PDA

View Full Version : Engineering problem? Hmmmmm



bernie
13-03-2002, 04:19 AM
Ive got a condensor that utilizes a discharge bypass valve(a-4-0\55),desuperheating tev(egs 1\6 L1), flood control(lac-4-215)and ranco fan cycling control set at 300psi.
This condensor has 2 fans, a 12# reciever and a suction accumulator.
Compressor superheat is at 70, at -5F box temp.
The discharge bypass line is locted and piped into the suction line in the condenser 2" ahead of the accumulator.
The desuperheating tev is located and piped in the suction line in the condensor about 10" ahead of the accumulator.
The sensing bulb for the desuperheating tev is located in between the DBV piping and desuperheating piping as they
enter into the suction line.
Condenser m#KH201L6HT3A-0790 (-30f SAT SUC TEMP @80FAMBIENT=
7670 BTUS PER HOUR.
EVAPORATOR M# KTW077ED-S2A-0566 (-20F EVAP TEMP @ 10F TD=
7650 BTUS PER HOUR, box design temp is -20F
EXPANSION VALVE (EGSE-1/2-ZP) IS INSTALLED.
SYMPTOMS ARE HIGH SUCTION SUPERHEAT AT COMPRESSOR.
WHATS GOING TO HAPPEN WHEN THE WEATHER TURNS WARM?
EVERYTHING IVE READ indicates I will not get the
proper mixing of the hotgas and liquid as it was piped from the factory. The sensing bulb is also located in the wrong place.
Why with a single evaporator would the even use this type of capacity control?
Ive been on the phone all week, trying to get an explanation from keeprite, Im getting lost in email

Had the box down to -15F and suction superheat at the
compressor was 57F
Looking at the data sheets for each piece of equipment,
and utilizing the expansion valve that they are. I
believe the tev is undersized for an oversized piece of equipment. Huh?
With a superheat that high at the compressor you would expect the box temp to be suffering but this is not the
case. It is my belief that the only time you can satisisfy your box requirements and still overheat your compressor is if the equipment is oversized.In this case the evap is oversized and the expansion valve is too small to flow the required liquid to maintain a reasonable suction superheat at compressor.
Am I making any sense of this, is my thought process flawed?
To top it all off the piping arrangment dosent match
any of sporlans guidelines or the manufactures own design drawing.
Anywho, I brought this to the attention of the keeprite engineering department, sent them a digital pic of the unit.
They in turn sent me a fax with the correct piping arrangement. told me to send a quote to repair. As soon as they got the quote I got an email stating that the piping would work just fine and all I had to do was change the location of the desuperheating tev bulb. What to do?
The darn guy backtracked on me, said he had numerous of these units on the west cost and they worked fine.

R-404a system
48-50F ambient
suction pressure 12-15 psi /suction temp at comp 42F
head pressure 225 psi /liquid line temp at outlet of reciever 92F
Box temp at time of reading -5F

Gary
13-03-2002, 06:52 AM
The subcooling at the receiver is 5F. This would indicate an undercharge. The evap TD is 19-23F, which tends to confirm undercharge, as does the high superheat. I suspect the coil outlet superheat is also high, which would leave no doubt.

I'm thinking this unit is undercharged.

I disagree with the notion that box requirements cannot be met with high compressor inlet superheat (assuming coil outlet superheat is right).

The desuperheating TEV bulb location seems wrong. Where did he tell you to relocate it to?

Gary
13-03-2002, 02:59 PM
I don't necessarily disagree with your analysis, Mark. however...

At 50F ambient, with fan control set at 300psig, the effects of the LAC should be minimal. It should not yet be in the picture, nor should the desuperheater and/or BPV be operating with the box temp at -5F.


...ensure there is sufficient charge in the system for the LAC to properly flood the condenser with still adequate refrigerant in the receiver for proper liquid delivery.

Therein lies the rub, and the Achilles heel of the flooded condenser system. How does one determine if the charge is correct (for all conditions), short of removing and weighing it?

bernie
13-03-2002, 09:54 PM
Ok Gary let me say this,
I reclaimed all refrigerant, sized up my condensor for winter
charge and compensated for the 50 ambient.
At -5F box temp the flood control was infact bypassing and the desuperheater was allowing liquid into the suction line due the high return gas temperatures.
As far as the fan cycle control is concerned it wont be a factor untill My ambients raise another 10F or so.
Personally I dont see the use for it, Its not needed in this application with a flood control set at 215.

