Results 51 to 78 of 78
Thread: trouble shooting
-
14-05-2010, 05:07 AM #51
Re: trouble shooting
-
14-05-2010, 05:42 AM #52
Re: trouble shooting
If everyone agreed with me, then by definition I would no longer be a rebel... which would screw up my self-image and spoil all my fun... lol
-
14-05-2010, 05:52 AM #53
Re: trouble shooting
I disagree.
I've been at this for the better part of thirty years, which is likely longer than you've been alive. Maybe you should start taking that into consideration before showing your ass any more than you already have.
Who is they? I believe the discussion has been between you and I. Are you a student?
I didn't say that they "did", I only said that they "can". Big difference. Huge difference. I also provided several examples, which you simply don't seem to understand.
-
14-05-2010, 01:15 PM #54
-
14-05-2010, 01:39 PM #55
Re: trouble shooting
LOL! I'm sorry, I didn't mean to laugh, but that was just too funny!
Here are a few of quiz questions for you.
If the suction pressure drops, but head pressure remains constant, will the compressor ampdraw go up, or will it go down?
What happens to the energy supplied to the compressor motor?
Will an evaporator pick up heat from the airstream when the evaporator pressure is 5 psig? 10 psig? 15 psig?
Will a liquid line pick up heat when the liquid line subcooling is zero and it's temperature is below the temperature of the surrounding air?
What will the effect on head pressure be when the condenser's effective internal volume is reduced to 1/4 it's normal operating value by an accumalation of excess liquid refrigerant within it. 1/8 its volume? 1/16 its volume? Etc.
Just a few to get you started in the right direction.
-
14-05-2010, 05:07 PM #56
Re: trouble shooting
I accept your surrender.
Here are a few of quiz questions for you.
Just a few to get you started in the right direction.
Cheers
-
14-05-2010, 05:14 PM #57
-
15-05-2010, 12:07 AM #58
Re: trouble shooting
Interesting questions, theory and practice can be different depending upon application and equipment type.
On question reduction in suction pressure same head pressure, practically the amperage changes very little (slight drop) on smaller closed couple compressers. This drop is not as great as one would expext, as the volumetric efficiency reduces along with the efficiency of the motor, ( changing the power factor, more amps for Kw drawn or should I say KVA)
For this reason your KW may drop more than the amps indicate.
The evap will pick up heat from the air if the evap is colder. If the evap and air flow remain the same the colder the evap the more energy will be absorbed as long as there is sufficient refrigerant mass flow to absorb the heat. (a bigger compressor is needed)
If the compressor is the same and suction pressure drops then less energy will be absorbed as mass flow will be reduced, with only a small proportion of the evap is actually used for latent heat(boiling of refrigerant) transfer. Refrigerant vapour superheating will occur.
If the air is warmer than the liquid line it will pick up energy regardless if it is sub cooler or not. If there is no sub cooling then flash gas will occur, which then increase liquid line pressure drop which futher increase flash gas.
Condensor presuming all points within the system remain constant, then reduction in volume, thus heat transfer surface area, then head pressure will rise.
-
15-05-2010, 03:35 AM #59
Re: trouble shooting
Ok. Good stuff. Now let's put some numbers to this.
I've attached a couple of performance calcs that for the sake of this argument should suffice. The first shows a typical 10 SEER 3 ton A/C system running under moderate conditions. We'll let the conditions be 23.9°C (75°F) return air temp, and 26.7°C (80°F) ambient.
Looking at performance calc #1, which represents the system before a restriction occurs, the condenser heat load contributed by the electrical input works out to about 7.609 btu/hr. The cooling capacity is 36,500, so the total condenser load is about 44109 btu/hr.
Looking at performance calc #2, which represents the system with a LL restriction introduced near the LL service valve, we similarly get a total condenser load of about 21,440 btu/hr.
As you can see, the condenser heat load ratio is approximately 2 to 1. Thus, if we can arrange for the effective condenser area to be reduced by the same ratio by introducing this restriction, then we will have satisfied the conditions required in order to produce the respective sets of system pressures represented in these two performance calcs. In this case the head pressure remains unchanged. In order to get the head pressure to increase after the restriction is introduced, we need only reduce the effective condenser are to a little less than half its original value.
With a long line set, this is not only possible, but again, I have witnessed the effect with my own eyes.
-
15-05-2010, 03:39 AM #60
-
15-05-2010, 04:43 AM #61
Re: trouble shooting
If we presume that the restriction has cause the head pressure to remain the same "flooding the condensor with liquid" then you have made a mistake in your calculations. This being the case you would have increased liquid sub-cooling.
