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Old 06-19-2007, 07:50 PM   #1
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physics question

I know many of you have engineering backgrounds - can you point me in the right direction?

I have a chiller unit and I am trying to determine the flow rate of the chilled water going through the unit. I know the incoming and outgoing pressures, but not the flow. Can I figure out the flow from the difference in pressure on the two sides?

Where could I go to figure this out?!

thanks,
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Old 06-19-2007, 09:53 PM   #2
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Differential pressure alone will not give you the flow rate. It is possible to have the same pressure drop on two different systems with greatly different flow rates.

If you have information on the pump you might be able to get a rough idea of the possible flow rate, but the actual flow will depend on system restrictions. A flow meter is the only way for an accurate measurement.

Is there any way you can open the system and catch the output flow and time the delivery for a know volume?

There may be data for your particular unit available online, but actual performance will depend on the installed system and piping restrictions.

A link with some rough estimates of chiller flow rates Chiller flow rates and guidelines for chilled water system piping
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Old 06-19-2007, 10:23 PM   #3
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Maybe I can help.

Let's start here:

Are the water pressure taps on the pipe going to and from the chiller? What are those pressures going in and coming out?

What do you know about the chiller? Is it a "shell-and-tube" type or a "plate-and-frame" type?

Is the water flow on the "tube-side"?

I will have more questions but gotta have these answers first.

Ed
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Old 06-20-2007, 06:01 AM   #4
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I dont know what type... i just look after the building in general.
I have two different contractors / repair guys trying to fix it and they are arguing whether or not there is enough flow. The pipes go in / out the side of the chiller, and the gauges read 56psi and 62psi. Neither wants to take anything apart to put a flow meter in there.

This thing shuts itself off and locks out on a regular basis, and it says "low pressure control" which is the refrigerant. Refrigerant has been added and is above what it should be. One contractor says that air flow across the coils and water flow in the pipes will also affect the temperature of the refrigerant which will change the pressure, and if there is no flow, then the refrigerant will continue to cool to a point where it contracts so much that the pressure drops to the point where this safety opens and locks out. The other guy thinks this one doesn't know what he is doing...

I just watch and listen, and hope that one of them figures it out!
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Old 06-20-2007, 07:36 AM   #5
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I am an Engineer, but without retaking thermo and fluids, I wouldn't want to touch this problem.

If you need the pump designed on a CAD system, let me know.
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Old 06-20-2007, 11:08 AM   #6
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Questions Does the chill water run in thoses two pipes?, What is the suction and discharge pressure of the unit while running? What refrigerant is used in the machine? What is the differintial tempature between the two pipes? What is the rated tonnage of the chiller? What is the tempature drop across the cooling coil(s)? The machine could be low on charge, have a bad cooling pump impeller, low water flow, plugged cooling coil(s) air or water side, or plugged air filters.
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Old 06-20-2007, 11:22 AM   #7
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This thing shuts itself off and locks out on a regular basis, and it says "low pressure control" which is the refrigerant.
Gosh do I miss submarine refrigeration systems.

One cause of this symptom could be low flow (peanut butter or other corrosion in the refrigerant pipes). The compressor is sucking really hard on the refrigerant pipe but can't get enough flow to avoid overheating. Blockages could include the refrigerant filter, the dryer/dessicant, or air/water leaks in the pipes that have rusted into atherosclerosis.

While it'd be endlessly fascinating to debate all the possible causes of the symptoms, their thermodynamic impacts on the refrigeration cycle, and their possible repairs, those experts must be charging you a pretty high fee for this engineering experience. Let me step back from the problem and ask a big-picture question-- how old is this system? Would it be cheaper in the long run to replace it with a new EnergyStar chiller and just do a postmortem autopsy on the corpse with a hacksaw? Calling Doctor Rich!
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Old 06-20-2007, 12:07 PM   #8
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I would use Google and track down the manufacturer and ask for the specs.
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Old 06-20-2007, 01:50 PM   #9
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it's a Trane 60 ton, Trane is involved, it is less than 10 years old and it has been like this from day 1... including Trane, I have 3 people working on it telling me 3 different things that are the problem... i dont really know where to go with this... it just goes around in circles!
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Old 06-20-2007, 10:27 PM   #10
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Originally Posted by pl05br View Post

Can I figure out the flow from the difference in pressure on the two sides?

Where could I go to figure this out?!

thanks,
The simple answers to the original question - NO, NOWHERE, and You're Welcome.

Liquid flow and pressure is analogous to current and voltage (people usually use the analogy the other way 'round, but I'm an electrical engineer, so there you go).

If I know the voltage on each side of a device, I cannot know the current (flow) unless I already know the resistance of the device. Measuring the current allows me to calculate the resistance (again, if I know the voltage/pressures).

You have two unknowns and one known. You need one more known - either the flow, or the resistance. And if you get the resistance from a data table, that won't tell you if the device is damaged or clogged, and not to spec changing the actual flow. Which, since it isn't working, is a possibility.

But, if you measure the flow, and calculate the resistance based on the pressure differential, you can tell if the device's resistance to flow is in spec. But of course, your problem could be elsewhere in the system.