Marc,
I relocated the desuperheating sensing bulb after the accumulator just ahead of the compressor. I have to edit my self here,
The suction temp I stated in the last post of 57F was infact after I moved the bulb.
Does the desuperheater provide a positive shut off as long as the suction superheat is correct?
What about the the piping of the two, they are reversed and Im concerned that the hot gas will get thru the accumulator before the liquis can properly mix.
The frost pattern on the end of the evaporator indicates that the
superheat is high, although my box temperature falls quite well
and cycles off at -15.
It seems to me that injecting liquid due to poor system performace(compressor superheat) will cause pump down problems when ambient is low. Could liquid get trapped in the refrigerant oil and cause lenghty pump downs?
I really appreciate all your input on this matter.
Thanks
Bernie Glasgow

herefishy
13-03-2002, 10:34 PM
Head pressure control, AND fan cycle???

Should I assume the fan is not running?

Is the compressor an air cooled semi-hermitic?

Can I assume that there is a sightglass in the liquid line and it is "full" (yes, I understand that that may not be indicative of liquid at the TEV)?

First thing that I want to know is what is superheat at the evaporator OUTLET (i.e. TEV setting / operation), and if adjustment required, does it respond?

Gary
13-03-2002, 10:42 PM
I reclaimed all refrigerant, sized up my condensor for winter charge and compensated for the 50 ambient.

How many pounds of refrigerant are in the system?

(I hate the new small font quotes)

The frost pattern tends to support my undercharge theory.


and if adjustment required, does it respond?

I wouldn't recommend messing with the TEV adjustment unless/until I am absolutely certain there is sufficient liquid at it's inlet.

bernie
13-03-2002, 11:07 PM
2nd fan is not running, the sight glass is full of liquid.
The reciever has a 12 # capacity. I agree with your undercharge theory, actually The evap and expansion valve do not matcha ccording to the data from keeprite, I should have a 1 ton valve in there.
Im not a big fan of adjusting tev's unless necessary, a 1 ton out of the box should be just rite for my application.
Knowing the operation of this system and that sporlan builds in some saftey factor to their valves, I could probably take a few turns out of the spring and achieve better results. I would prefer just to replace the cartridge a half size larger. How does this sound?
Total system charge is 12.5 #
compressor is hermatic recip # AVA2490Z.
It seems to me the manufacturer had a sale on the units,
I cant understand why they'd sell a condenser and evaporator
with an expansion valve that isnt rated to carry the load or the
liquid that the evaporator needs to properly work.
I tried to discuss this with them direct and they stated all is good
and let them do the engineering. Hmmmmmmm

Gary
13-03-2002, 11:44 PM
Hmmmmm... The plot thickens.

We have a full sight glass, and sufficient head pressure to push it through the TEV. If we can assume that the receiver is sized properly for the application, then the charge may in fact be a little excessive. Whatsmore the pulldown time is good.

I am coming around to your way of thinking, Bernie. It's looking more and more like an undersized orifice.

bernie
13-03-2002, 11:59 PM
Well I took my time on this one. I initially added 9# of refrigerant
and started the machine.
After about 20 min runtime the sight glass was half full.
At that point I began charging liquid thru my lo side charger.
When the glass cleared I did the math out of my sporlan
book on winter charges and compensated for 50 ambient.
It was like 1.6# extra after that.
Im starting to come around to my way of thinking also Gary,
Thankyou.
All thats left is the DVB and desuperheater. I want to eliminate
it. I feel this set up is for units that are designed to run all the time and have mulitple evaporators. I cant see the harm in taking it out as long as the orifice and expansion valve provide proper suction temps year round.

Gary
14-03-2002, 12:12 AM
I cant see the harm in taking it out as long as the orifice and expansion valve provide proper suction temps year round.

The "harm" is giving keeprite an excuse to void their warranty. On the other hand, (letting them do the engineering), their own charts call for a 1 ton valve, right?

bernie
14-03-2002, 12:20 AM
In addition their schematic drawing of the condensor piping arrangement does match either.
They must have a lot of these out there and dont want or cant
afford the warranty claims.

herefishy
14-03-2002, 01:11 AM
Originally posted by bernie
WHATS GOING TO HAPPEN WHEN THE WEATHER TURNS WARM?


TEV capacity is going to INCREASE due to higher pressure drop across the valve.

bernie
14-03-2002, 01:13 AM
Units are shipped from the factory with 2 orifices loosely.
They are for r-22 and 404a #'s l3/4 or l-1 for 404a.
Im not sure which one is installed as I didnt perform the original installation. I can assure you in the coming days when I return I will check it when I replace the cartridge in the valve.

herefishy
14-03-2002, 01:15 AM
Originally posted by Gary
I wouldn't recommend messing with the TEV adjustment

Well, you're gonna check it, you don't have to mess with it..... what is the Evap outlet superheat? Inquiring minds want to know.

herefishy
14-03-2002, 01:17 AM
bernie...

we're on line at the same time, and you're responding to my posts before I edit them.... read the above, and I'll wait till I see a post from you.

bernie
14-03-2002, 01:21 AM
Evap outlet superheat is at 48.
Its crazy, superheat that high yet I can satisfy the box requirements, which is -10F

herefishy
14-03-2002, 01:27 AM
bernie....

