Engineering problem? Hmmmmm

[herefishy]

As such, unbeknownst to me, is the reason that I use the term "Design TD", when sizing a system up, relative to desired (RH) conditions?


(Dear God, please let me not have stepped off the cliff, again. Amen.)

[herefishy]

I have never cared about dT on the evap side of the thing. I don't do air conditioning, and typically I want NOT to dehumidify in a majority of the applications I engage in. Except for the meat plant loading dock, for example.

[Gary]

I don't think you have stepped off a cliff at all, herefishy. Your reasoning was sound, just incomplete, not considering all of the factors involved.

In general, increasing evaporator airflow increases sensible heat removal while decreasing airflow increases humidity removal.

A lot of people believe that increasing surface area increases humidity removal, but it in fact amplifies the above effects.

[Gary]

Fora constant volume flow I see reduced bypass factor and therefore increased dt and raised SST and very likely a raised ADP and therefore SHR too. However, since total duty would have increased with sensible duty increasing by a greater proportion to latent increase, does the absolute latent duty increase of decrease or remain?

Decrease due to shorter run time.

[bernie]

Ok let me jump in in much simpler terms that I can understand.
I'll be honest I'm not up to speed on all the abbreviated terms so here goes.
I think of a systems capacity as how much work is it doing.
When we consider an evaporator, and what its job is, it seems simple to me.
Remove heat from space.
The more heat the more work it has to do.
If we use a low temp evap for an example, and lets say its just out of defrost, what happens? On direct expansion coil the liquid enters thru distributors tubes and picks up as much heat thru absorbsion untill the last drop of liquid boils off, all of this is happening rapidly, there are no breaks our lunches during this time, much work to be done. The more heat the more absorbsion that takes place and the greatest volume of refrigerant is needed.. Now that same box an hour later, whats happening? Liquid actually begins to back up in the coil. There's
not much work to be done , suction temp and pressure have fallen, box temperature has fallen, and so has the capactiy of the
evaporator. Think about it, Its worked so hard the last hour and comsumed so much heat. Its full of liquid now, there hardly any absorbsion taking place. Things are starting to slow down inside the coil of that evaporator, refrigerant velocities thru the tubes are at their lowest, and to further complicate the issue some oil has taken a break from the refrigerant and justs hangs around in the evaporator getting inthe way of our dedicated refrigerant. This is how I know capacity is reduced when the s.s.t temp is reduced. I know it kind of a simplistic explanation for the qualified techs in this forum, but it works for me. Im prior service and the first thing I learned was the acronymn K.I.S.S.

As far as my problem with Keeprite goes, Im in a holding pattern waiting to see who is going to pay for all this. Keeprite is dragging their feet and probably will hire their own people to correct their mistakes. I hope I get the chance to at least finish the store I started. I should have an answer tomorrow as to the next course of action. tHIS HAS BEEN A GREAT READ !!!

[herefishy]

bernie.....wassuuuuppp???????

[bernie]

Funny you ask, Im working on an update right now :>)

[bernie]

Picture if you will looking down upon a typical condensing unit.
Air cooled design, 2 fans etc. Notice the suction line as it enters the suction accumulator. immediatley ahead of the accumulator are 2 seperate copper tubes that are brazed into the suction line at the 3:00 position. These tubes are the 1/4 sensing pressure tube and the 3\8 tube is carrying hot gas from a A4 0/55
discharge bypass valve.
Now if we move even further upstream of the accumulator there is another tube that is brazed into the suction line at the 9:00
position. This tube is adding liquid from a desuperheating TEV.
The seperation between the discharge bypass valve and the desuperheating TEV as they enter the suction line is approx 12".
The desuperheating TEV sensing bulb is strapped to the suction line between the 2 controls. I have been advised by the manufacturer of this unit that all I need to do is move the sensing bulb 6" from the suction inlet at the compressor after the accumulator. Will this work? Will the hot gas travel thru the accumulator and on to the compressor before the liquid has a chance to mix with it?

[bernie]

Here is a photo of the described unit.

[herefishy]

Is it just me?, Or does that de-superheating TEV look like it has an external equalizer connection.... WITH NOTHIN' HOOKED UP TO IT? Is that a flare cap I see on the side of the valve?

It seems to me that we have dual protection, here. De-superheating TEV, and suction pressure control.

I might agree with the factory recommendation. The de-superheating TEV I believe to be applied to offset the hot gas superheat during "unloading". It is the conditions at the compressor that are of concern, and during unloading the additional SH from the bypass is wanted to be negated by the de-superheater. I would imagine a superheat setting (at compressor) of around 20degF, perhaps.