We all know in refrigeration at time very strange things can happen, I think if i saw what you are describing and would sumise that the system was "slightly" overcharged originally.
With the simulations given, under normal circumstance I would expect the head pressure to fall. (full floating head pressure)
-
15-05-2010, 05:31 AM #62
Re: trouble shooting
Is this what you meant? (See attachment: performance3.jpg)
That correction actually enhances my argument. And as you can see, it provides very little change to the numbers.
FWIW, Carrier did not include the warning about excessive head pressure (during pump-down conditions with long line sets) as a result of mistaken notions, or as a result of incorrect math. It's an empircal fact that when refrigerant migrates out of the evaporator and in some cases also the LL (and accumulator if applicable), then it is necessarily piled up in the condenser.
A 150 ft lineset with 3/8" LL requires an additional weighted charge of 5 lb. If the system's factory charge is 5 lb, then the system now contains twice its factory charge. If 5 lb of that 10 lb total charge ends up in the condenser due to a LL restriction, then what percentage of the condenser coil will be available to desuperheat and condense the incoming vapor?
I hope you aren't expecting a live demonstration of this effect, because I'm not quite interested enough in convincing any of you. I suggest instead that some of you volunteer to provide a sound argument forbidding an increase in head pressure with a LL restriction. "I can't see it happening" is somewhat less than a valid argument.
Though I once argued from Gary's position, and I'm sure that I argued that position with him in the same thread on an occasion or two, I cannot be convinced now that this effect cannot occur. I've seen it in person, how do you expect me to argue against that, or to take seriously any argument to the contrary?
-
15-05-2010, 05:49 AM #63
Re: trouble shooting
A rise in head pressure presumes the condenser capacity is insufficient for the heat absorbed in the evaporator. So... there are two factors involved, i.e. condenser capacity and heat load.
Given the absence of a receiver, a restriction might back liquid up into the condenser (more likely if overcharged)... but that same restriction should reduce the heat absorbed in the evaporator.
How is that evaporator heat load maintained?
-
15-05-2010, 06:18 AM #64
Re: trouble shooting
Thats the nature of our game, occassionally things happen out of the ordinary. I would never say that a restriction in the LL is an impossible cause of increased head pressure, but to claim it as a normal condition is in itself flawed.
So if I ask the question what is "most likely" to raise head pressure
An increased air "on" (ambient) to the condensor or
A restricted liquid line.
I suspect when you have seen this occur, you had high ambients and a high evap heat load (this would be normal if a fault has occurred), then other factors come into play, increased compressor power draw, reduced condensor efficiency due to air density and its thermal properties.Last edited by mad fridgie; 15-05-2010 at 06:27 AM.
-
15-05-2010, 06:36 AM #65
Re: trouble shooting
How many times must I explain this. Go back and read through my replies, starting with the first one. You should find the answer easily enough.
Good grief. You guys are making me weary. Let me save you the trouble. Here are a few quotes from some of my previous replies:
"C is true in the vast majority of cases, whereas the others are only true under certain uncommon conditions."
"To the author's credit he asked for the condition that "will likely" increase head pressure, which to me is synonymous with "has a higher probability of". In that case, C becomes the obvious answer."
"Stretching the argument, as I did above, is an example of what not to do on a multiple choice question."
In response to Sneep:
"Do you remember this? "Care to flesh out what you are saying?" That was YOU asking for those exceptions that I had in mind. Now all of a sudden I'm wrong for obliging you?"
"I didn't say that they "did", I only said that they "can". Big difference. Huge difference."Last edited by hvacrmedic; 25-07-2010 at 02:04 PM.
-
15-05-2010, 07:02 AM #66
-
15-05-2010, 08:03 AM #67
Re: trouble shooting
To take this a step further:
The same restriction that is reducing the condenser capacity should be starving the evap and thus reducing its capacity to match the condenser... and your attachments seem to agree.
If there is enough refrigerant to maintain the evaporator capacity while at the same time backing up into the condenser and reducing its capacity, well... that would pretty much be the definition of an overcharged system.
I wouldn't go so far as to say that it can't happen. After all, you have seen it with your own eyes. Can't argue with that.
Maybe a long line set can make the difference. Possibly the liquid line could function as an extension of the evaporator... keeping the evap load up?
Stranger things have happened.Last edited by Gary; 15-05-2010 at 08:26 AM.
-
15-05-2010, 08:40 AM #68
Re: trouble shooting
You haven’t been right yet. Restating it doesn’t make your points true.
