No liquid line service ports

[cxagent]

I'm having trouble with a 100% outside air unit. The leaving air is running 55 to 65 F dew point most of the time. The system has three complete refrigerant circuits with hermetic compressors and TXVs on each circuit. The TXV are all hunting badly. The bottom of the suction header (where the TXV bulb is clamped) temperatures vary from 50 to 65 as the TXV hunts. The top of the suction header is running over 70 F on all three circuits. Super heat is 8 to 15 degrees on each circuit. All three sight glasses are running clear (no bubbles).

I think the TXVs need to be adjusted to reduce the hunting. But before I do that I want to verify the refrigerant charge is right. But there are no liquid line schrader valves or service ports. I can get the hot gas line pressure but not the liquid line pressure.

Is there a rule of thumb (sorry dash) or other method to estimate the pressure drop through the condenser coil? Is there any other method you would recommend?

[R12rules]

Have you worked on a system like this before?
If yes, just sweat on C&D saddle valves for your access ports, wherever necessary.

If the answer is "no", then get assistance from someone in your area whom you trust.
ASK for someone if you dont currently know anyone who will walk you thru this system.

[cxagent]

This system is still under the manufacturer's warranty so I will not be adding or changing anything until the warranty is over. Since the unit has never performed right, the warranty period hasn't started yet. The manufacturer seems to be waiting for the warranty period to be over so they can wash their hands of the problem. I have number of data logs that I might need to post on the Wall of Shame.

At least the installer has recognized that the unit is not working right and is willing to make corrections. However, they are very hesitant to make many changes so the manufacturer cannot void their warranty responsibility. Sadle valves would be just the thing to void the warranty.

I probably need to fill in some of the details so you understand the situation. I am the commissioning agent for the this project. I am a licensed and practicing (as my time permits) HVAC contractor. I also have more other licenses and certifications than you can shake a stick at. (Just for the record I currently find more design problems than I do installation problems. The contractors are frequently put in a no-win situation because the design cannot work.)

As the commissioning agent, I report to the Owner of the project. Since this unit was installed I have been reporting it has NEVER worked as was scheduled. That started July 2003. It didn't take long to verify that the package OA unit was not performing and the installation had nothing to do with the problem. So it landed in the lap of the manufacturer to correct. Over the winter, the unit didn't need to do any dehumidification so there was no way to verify performance or make corrections. When the spring humidity started again, the responsible parties were in denial ("It has run fine for months..."). Now that the OA humidity is back and the space is running over 70% RH, it is a huge problem and needs to be addressed. The manufacturer is starting to recognize there is a problem, but doesn't seem to want to make corrections. The Owner asked me to start diagnosing the problem, but be careful to not void the warranty.

[Edited by cxagent on 07-26-2004 at 11:07 AM]

[absrbrtek]

Have you checked the airflow on this unit to make sure its in spec? A little too much airflow on a 100% ODA unit can really screw you up. Id start there first before adjusting anything.

[seaboard]

You're going to have to know what the liquid pressure is so you'll need to add the saddle valves as R-12 suggested. He is describing a type that is specific for the line size you have. You braze it on the line and screw in the top to pierce the line. You should seek the manufacturer's and owners permission to do this and hopefully they'll grant you permission. They should all want to fix the problem and not the blame but it doesn't always work that way does it! It sounds like this is a very thorough job, was a certified air balance done to assure the unit is taking in the correct amount of air? Also what brand of TXV's are used. These applications require a lot of tonnage variations so the TXV must be a good quality to make sure it can react to different conditions. One more thing. Are the circuits on the evaporator intertwined or seperated. It ain't going to work well at all if they aren't intertwined.

[cxagent]

Met with the engineer on the project there today. Found the three circuits hunting even more - 45 to 70. Called the manufacturer from the job site and went over everything again. Manufacturer is sure there is someting wrong with the refrigerant charge. They will schedule someone out to look at it...