In regard to your "will it mix with it?" question, the liquid from the de-superheater will be evaporating, it doesn't "mix" in that sense.

[Dan]

There is some sort of stub on the desuperheating valve that goes nowhere, but it looks like a casting for an equalizing line connection to me.

Hmmm. Wish I could see the compressor model No. so I could check its parameters. I still don't see why we have all this "protection" designed into what I would consider a standard application that should not have hot gas bypass, liquid injection and a MOPD powerhead. The combination of hot gas bypass and a ZP powerhead seem to me to be working two ends against the middle. Perhaps the compressor is operating below its rated envelope without liquid injection, but I see competing theories regarding protecting the motor.

I'll stick with making all the fancy stuff history and let the unit work like the other millions of condensing units achieving the same duty with basic pumpdown controls.

Why on earth do we have hot gas bypass on this unit?

[Dan]

I looked up the unit and specs. Is it an AVA2490z compressor? Hmmm... Tecumseh. But the standard features and options do not include hot gas bypass nor liquid injection. "More options available upon request." I will search, but last time their website had nothing to offer.

Methinks somebody requested too much - liquid injection, maybe, but not hot gas bypass.

[Dan]

Their website still has nothing to offer. I will just trust that their ratings include liquid injection. 9500 btu/hr @ -10. Actually, if that rating does not include liquid injection, then I don't think the 7600 btu/hr at -30 does either. Hmmm. I keep going back to taking the hgbp and zp charge out of the picture. And if I could get my hands on the setting for the liquid injection, I would make sure that I am no higher than what is called for in Tecumseh's literature.

Ok. I am done.

[bernie]

And if I could get my hands on the setting for the liquid injection, I would make sure that I am no higher than what is called for in Tecumseh's literature.

Im assuming your refering to the compressors suction superheat.
If this is true, I'd say that a properly sized expansion valve, well
insulated suction line and a check at final temp should be all that is required. If my super heat was too low I could always add a liquid heat exchanger.
Personally, I believe they had these left over from a previous order that canceled. They inturn sold them to an unsuspecting consumer praying that the installing contractor was qualified to set these controls up properly. As you've learned from this post this was not the case. :>) So I am forced to swallow these
explantions from keeprite, go back and move the bulb and hope for the best.

[bernie]

Did I mention I am NOT THE INSTALLING CONTRACTOR.

[Prof Sporlan]

There is some sort of stub on the desuperheating valve that goes nowhere, but it looks like a casting for an equalizing line connection to me.

Dan, you are correct. The valve is an internally equalized Sporlan Type G or EG valve. We use the same brass body forging for both the internal and external versions (it is a cost savings issue). And in the case of the internal version, we obviously need not machine the fitting.

The outlet out the desuperheating TEV looks to be piped too close to its sensing bulb.

Is the AJ valve is feeding downstream of the desuperheating TEV sensing bulb?????

[Dan]

So I am forced to swallow these
explantions from keeprite, go back and move the bulb and hope
for the best.

Bernie, Forced? Hope? You are not to assume the posture of a victim. If you worked for me, you would be advised to take out the hot gas bypass and change the powerhead. I would take the liability and expect no payment from Keeprite, although I would submit bills.

I just want you to make this sucker works right so you can walk away and venture into another trouble.

I am not sure I said that right.

[bernie]

Ok, Let me rephrase my earlier post. I have expended so much
energy thinking about what Im going to do to resolve this situation and get paid. I know what has to be done, and know what I should do. With that said I've been told what I need to do
by my customers field engineer. His recommendations are not what we have or I have decided to do. the professor points out that the hot gas is discharging down stream of the liquid injection point. Im not an design engineer but I can read. I have yet to see documentation that states it is permissible to pipe a system this way. Yes an accumulator is a good mixing chamber for the liquid and hotgas. BUt in our case the hot gas is flowing almost directly into the accumulator. It seems to me that the liquid will be chasing the hot gas and not mixing properly. As far as being a victim, I guess I just want to give them what they want to be spiteful. It seems knowone wants to admit they dropped the ball on this one and they are doing everything they can to cover their a##. Please let me know if you gentlemen agree with my concerns about proper mixing. Thankyou
Bernie

[Dan]

I agree with you about proper mixing to a small degree. But picture this: If you have cold expanding liquid being injected into a warm suction stream with or without a chamber such as an accumulator, doesn't the stream ultimately reflect the sum of the mass temperatures? If not, where does the "cold" go? Are you picturing a laminated suction vapor entering the compressor and worrying that there is a hot portion and a cold portion that doesn't do its job properly?