-ERD50
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Old 06-21-2007, 12:09 AM   #11
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i don't even know why i opened this thread in the first place.

but while i'm here i just want to say that only nords could make refrigeration sound interesting.
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Old 06-21-2007, 10:25 AM   #12
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Is the machine air cooled or water cooled? If things are working properly your flow should be 120 gpm +- 20 gpm for normal cooling delivering a 55 degree f air temp. Assuming the cooling medium is water 120 gpm x 8.33 = 1000 lbs of water per minute. 60 tons cooling = 60 x 12000 BTU/Ton = 720000 BTU /hr or 1 ton per minute of cooling If you know the delta T of inlet to outlet water both at the cooling coil and the chiller you can determine what the machine is supplying and what the cooling coil is delivering to the space.
The design engineer might have made a mistake. The install might be incorrect. Or the machine could be oversized.
Plus all the stuff Nords said.
As a Coastie I attended US Navey AC&R School in San Deigo 1974. Some of the best training in the world for ACR Mechanics. When your 200 ft under water and your AC quits its a little hard to use the Yellow pages to get help.
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Old 06-21-2007, 10:56 AM   #13
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but while i'm here i just want to say that only nords could make refrigeration sound interesting.
Not to speak for Nords, but I'd bet it's much more interesting in hindsight...
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Old 06-21-2007, 01:44 PM   #14
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but while i'm here i just want to say that only nords could make refrigeration sound interesting.
Years of giving training on nuclear power. Poking four-knot holes in the water for 90 days at a time is like attending the Jerry Seinfeld School of Stand-up Comedy Training with George Carlin heckling from the cheap seats. Of course we all tended to look more like Larry Miller.

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Not to speak for Nords, but I'd bet it's much more interesting in hindsight...
Things that weren't funny at the time:
- Submarines used to carry spare bottles of R12 at sea until a medical study noted that R12 sucked through a lit cigarette is catalyzed to phosgene gas. Suddenly all those bottles ended up stored in the airtight escape trunk or on shore...
- R12 is tremendously hygroscopic and corrosive. During an inspection a CO picked at a piece of "dirt" on a pipe, saying "What's this?" as the chief cringed at realizing he was scraping on rotten refrigeration piping. Sure enough, "Ah, it's just loose rust" followed by "Hey, what's that hissing sound?"
- Don't let people scrape frost out of the freezer with sharp knives. Especially don't let them stab icepicks at the stubborn chunks of ice covering the R12 lines...
- 1980s R12 "detectors" were so finicky, expensive, & unreliable that they were ignored. Instead, machinist's mates would duct-tape condoms to suspect valves or piping to see what came up (so to speak). Most people are used to visually associating condoms with a completely different type of scenery, but I can only imagine what this libidinous imagery did to the psyches of a generation of these mechanics & their loved ones.

I could go on all week. Luckily I can sleep pretty much all the way through the night now.
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Old 06-21-2007, 05:24 PM   #15
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Speaking of phosgene...

Back in the good ol' days, we use to deposit doped SiO2 using atmospheric chemical vapor deposition, in a tool we lovingly called the "flame deposition" tool, so-called because of its tendency to clog, screwing up the balance of gases, then literally shoot flames out the sides of the mixing head. Imagine a metal plate, heated by internal heating rods, hooked to a conveyor, which slowly made passes back and forth beneath a mixing head, consisting of numerous thin plates bolted together. Now imagine flowing silane, oxygen, and a dopant gas through the mixing head. One many occasions, the head would clog with residue from the reaction, throwing off the ratios. Silane, in particular, is prone to spontaneous explosion when exposed to air, but sometimes it just "powders" everything with oxide dust. In either case, looking up to see flames shooting out of the thing was quite exhilarating...

To make a long story longer, we used arsine, phosphine, and diborane for dopant gases. These were stored in a SS gas cabinet in the SCRAM room with the "flame deposition " tool. One day, we noticed an odor out in the main part of the fab, and, upon investigation, we discovered it was a diborane "leak". I say "leak, but what really happened was the installer had put an in-line filter in the gas jungle inside the gas containment cabinet which was not welded, but was two-piece with an o-ring. Problem was, after some period of time, the o-ring deteriorated, and, when it gave way, an entire bottle of diborane gas was vented to atmosphere. Thankfully, the cabinet was well exhausted, and most of the gas went up the stack to the scrubber system. I say thankfully, since diborane is HIGHLY toxic... As an aside, the toxic gas detector for the SCRAM room was in "calibration mode" at the exact time of the leak, and didn't issue ANY warning, nor provide ANY record of the concentration level... I fully expect to grow a extra limb someday, as a result of the exposure...
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Old 06-21-2007, 06:43 PM   #16
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Ah yes phosgene, memories of laying in the bilge of a Cutter brazing up a leak in an unpurged R-12 line. Couldn't breathe right for a week. Or walking into the Ammonia compressor room coming on shift at the Dairy and going instantly blind from tears caused by a small ammonia leak. Or at the big hospital working on a 4000 ton R123 chiller plant located one level ABOVE the boiler room knowing that if a bad leak happened all of that heavier that air R-123 would drop into the boiler room displacing all the air. Yep those were the days......

Speaking of which 5 and a wake up to permanant vacation.
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Old 06-21-2007, 07:23 PM   #17
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In either case, looking up to see flames shooting out of the thing was quite exhilarating...
It's amazing that any of us survived our attempts to entertain ourselves earn a paycheck.

Submarine firefighting trainers
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Old 06-21-2007, 10:39 PM   #18
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ERD50 wrote:
Quote:
And if you get the resistance from a data table, that won't tell you if the device is damaged or clogged, and not to spec changing the actual flow. Which, since it isn't working, is a possibility.
I am afraid ERD50 is right.

kcowan has a good idea.

The first thing I would do is try to get hold of a manual for this package unit. It is a standard design, so Trane should have one. It should have normal operating parameters in it and troubleshooting information. One should have come with the machine--check your files.
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