The certified TAB was 2100 cfm on a scheduled 2200 cfm. The TXVs are Sporlan. There are three refrigerant circuits each going to a separate section of coil (split face - not intertwined). I don't think intertwined coils will work very well on this type of constant volume OA unit because if one compressor is off or down, a large part of the evaporator coil will be warmer than the desired dewpoint.

Looks like I will have to get some service ports installed. I already have to get the filter drier replaced on the circuit that had the TXV replaced.

Thanks for everyone's help. I keep thinking it shouldn't be this hard to get anything done. Shows what I know...

[jon harmon]

Was it Pascal that said "A confined fluid expands evenly in all directions"

If you know your hot gas pressure, how could you not know your condenser pressure?

[jon harmon]

What does Pascal's law say? How could you know the hot gas pressure and not know the condenser pressure?

[jon harmon]

I love it when I double post!!!!!!!!!!!Brilliant

[R12rules]

hark ...... I hear an echo...................

[twcpipes]

Take the head pressure reading from the compressor and deduct 10psig for the piping and condenser pressure loss. Then read the liquid temp near the TXVs. You will be within 4 degrees of true subcooling.
a word of caution regarding another situation: before you tap in liquid line access valves get the factory to agree to it. You will be "technically" breaking into their sealed refrigerant system and they may tell you, "you are on your own now". Either get a fax, use a conference call, use a speaker phone with the owner present, or tape the conversation. I realize this sounds a little over-kill but the owner has expressed their desire you don't, in any way, void the warranty. Good luck.

ps: with the somewhat exteme swing it the superheat, the bulbs could be dirty or un-insulated or not clamped tight, the subcooling goes and comes, and/or oil is traveling around the system in excess.

[cxagent]

twcpipes - thanks. That's what I was hoping someone would have - a rule of thumb that they felt comfortable with.

I've already checked the TXV bulb clamping, isulation, etc. All looked fine. I suspect a charging problem but you may have nailed it on the oil moving through the system. One reason I suspect low charge is the bottom of the suction riser (collects all the suction gas connections from the coil) is cold at the bottom and warm at the top. It looks to me that if the riser is warm at the top there is not enough refrigerant to get the top of the evaporator coil cold. But I needed subcooling to verify...

I will be getting the liquid line taps appoval in writting before doing anything. I've been burned before and it sounds like you have too.



[Edited by cxagent on 07-27-2004 at 02:39 PM]

[cxagent]

Originally posted by jon harmon
Was it Pascal that said "A confined fluid expands evenly in all directions"

If you know your hot gas pressure, how could you not know your condenser pressure?

I believe that only works if the fluid is not in motion. Once the fluid is in motion there are pressure losses due to friction with the tubing the fluid is flowing through.

So a different way to ask this question was what twcpipes answered - what pressure drop would be expected through the condenser coil?

[jon harmon]

How large of a system are we talking about? Line loss on any type of packaged equipment would be neglible. For that matter, it would have to be a pretty good distance to make any difference on a split. To use 10 lbs. on all applications does not sound accurate. Is there 10ft. of line set? 100ft? how many feet of rise is there? Are there double hot gas risers? The point I am trying to make is the Manufacturer "should" have sized the liquid lines and condenser to account for pressure drop. If your goal is to determine if your charge is accurate then I assume( dangerous to do I know)you are trying to determine your subcooling. If I read your original post correctly you have TXV "hunting" but you can acheive as low as 8 degrees SH then it swings to 15, correct? If you can stack liquid in the condenser and maintain sub-cooling then line loss is not your problem. I need to go back and review your post again to make sure I did not misinterpret but it sounds like you need to check the bulbs and the airflow. By the way, what is the design superheat? 15 degrees is not outrageously bad. I'll check back later tonight, hope this helps.

[jon harmon]

How does one measure sub-cooling?

Saturated condensing temp. minus temp. measured at the condenser oulet.


Now, define saturated condensing temperature.