I cannot think of a better mixing chamber than a rotor spinning at 3600 rpm,s.

I guess I just want to give them what they
want to be spiteful.

I certainly understand that sentiment, but I have learned it to be more satisfying and profitable to fix the problem and move along even if I eat the cost and get criticized for design changes. But I know where you are coming from. And heck, I could be wrong with my suggestions. But an hour with a torch beats the hell out of an hour on hold with a factory. I favor wrestling the problem to the floor even if I am the one who gets pinned.

[Prof Sporlan]

I have yet to see documentation that states it is permissible to pipe a system this way. Yes an accumulator is a good mixing chamber for the liquid and hotgas. But in our case the hot gas is flowing almost directly into the accumulator. It seems to me that the liquid will be chasing the hot gas and not mixing properly.

With the desuperheating TEV sensing bulb upstream of the hot gas injection point, the desuperheating TEV is, in effect, controlling the wrong superheat. This would be analogous to a temperature sensor in a refrigerated case positioned such that it cannot measure proper air temperature, and expecting it to maintain desired case temperature.

With the desuperheating TEV sensing bulb downstream of the hot gas injection point, which will have to be downstream of the accumulator the way the system is piped, the TEV will sense superheat entering the compressor, and respond accordingly. The fact that liquid is being injected upstream of the hot gas injection point shouldn't be a problem as long as there is sufficient volume for the hot gas and liquid to mix, which should be the case in the accumulator.

A better way to do this is to pipe the lines leaving the hot gas bypass valve and desuperheating TEV together before entering the suction line, i.e., have the liquid and hot gas mix before entering the suction line. The current location of the desuperheating TEV sensing bulb may be ok with this arrangement. But the fact this system is so close coupled makes placing the sensing bulb downstream of the accumulator almost a no-brainer.

I favor wrestling the problem to the floor even if I am the one who gets pinned.

Makes for better war stories when discussing refrigeration problems...

[bernie]

Actually I pictured the higher velocity hot gas traveling thru the mixing chamber without giving up much to the cool vapor before reaching the compressor. I was also concerned that the liquid being injected, even though it is at the same pressure, is actually is moving slower thru the suction line absorbing heat from the warmer vapor. The combination of these creates a constant demand at the sensing bulb for more liquid than is necessary to maintain proper superheat at the compressor. That being said, Im quite confident in the professors logic on this subject, I would also compliment Dan on his view points too.
Everyone here on this board has been generous with their knowledge on this subject. As of today I'm still waiting for instruction from the engineers involved as to when we will proceed with the repairs. Time is important, These units are running 70F superheats at the compressor. TICK TOCK :>)

[herefishy]

I now fully understand this C.U. that bernie has, because I just completed a change-out in which I had to apply hot gas bypass and de-superheating.

The 5-hp A/C condensing unit that was serving the 5-case deli line which apparently had gone through two compressors in it's short (4-yr?) lifespan (mind you, I never replaced a compressor in it), failed again. I got the capacity requirements for the cases, threw the A/C off the roof (kersplat!), and replaced the it with a proper (3-hp) refrigeration condensing unit (15,000 btuh approx.).

At the time we did the replacement, each of the five cases was operating independantly, cycled by suction stop solenoid valves via t-stat. We coupled up four of the cases to create a 3-circuit system, with the fish case (lowest load) all alone, and installed EPR's set at case mfgr recommendation (18degF min. evaporator temp.). Yes, there were none installed (it was a used equipment installed by Johnny the Jackleg in the first place).

Upon starting up and checking out, we discovered (I anticipated) that at worst, when only the fish case was calling (2,400 btuh rated capacity), the C.U. SST would be at -10 (below minimum C.U. rating of +10), and superheat would rise up to 80degF. Working with my wholesaler, we spec'd and installed an ADRE hot gas bypass valve set at a minimum suction pressure of around +14 degF SST. The TEV employed was a Sporlan Q valve with appropriate orifice and a KT43-L thermostatic element.

Relays had to be installed downstairs in the cases (controlled by the thermostats), wired in parallel, in order to control the Hot Gas and de-superheating TEV solenoid valves, for (unloading) only when a case was calling. Otherwise, pumpdown would never be achieved when all cases were satisfied. Fortunately "Johnny" had enough spaghetti of wiring to control his A/C-C.U. to accommodate the control circuit from the cases to the new C.U.

When 'dialed in", the ADRE and TEV worked like an orchestra. We never saw an entering gas temperature to the compressor above 60degF (max rating 65degF), and superheat never exceeded 40 degF.

Happy compressor!!!

Happy customer !!!!

Happy compressor manufacturer !!!!!