacer2]

Hi all.....

I am a Mech Eng with no HVAC experience, but have been asked to help with a 25yr old Glycol chiller. System has been "badgered" over the years, and documumentation is almost non existant. I have been trying to reverse engineer the thing, but have a few questions. First - what I do know:

Continental NUPAC air cooled chiller (w remote condenser)
9 ton (nominal) capacity
R134a
fluid: 40% EG
flow rate: 50gpm
Ent temp: 15 deg F
Lvg temp: 10 deg F
Ambient Temp: 65 deg F

OPTIONS:
Low Ambient to -20 deg F
hot gas bypass
suction accumulator
oil seperator
head pressure control + 2 speed condenser fan

My first thought was to get into the pressure enthalpy diagram. Is a good assumption for the design high pressure side to be:

65 + 30 = 95 deg F - corresponds to 129 psia

What is a good assumption for the low pressure side design temp/pressure? Would it be 10 deg F (leaving glycol) minus some standard # for the evaporator?

I have dug up alot of information on all of the individual components in the system. Just not the "nominal" operating conditions. Any suggestions or help with a "plan of attack" would be much appreciated....

Just FYI - the system is not meeting spec, and I have been asked if I can figure out why not.

[klrogers]

Hi all.....


Continental NUPAC air cooled chiller (w remote condenser)
9 ton (nominal) capacity
R134a
fluid: 40% EG
flow rate: 50gpm
Ent temp: 15 deg F
Lvg temp: 10 deg F
Ambient Temp: 65 deg F



Are these the specs. or the present operating conditions ?

[hvacer2]

these are numbers i found on an old spec sheet......

[wolfdog]

klrogers is asking a good question. At first glance your GPM is way high.

[Heavyevans]

I would need more info on the refrigeration circuit to be certain. Pressures, superheat, subcooling, etc...
I would probably take a close look at a 25 year old chiller using 134a. I don't remember 134a being used until the early 90's.

[hvacer2]

50 gpm was the spec....

I have measured (with several different methods) and get numbers in the mid 40s...

If I am not mistaken, 50gpm x delta 5 deg F x S.H. of EG is approx 9 ton

[hvacer2]

ahhh yes... my first small victory!

when I was handed the folder, I was told "25yr old system". I just looked at the date on the original spec sheet and see it was built in 93.

Sorry - my bad.

As for Pressures, superheats, subcooling, - well, these are what I am trying to figure out (well - at least what they should be).

The only thing I could find was that the TXV was set at 13 deg F (factory). The system has a reciever in it if that helps (subcooling wise).

[wolfdog]

..

[Heavyevans]

Unless I can put my hands on it, I'm usually no help. Try here http://www.air-conditioning-and-refrigeration-guide.com/water-cooled-chiller.html

[gasnowman]

Evaporator approach defined is 10 degrees colder than your leaving medium. Have you checked the specific gravity of EG? Is this system capable of running? If so we would need more trends.

[wolfdog]

If you are looking for Continental chillers....http://www.continentalteam.com/index.html

If they are like some other process chiller manuf. they will have a file on the original design/build for this one.

[hvacer2]

The system produces chilled glycol at ~45gpm right now..... That glycol is pumped to three seperate circuits. Two of them have AHUs and the third is to pipes embedded in the floors and walls.

The glycol flow is not really the problem (I don't think). The chiller does not seem to be able to get down to the specified temperatures (even when removing much less than 9 tons).

[mrhvacmechanic]

Please correct me if I'm mistaken, but due to the weight of the glcol solution you need a higher flow rate than the 2.4 gpm for water.

You said the system is 25 years old. What is the present condition of the solution?? Has it been chemically maintained over the years? If not, you may be pumping more of a jel than a solution? You may not get the heat transfer you should be getting.

[wolfdog]

.

[jayguy]

i have:

40% EG
SH: 0.849
SG: 1.058

(5F * 50 GPM * 0.849 * 1.058) / 24 = 9.36 tons

what is the spec that it is not meeting? leaving temp? flow? process? delta T?

good luck.

[hvacer2]

Thank you Jayguy!

As I have said before, I don't think the glycol loop is the problem:

9 ton
delta T = 5 deg F
10 deg glycol produced
15 deg glycol returned
40% EG

These are all specs, and they all correlate. I assume when the chiller was installed and first started up, it would meet these or not get signed off on (I know, I know.... don't assume)

As it operates now, the flow rate is down a bit (measured 45gpm), but for an old system, I figured this might not be unusual.