Start at the TEV... see if it responds.... One thing that I've noticed in a same application id the failure of TEV element head at those temperatures.... I have a -30degF application, in two years, I've gone through (2) thermostatic elements. I'm considering the application and still figuring out what the deal is.... is it a Sporlan KT53ZP element?

superheat, way too high!!!!!!!!!!!!!!!!!!!!!!

herefishy
14-03-2002, 01:29 AM
In my situation, the customer doesn't call until something else manifests itself...... I'LL BET MY BOTTOM DOLLAR, A NEW THERMOSTATIC ELEMENT WILL DO IT IF THAT THING IS A KT53!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Which it should be, I believe, being a "G" body. That's what I've employed in my application that I refer to.

Dan
14-03-2002, 01:34 AM
I would lose the zp powerhead and replace it with a Z. I would eliminate the DVB and desuperheater.

I don't follow how a 7000 btu/hr load is too much for a 1/2 ton valve. Isn't the condensing unit sized to handle a 7000 btu/hr load? If so, why are only achieving -25 with a starving coil? The desuperheater, methinks. No reason you cannot accomplish sufficient desuperheating in the coil.

The prof might hate me, but zp charges should only be used when a manufacturer intentionally slights the motor size for the displacement.

Dan

bernie
14-03-2002, 01:35 AM
This equipment is less than 6 months old,
the element is a kt 43 zp
I have also had problems with the 53 elements

herefishy
14-03-2002, 01:38 AM
I agree. You may have a point there. Maybe the cross-charge is what is screwing you and I, huh?

I spec'd the equipment in my application, myself. What needs to be taken into consideration, is the maximum suction temperature rating of the condenser. Knowing Keeprite..... I'll bet you BETTER have that ZP charge... because they probably didn't leave much room for higher suction ratings!

herefishy
14-03-2002, 01:40 AM
Have you attempted adjusting the TEV?

bernie
14-03-2002, 01:42 AM
From the data sheets this evaporator will provide 7650 btus,
Granted if I turn the valves adjustment out I'll probably
be close on the superheat.
The kicker is they reccomend a 1 ton valve for this specific application in those same data sheets. Yet, it shipped with a 1/2, go figure.

herefishy
14-03-2002, 01:44 AM
NO..... in my book, the #1 valve is a no-no.

herefishy
14-03-2002, 01:46 AM
mind you, the valve with the number "1" on it is not a one ton valve!!!! The rating of the valve is dependant on the application!!!! the "1" is "nominal".

herefishy
14-03-2002, 01:49 AM
In regard to the evaporator.... it does not provide 7,650 btuh. It provides 765 buth for every degreeF of T.D. between the S.S.T. and the "room" temperature. I can mathematically make that coil produce 1,000,000 btuh refrigerating effect!

bernie
14-03-2002, 02:06 AM
Dam I hate math,
If I use a SQE valve body
A BLUE q valve cartridge
and kt 43 szp or rzp,
Ok, the element decides capacity along with cartridege selection.
I gotta ask, What book are you refering too?

bernie
14-03-2002, 02:23 AM
Well said Mr O'Brien,
Are you always up this late or you a earlier riser, Either way
thanks for your input.
Were's the prof these days?
Hes lurking somewhere:)

herefishy
14-03-2002, 02:32 AM
I am certainly interested in this TEV application scenario....., because Marc, if you are right, that is probably the nature of my problem.....

but I stand by my guns, however the track record doesn'r support it, I'll admit.

I may be changing a TEV spec on the result of this diagnosis.

.....BUT I'M READING MY "201"!!!!!!!!!!!!!!!!!!!

Sporlan support ph. # 636-239-1111

I hope I don't get in trouble for that........

But I think the 1/2 is good app.

herefishy
14-03-2002, 02:38 AM
Originally posted by Fridgetech

There is no doubt that you should have the 1 ton orifice in there...


...Then because the valve is undersised


What's undersized?... the TEV, or the orifice?

What I am really trying to visualize... is that when you decrease S.S.T., you increase Pressure drop and valve capacity increases accordingly. I don't get anyone spec'n a "1" in this app. in relation to the figures in the table.....