[cxagent]

Jon,

There are three hermetic compressors that I think are 3 tons each (I'm not where I can get to my notes). The hot gas line is 10 to 15 feet of 3/8 copper with less than 4 feet of rise. There is one hot gas line for each circuit, but I'm not sure what you're asking about double hot gas riser. One of the circuits has a pressure controlled hot gas bypass valve (I've verified it is staying closed).

The pressure drop would be the drop in the lines PLUS the drop in the condenser coil. I would think the manufacturer designs for some amount of pressure drop in any system. That number could change by manufacturer or even unit to unit.

Already checked TXV bulb clamp and insulation. Airflow has been checked and verified. Superheat is varying from 5 to 15 as you understood.

New development - Manufacturer says SH should be 18F and the variations are because the refrigerant charge is off. I'm still asking them how to determine correct charge without liquid line ports.

Your other post -
SC - Saturated condensing temp. minus temp. measured at the condenser oulet, just as you said. How do you measure saturated condensing temp? Measure condensing pressure and convert to temp using temp/pressure chart for the refrigerant. How do I get the condensing pressure without liquid line ports? Either I use a 'rule of thumb', assume their design pressure loss is right, guess, ??? Best solution, add ports and KNOW what the condensing pressure is. Both SH and SC tell a whole lot of what is really going on with a system.

Thanks for the help.

[jon harmon]

My whole point is that what you read for compressor discharge is essentially your condenser pressure. Whatever pressure drop, if any will be irrelevant. The distance you stated is minimal and I don't believe it makes a difference. I asked the question about SCT to illistrate the point that the only pressure we are concerned about is the point in the condenser where the refrigerant has reached it's saturation point. The hot gas is a confined fluid at this point for all practical purposes due to restriction of the TXV and the column of liquid in front of it. If you add those ports it will tell you nothing that you cannot already know with what is available.

Take your pressure reading at the compressor discharge, convert it to SCT, measure the actual temp of the liquid line at the cond. outlet and the difference is your subcooling. End of story.

Step 2 if you think the system is under charged then monitor the TXV inlet and the Evap. outlet at the same time. Ir the system is low on then you will see the inlet to the TXV temp raise significantly just before the evap SH increases.

Next question. Are you staging this unit by room temp. or discharge temp.?

[cxagent]

jon

The unit is controlled by evaporator leaving air temp. It has a heat pipe wrapped around the DX coil so the LAT sensor is between the evaporator coil and the reheat side of the heat pipe.

I hadn't thought of watching the TXV intlet temp and the evaporator outlet at the same time. Makes sense when I think it through.

[twcpipes]

I used the 10psig as you said you had a package unit. That was what is was to be used for not splits. If anyone thinks they are getting through that condenser for free with no pressure drop, a shock is coming. An easy way is to get on a unit with a king valve in the future and measure the difference. Try and use an average discharge pressure of 225psig for fairness to the test.
Also if you think for one minute a manufacturer allows for adequate pressure drop, another shock. They know exactly the capability of any given compressor. If the can downsize condenser, discharge, and liquid line tubing from, say 5/8" to 1/2" in the condenser they will do it. If they save $5.00 per unit and make 100,000 units you can do the math.
If there is flow there is pressure drop. And the amount of flow is directly related to that pressure drop against a fixed pipe size. If all was perfect in the package unit world, other than deducting the expansion devices and cap tube losses which for R-22 would be around 130psig drop then the suction would be 130psig under discharge. So at 225psig discharge the suction would be 95psig under unit design conditions. But for some unexplainable reason it never is. Perhaps there is pressure drop losses throughout the piping system due to flow.
If somebody doesn't like the 10psig average drop, then call some manufacturers and ask them what they designed the drops at, or better yet ask what number for said drop should be used.
And I got no more to say about it.