The problem is the glycol temp - cannot seem to get it down below 20 deg F. Even when the delta T is less than 3 (ie less than 60% of full capacity).

I have been looking into the refrigeration cycle to see if I can identify any problems there, but I need to have some nominal pressures to start with (high side and low side). Then I can look into all the good stuff like subcooling, superheat, offloading, bypassing, etc.

[jayguy]

I have been looking into the refrigeration cycle to see if I can identify any problems there, but I need to have some nominal pressures to start with (high side and low side). Then I can look into all the good stuff like subcooling, superheat, offloading, bypassing, etc.

to get some nominal/normal press/temps...we will need to know more about the refrigeration system itself (for diagnostic purposes, anyway). recip, scroll, screw compressor? compressors? unloading capability if any. unloading type (piston unloading, shut off compressors, etc). flooded, fixed orifice or txv?

it appears from your post that you are getting the temp down below 20F...so what is the problem again?

double check your evap approach temp (leaving glycol temp minus saturated evap temp)...less is better...flooded is approximately 4-8F, txv is approximately 10F to 15F. much more than these numbers and you may have too much gpm, dirty glycol-side tubes, etc.

good luck.

[hvacer2]

Ok.... thats one number I am looking for

If flooded sat evaporator temp should be 4-8 deg F (lets say 6 ) above glycol , then when system is meeting spec:

evaporator temp should be 10-6 = 4 deg F
evaporator pressure should be ~23 psia

Any comments on my condenser side pressure estimates?!?! Does ambient + 30 deg F seem reasonable? (i.e. 95 deg F corresponding to ~129 psia)

Now a few more details of the system:
-reciprocating semi-hermetic compressor (25hp)
-r134a refrigerant (nominally 160 lbs)
-3 staged unloading (in compressor)
-hot gas bypass
-TXV with 13 deg F superheat (factory set)
-head pressure control valve
-reciever
-6800 cfm condenser

[jayguy]

ambient + 30F seems reasonable for an air cooled cond

you have a txv system, so a 10 to 15F approach is more likely. so that makes for a 19 psia sat evap temp.

do you have to use R134a? you are starting to get to the point where R134a is not as efficient as some other refrigerants. R404a or R507 (my personal favorite) would be a better choice. uses the same oil as R134a and is as long term (environmentally) as R134a. you will probably have a larger range of products to choose from as well.

good luck.

[jogas]

I believe in checking the simple things first.
When you load the chiller up to 100%:
What is the HGBP doing? It could be keeping your evap temp/pressure too high. I often find them set wrong.
What is your superheat/subcooling/approach temps? Full charge?
How does the compressor amp draw compare to compressor % loading? Does the amp draw change with each load stage change? You could have bad unloader(s). What's the amp draw verses FLA?
It's a recip? Does it pump down?
Running the chiller at 100% capacity, then logging data will give an indication of true capability.
jogas

[hvacer2]

Armed with my new "design" high side and low side pressures, I will visit the facility in the next week or so, and try to gather as many of these numbers as I can......

BTW - the HGBP is suppost to be set at 10deg F. (as per original design). How would you go about determining if this has been set improperly?!!?

[jayguy]

Armed with my new "design" high side and low side pressures, I will visit the facility in the next week or so, and try to gather as many of these numbers as I can......

BTW - the HGBP is suppost to be set at 10deg F. (as per original design). How would you go about determining if this has been set improperly?!!?

you will need to have a low load condition...and see if the HGBP maintains

good luck.

[crab master]

I am confused and I've tried re-reading this multiple times. You keep saying these are the specs and then in another post you say they all correlate - what are the actual readings of everything?
I understand you have verified flow to be just a bit less than the 50 gpm, but then you state you can't get the glycol temp below 20 deg. Can you please post your actual readings?
I assume you've checked and made sure all three circuits are original circuits and they have not been added to?
Next you seem completely convinced it is not a loop problem but did you take a refractometer and check the glycol percentage or are you just going off of the specs? Many older systems I have come across developed a glycol leak sometime before I/our company was involved and someone ended up pouring straight glycol in as the "fix." We were hero's making their system work properly diluting their glycol and "giving" them a few "extra" pre-diluted containers to refill when/if needed. With less glycol the system would pull down to the temperatures as it was designed to do. Anyway maybe I am just missing a bunch of points.

[hvacer2]

Okay... Here they are once more......