I'm going to my Sporlan software, now.

bernie
14-03-2002, 02:40 AM
I guess Im ignorant, but tell what the 201 and Ill get a copy of it for sure. Thanks herefishy
Hey Dan Im all in favor of dumping the zp charge, although with the frequent defrosts I dont think the accumulator is enough protection. Some say an pump down is all the protection you need. I dont like CPR in this specific application due to the increased pull down times also.

herefishy
14-03-2002, 02:43 AM
refer to edited post

herefishy
14-03-2002, 02:48 AM
What max. ambient do you see in chicago?

at 110 condensing the software selects the 1/2.....

In Scotland you would use a "1"

Marc.... try it at 110degF condensing, then 90degF condensing.

herefishy
14-03-2002, 02:57 AM
............TEV..................

call or figure it out. It's either malfunctioning or misapplied.

....I say malfunctioning......... I'm out of here, I'll check y'all out tommorow.

see ya',

Mark

herefishy
14-03-2002, 06:23 AM
[i]Originally posted by Fridgetech[i][/B] When you decrease SST you tend lose out on net refrigeration effect faster than you gain on pressure drop. If the compressors capacity is reduced by a greater amount, which it normally is, then the net effect on comp-tev ballance is for reduced superheat operation. [/B]

But Marc, in the situation that the pressure drop decreases (low ambient situation), that is when the condensing unit (compressor) has it highest capacity due to the increased volumetric efficiency resulting fron the lower head pressure, and furthermore the increased T.D. of the condensing coil!

As such, you should still maintain your superheat.

As valve capacity decreases, C.U. capacity decreases according to the drop in S.S.T.

As valve capacity increases, C.U. capacity increases due to resulting increase in S.S.T. As such the system maintains a balance that is inherant under any given conditions whether right or wrong (in consideration of the design criteria producing the desired effect).

Gary
14-03-2002, 02:34 PM
Reading some of those old articles on alt.hvac I see that Gary has always had way with summary's, they a joy to reread after all these years

Is this a hint or what?...LOL

Okay, I'll give it a shot:

In a stabilized closed loop series system, the mass flow is identical at all points, with the mass building up behind the chokepoint(s).

Imagine that once each minute I hand you a dollar and you hand me two dollars. Before long our closed loop system will stabilize with the rate of flow at all points being one dollar per minute, and the mass buildup is in my pocket. Your ability to transfer funds (increased capacity) is overridden by my ability to hold onto them (decreased capacity). I am the overriding chokepoint.

In a refrigeration system, (assuming no other restrictions) the overriding chokepoint can be either the metering device or the compressor. And like a dam in a river, the mass rises before and lowers after.

herefishy
14-03-2002, 04:42 PM
Originally posted by Fridgetech
[I]... if you're letting head drop with ambients then as the compressor power reduces the condensers total heat rejection reduces and so sat cond temp is allowed to drop closer to ambient i.e. reduced TD.

... up to the point that head pressure/temperature control is active. At that point the "potential" capacity of the coil surface would be greater. However, I understand that the use of condensing pressure controls negate that "potential' via decreased airflow, refrigerant flow, or decrease of the active area of the coil. So I concede.... somewhat


If you look at a PH chart, you can see that for any given head pressure and liquid temperature, when the sat suction drops the net refrigeration effect is reduced.. and you see the compressors capacity is reduced by the lowering of vapour density and too the reduced volumetric efficiency of account of the increased compression ratio.

Yes, however due to the lower evaporating temperature, the net refrigerating effect of the unit cooler is increased. And there lies the effect which balances the system.

(I don't know how I got that "bold" print turned on. I don't mean to yell, sorry)

herefishy
14-03-2002, 05:08 PM
Evaporator capacity is directly related to the surface area and the temperature diffence between the surface, and the medium being cooled (refrigerated space).

Say for example, a BOHN ADT 090. The capacity of the coil in consideration of the airflow created by the fans is 900btuh per degF T.D. In a typical 10degF application (i.e., 35degF space, 25degf S.S.T.) the unit cooler will have a net refrigerating effect of 9,000 btuh.

But in a 12degF T.D. (S.S.T. @ 23degF in the above example) The net refrigerating effect of the unit cooler will be 10,800btuh

herefishy
14-03-2002, 05:22 PM
Marc,

Really I'm interested in learning. I know you've got a lot to offer.

LMTD?

SHF?

Anyway, have we determined that the TEV is bad yet?

herefishy
14-03-2002, 05:33 PM
Originally posted by Fridgetech
A reduced SHF will tend to increase LMTD for a given TD by reducing dt i.e. raising leaving temperature.