[jon harmon]

The point of asking what the pressure drop through the condenser would be is to determine adequate charge, correct? If that is the case, then that info is is not pertinent to determining accurate charge.

example #1 A home has domestic water pressure of 60 psi. Joe Homeowner attaches a garden hose to the outdoor spigot with a pressure gauge and shutoff. If Joe opens the shutoff 100% there would be pressure drop between the house inlet and the hose outlet. If Joe closed the hose to 50% there would probably still be some pressure drop. If Joe continued to close the hose incrementally at some point well before the shutoff point the pressure drop would cease to exist. That is the point where the supply has the ability to overcome or keep pace with the exiting water.

This same logic applies to refrigeration with respect to the relationship of the compressor discharge and the rate of flow through the metering device.

I am not denying there is some pressue drop through a coil, my point is that is negligible and irrlivant with respect to determining correct charge. Secondly, what is an acceptable pressure drop across a liquid line filter drier? If I am seeing 10 psi across one it is getting changed. The next time anyone is out on a unit with access to head and liquid pressures check them and see. Note, you must verify there is no other reason for any large drop ie...plugged LL filter etc...

If anything, I would expect to see pressure drop if the unit is very low on gas. This would be due to the fact that the EXV wide open. I don't think this is the case due to the fact that SH's are lower than what manufacturer wants.

If the pressure drop were excessive so would the subcooling. Probably in the range of 30degrees or so.

CXAGENT Please log the unit and we can help you better.

List the following:
OAT, Comp. head and suction pressures, Liquid line temp at cond. outlet, suction line temp at evap. outlet, delta T across evap., how many row coil is it, are the in and out correct, what refrigerant, can you verify the total air flow(not just what the report said). There should be a blower curve available from the manufacturer. Does anyone else want additional info?

Get the info from the sporlan valves and power heads and make sure the bulbs are strapped in the proper place. I am not an expansion valve expert but there are plenty of guys on here that know the numbers by heart.

[fastfred]

Have you tried recovery and weighing the recharge as per the factory spec?

[cleancondenser]

Unless your evaps. are starved or your head pressure is very high the discharge pressure should be enough.

We had this problem once but sporlan had us change the power heads to a crossed charge type to reduce the hunting

and check the location of the feeler bulbs

Check your airflow accross the coils,.

They were opposed to us adjusting the superheat setting,
The new power heads worked good

Thanks

[cxagent]

Just got a chance to get back out to that job and get/verify details. Note - the TXVs are NOT Sporlan as I had previously posted.

Gory details -

3 Copeland CR38K6-TF5-270 compressors

TXV Alco head R0240 body 2669 H2B on one sticker & 2344 V13B on a second sticker

Compressor #2
suction 81psi 48 Saturated Suction Temp
head 285 psi 127 Sat Condensing Temp
suction line 65F Superheat 17F
liquid line 98F Subcooling 29 (Assuming NO pressure drop through condenser)

Compressor #1 (left)
suction 83psi 49F Sat Suction Temp
head 299psi 130F Sat Condensing Temp
suction line 59F SH 10F
liquid line 105F SC 25F (Assuming NO pressure drop)

Air flow 2200 cfm scheduled 2150 cfm measured.
(NOTE - this is a 100% OA unit so 400 cfm per ton will not apply!)

Outdoor temp was 85F and 75F DP. There is a heat pipe wrapped around the evaporator coil. So the entering air condition will be cooler than the OA temp. The DP will be the same as OA. I do not have access to the evaporator entering temperature without drilling a hole (not happening during warranty).

The unit was running better than I had seen it run. The TXV's operation was relatively stable. This was the first time I had checked it in the morning when the OA temp was cooler. The manufacturer MAY have been working on the unit since I last checked it. (No one could say for certain.)

I only had time to log the two circuits. I picked the ones that were the best (#1) and the worst (#2) when I was there last. The suction line temperature of both circuits is way too warm. To have leaving air dew point of 54 F, I would expect the SST to be below 45F. Hopefully, the evaporator is full of refrigerant (so there are no warm sections) so I would expect the scution line temp to be about 50F.