SPECS:
9 ton chiller - R134a refrigerant loop
40% EG mixture
Supply 10 deg F
Return 15 deg F
50 gpm

Maybe "correlate" was the wrong word.... hows about "calculate":

EG @ 40%:
SG = 1.045 ---- density = 1.045 * 62.43 = 65.24 lbm/ft3
SH = 0.8218 BTU/lbm/F

(50 gpm) * (60 min/hr) = 3000 gph
(3000 gallon / hr) * (.13368 ft3 / gallon) = 401.04 ft3 / hr
(401.04 ft3 / hr) * (65.24 lbm / ft3) = 26160 lbm / hr
(.8218 btu / lbm / F) * (26160 lbm / hr) * (5 deg F) = 107491 btu/hr
107491 btu/hr ~ 9 tons as is the specified tonnage.

I have checked the glycol with the refractometer, and we are at about 38% - Again pretty darn close to what we are suppost to have. The three circuits that we run to are the original circuits. The only changes are that I added some paddle wheel flow meters to confirm glycol flow.... In general, the lowest we seem to be able to get the glycol down to is about 20deg F

OK - here are some actual readings reported this morning:
Glycol flow = 44.5 gpm
glycol supply = 23.6 deg F
glycol return = 26.8 deg F

As you can see, the heat removed is approx 5.1 tons (well below capacity) and the chiller is only getting down to 23F

This week when I check it out on site, I will gather a set of data with no loads (very small load - I will bypass all AHUs), and a set with full load (all AHUs on and open to high outside air temps)..... Hopefully these two sets of data will be illuminating.

Thanks for all the help everyone, and I will report back with some numbers when I get them!

[crab master]

Okay thanks for the clarification - I'll be interested in what you find, as I can think of at least a dozen more questions at this point.

[beenthere]

Do the unloaders energize, or deenergize to unload. And has anybody checked if they are working right. You could have a burnt out selenoid on one of the unloaders.

[hvacer2]

We checked the unloaders with a screwdriver (touched solenoid heads to see if they were energized). They were all non-active. We went through the schematics, and confirmed that they should energize to unload.

Hmmm.... Just as a sanity check, I will review the compressor documentation to confirm that energized = unloaded. We did replace the compressor several years ago and I guess it is possible we got one back with "de-energized unloaders" vs "energized unloaders".

I will add this to my list of things to check this week!!
Thanks beenthere!!!

[madhat]

You need to go here and fill out their form for a free analysis kit, for a rush job there is a charge. 1993 Glycol with no maintenance done on, you have big problems. Your refractometer is useless on broken down glycol, as you cannot get a consistant sample, and will be reading what ever has settled around the valve. http://www.dow.com/heattrans/contact/

[hvac248]

50GPM 9 tons ????

[gonefishing]

Those un-loaders, particularly the hot gas type, suction cut-off are nearly as bad
are guilty of allowing some by-pass killing efficiency. Also, might check for a week
or slightly mis-adjusted hot-gas by-pass valve.

GF

[jogas]

you will need to have a low load condition...and see if the HGBP maintains

good luck.

Or throttle liquid line 'till suction drops and HGBP starts opening. Observe and set as needed.
jogas

[hvacer2]

MADHAT:
I checked with our guys, and there was a complete system flush about 5 years ago. I did look at the link to what you sent, and it looks like a good service for us to start using regularly.....

HVAC248:
Yep - 50gpm / 9 ton. Seems to be a sticky point with lots of readers, but I confirmed with the manufacturer that this is the spec.

GONEFISHING / JOGAS:
HGBP and offloaders are going to get the "special attention" you suggest, on my next visit to the facility.

[bertoh]

what kind of compressors? what kind of chiller??? what are these temps , flows etc being measured with?? dont rely on controls on chiller verify....what sort of ahu's are taking 23 deg glycol???? are the defrost 's working on ahus- maybe you dont have a load for this chiller... is it fully loaded at 23 deg.. can you close the HGBP and watch suction press or LWT. any oily looking spots around the chiller

would help to have the press temps SH SC etc

[flange]

While it is not unusual to see these types of process chillers in the marketplace, when some mechanics see these types of machines they get glassy eyed. So what i am hearing is that you have a flow rate that is less than design and you cannot make temp right? Is there any possibility that the tubes are fouled? Can you shut down and pull the heads? Is it possible that the high flow rate has eroded the divider plate or gasket inside the head and is allowing your brine to bypass the evaporator? My guess to these questions is no in both cases, or you may just short cycle the compressor. Now, have you taken good readings on your refrigerant circuit? pressures, temps, SH, SC. You can pull the coils off of the unloaders to check that, but I doubt that is your issue. I am guesing fouling of the tubes or refrigerant charge.