Actually, that occured to me as I posted it.

But wouldn't the (properly applied) TEV's (possible) reserve capacity allow such a scenario (maintaining SH) up to the point that the unit cooler drop in T.D. would become "static" when having exceeded the range of the (expansion device)?

The system is going to balance. The whole LOT cannot simply drop, unless something is broken. Right?

herefishy
14-03-2002, 05:53 PM
Originally posted by Fridgetech
less vapour from the fed liquid, the reduced superheat closes the TEV some and the subsequent/coinciding lowering of SST reduces suction vapour density which reduces system mass flow and so a new ballance point is found at a reduced TD and SH with reduced SST and mass flow.

..... which is balanced by increased evaporator capacity......

herefishy
14-03-2002, 06:03 PM
I probably do, but am not familiar with the acronym.

herefishy
14-03-2002, 06:13 PM
Originally posted by Fridgetech
Lol, the reduced return air temp reduces evap TD and so it reduces evap capacity

... and increased return air temp increases evap TD so it increases evap capacity

herefishy
14-03-2002, 06:19 PM
Are you saying, that simply evaluating the TD in terms of sensible (dry bulb) temperatures is the basis of my "misunderstanding".

That actually the same amount of work is being done but as conditions near the design temp, more of the work is latent. As such the variances in the sensible TD's?

herefishy
14-03-2002, 06:43 PM
Originally posted by Fridgetech
if the compressor is losing capacity how can the evap be increasing capacity?


Okay, only to keep beating this dead horse.... I am assuming INCREASED compressor capacity due to lower ambients. You agree with that, right?

So, compressor capacity increases, resulting in increase of evap TD, as such evap capacity increases.

Where this started was you're statement regarding lower compressor capacity resulting from lower SST due to lower PD across TEV.:p

Is it me, or are we debating our agreement?

:rolleyes:

herefishy
14-03-2002, 07:37 PM
I am not proposing a "starved" coil condition in regard to increase in evap TD.

herefishy
14-03-2002, 09:54 PM
Okay, Marc.

I admit it. I've never considered anything along the line of SHR. I understand the basic concept of LH+SH (since you explained it), and no, I don't know what the acronym LMTD represents.

Yes, as SST drops, so does compressor capacity.

I was engaging only on the basis of the TD*btu/hr. degF thingy.

I visualize the SST and compressor capacity reaching a "static" condition in relation to the load. And I think that the way I was addressing the subject, someone could possibly have concluded that capacity would increase infinitely as SST dropped.

Yes, as SST drops in approaching design conditions the TD is not increasing it is decreasing.

I think I may be having trouble expressing myself. My only hope is that I am at least amusing.

herefishy
15-03-2002, 12:15 AM
Marc, I've been to your site before, and checked out the optichiller info. Good concept. However, I'm brousing your page, now, and I'm having trouble locating the information regarding to what you reference.

Should I be observing the tabulated data page?

Gary
15-03-2002, 12:23 AM
I was engaging only on the basis of the TD*btu/hr. degF thingy.

You were ignoring the *btu/hr side of the equation, and focussing only on the TD. A drop in SST indicates that the btu/hr has decreased more than the TD has increased. Unless, of course, the SST has dropped because the compressor is pumping faster. :)

herefishy
15-03-2002, 12:40 AM
Thanks.

Has bernie been back out to that job? I hate it when someone starts all the racket, then walks out of the room while we're beating each other up!

herefishy
15-03-2002, 12:54 AM
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
15-03-2002, 01:01 AM
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
15-03-2002, 01:27 AM
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
15-03-2002, 02:41 AM
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
15-03-2002, 04:44 AM
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
27-03-2002, 06:22 PM
bernie.....wassuuuuppp???????

bernie
28-03-2002, 01:20 AM
Funny you ask, Im working on an update right now :>)

bernie
28-03-2002, 03:45 AM
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
28-03-2002, 03:52 AM
Here is a photo of the described unit.

herefishy
28-03-2002, 04:08 PM
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
29-03-2002, 01:35 AM
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
29-03-2002, 01:55 AM
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
29-03-2002, 02:17 AM
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
29-03-2002, 02:38 AM
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
29-03-2002, 02:43 AM
Did I mention I am NOT THE INSTALLING CONTRACTOR.

Prof Sporlan
29-03-2002, 04:55 AM
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
29-03-2002, 05:21 AM
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
29-03-2002, 05:46 AM
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
29-03-2002, 02:53 PM
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
29-03-2002, 10:32 PM
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
30-03-2002, 06:10 AM
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
26-06-2002, 03:52 PM
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 :rolleyes: 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 !!!!! :D