Since the TXVs weren't hunting, I did not log TXV inlet temp and suction line temp over time. That would be interesting to do.

[twcpipes]

Hi again, you posted in part:

>>>The suction line temperature of both circuits is way too warm. To have leaving air dew point of 54 F, I would expect the SST to be below 45F.<<<<

Do you know the coil capacities and latent to sensible ratios?
I might have at least wanted to see somewhere close to equal suction sat. temps but it would be hard to say what they should be without taking actual coil in/out db/wb and actual airflow readings and comparing those readings to manufacturers' data. Unless I am wrong.
The difference in suctions could very well be attributed to TXV feeding and/or coil arrangements especially if the evaporators are stacked by face, with the warmer air entering the top of the coils should there be ample space for inlet air stratification to said coils with the coils being draw-thru ahead of the blower(s).
Sounds like you're on the right track though.
tom

[lenb]

Originally posted by jon harmon


example #1 A home has domestic water pressure of 60 psi. Joe Homeowner attaches a garden hose to the outdoor spigot with a pressure gauge and shutoff. If Joe opens the shutoff 100% there would be pressure drop between the house inlet and the hose outlet. If Joe closed the hose to 50% there would probably still be some pressure drop. If Joe continued to close the hose incrementally at some point well before the shutoff point the pressure drop would cease to exist. That is the point where the supply has the ability to overcome or keep pace with the exiting water.


You would still have a pressure drop, just too small to notice. The laws of physics demand that with any flow there has to be a pressure drop. In a condenser, the liquid line pressure would be closer to the saturation pressure than the discharge pressure would be. The refrigerant would be condensing toward the end of the condenser. If I don't have access to the liquid line, I'd guess about a 5 to 10 # drop thru the condenser on a small system. But it could be quite different from that depending on size of the condenser and how much load it's running at.

[wavery2]

Sorry guys, I have read thru this post with amusement... Just had to add my two cents. The high side pressure is the high side pressure (unless there are problems with the system causing restrictions)....that is how the systems are designed to work. High pressure vapor is condensated into high pressure liquid...at the same pressure...period! Yes you can argue about a psi or two loss in certain situations but this is neglegeable. I believe the origional question was where do I read the high side of the system.... The answer is: right where you read it. The high side is the high side..(cant stress this enough). It sounds to me like someone is venturing into the world of refrigeration (where this pressure tap is not uncommon) and he is not sure about a few things...or he has run across a unit that has the tap in this place. Bottom line....take the reading and be confident that this is the high side reading you are looking for.

Hope this helps...Avery
( just my two cents - so dont waste your dime )

[weezer]

Even if the txv bulbs are clamped snug and insulated properly, they won't work properly if they are positioned wrong. If they are at a 12oclock positon on the pipe they will only sense gas temp. If they are positioned at 6oclock position they will only sense liquid temp. I ran into this problem on alot of old Lennox units. Changed position of bulbs and units ran great for once in 15 years. Stopped having to replace compressors, and customers were happy. Imagine that! You have to have them sensing a mix at 2-4oclock. Also, if in series, they might need to be set at different SH. If they are all at same adjustment, you will have wild swings in rh and temp. Start at the beginning with a proper CFM like 'absrbr' said. Get the proper specs on pressure drops for the condensers as brought forth by 'twc'. You don't need to break into the system to get a fairly good measurement of SC and HG pressures. Is the building properly exhausted?(Weak draft=jungle) In the end this machine will only do what it's design allows it to do. Good luck.

[weezer]

Just read some more posts by 'agent'. Sounds to me like that unit is doing it's best and then some. Your head pressures seem good. Suction pressures are high. And subcooling very high accordingly. Sounds like system is undersized. Remember, MOA's are not sized the same as regular ac units. There is much more cfm involved! A six ton space for return air system might need 2200 cfm, but the same space with a constant rh heat load might need twice that and more refrigeration.