In response to Sneep:
]"Do you remember this? "Care to flesh out what you are saying?" That was YOU asking for those exceptions that I had in mind. Now all of a sudden I'm wrong for obliging you?"
Mad boi, if that doesn't clear it up for you, then pardner, you are just S.O.L.
It does not show the results of a restriction. Thats not the purpose of that calculator.
You fail to grasp what everyone is saying.
1. When you introduce a restriction you reduce the refrigerant getting to the evaporator.
2. Less refrigerant in the evaporator means you can’t pick up as much heat.
3. That means less to reject in the condenser.
4. Temperature goes down, pressure goes down.
Backing the refrigerant up into the condenser from a restriction and adding long lines with the extra charge it takes to fill them will not maintain a increase in head pressure beyond what it takes to stabize the system to the feed rate of the restriction. If you can’t pick up the heat you won’t have it to reject.
You might want to chew on what has been said before you post again.
Cheers
-
15-05-2010, 02:02 PM #69
Re: trouble shooting
You haven't listened to a word thus far, nor are your replies very coherent in general. Your grasp of thermodynamics is not adequate enough for you to formulate a valid argument on your own. I don't expect that I can fix that for you, so I'm going to have to depart from the conversation. Cheers.
-
15-05-2010, 02:26 PM #70
Re: trouble shooting
The attachments paint a different picture than you're seeing Gary. Though the cooling capacity has dropped to just over 1/3, the condenser heat load has dropped to 1/2. The changes in the two loads are not self canceling as you have presumed. You're ignoring the compressor kw component, which according to the performance calcs, has not changed at all in this instance.
Nor does the amount of refrigerant in the evaporator relate directly to the cooling capacity of the evaporator coil. I suppose that with a saturated suction temperature of 1°F and pressure of 25psig, the same lb of R22 might be able to absorb more heat than it did at 41°F and 70psig. At half capacity, the evaporator coil has less than half the initial mass of refrigerant within it. Just a thought.
-
15-05-2010, 02:55 PM #71
-
15-05-2010, 03:06 PM #72
Re: trouble shooting
Sorry, I buggered that up a little bit. Though the refrigerant will absorb more btu/lb at the lower pressure, it is a negligable change. What I meant was that it will abosb heat a much greater rate, due to the higher temperature differential. Half of a coil at 1° F will absorb, per unit time, more heat than a fullly saturated coil at 41° F. Since the cooling capacity of the coil is only 1/3 the initial value, it should follow that the saturated portion of the coil has dropped to much less than 1/3 of the total, leaving less than 1/3 the initial refrigerant mass in the coil.
All of this is immaterial though. Even if these things scaled differently than in my example, it would still be possible to for a high head pressure with a restriction. One would need only reduce the size of the condenser coil relative to the evaporator. In refrigeration systems this is not an uncommon condition, which I presume is one of the many reasons for using a receiver on those systems. That was in turn one of your initial counterarguments, was it not, that the reciever would prevent this?
-
15-05-2010, 04:44 PM #73
Re: trouble shooting
Logically then, the greater the restriction, the higher the head pressure would go... and a pumpdown would send the head pressure right up through the roof. Possibly the extended liquid line could hold enough extra liquid to completely fill the condenser?
I'm thinking the manufacturers have a limit on how far the liquid line can be extended. Maybe this is why?
-
16-05-2010, 12:26 AM #74
Re: trouble shooting
Yes, the total charge can be easily be too large for the condenser coil to accomodate.
NOTE
: All outdoor unit coils will hold only factory supplied
amount of refrigerant. Excess refrigerant, such as in long line
applications, may cause unit to relieve pressure through internal
pressure relief valve (indicated by sudden rise of suction pressure)
before suction pressure reaches 5 psig (35kPa). If this occurs, shut
unit off immediately, front seat suction valve, and recover
remaining pressure.
Last edited by hvacrmedic; 25-07-2010 at 02:10 PM.
-
16-05-2010, 11:02 PM #75
Re: trouble shooting
[quote=sneep;188124]
Hydraulic pressure is what you’re seeing. You can’t have high head pressure with a restriction.
Did I miss something here?
High pressure is high pressure, doesn't make any difference whether its hydraulic or a gas or a bag of exited aliens... high pressure is High pressure.
Reality is an elusion created by alcohol deficiency. Quaff and enjoy. [Yorkshire, UK]
-
17-05-2010, 06:25 AM #76
Re: trouble shooting
1.filter drier chokeup.
2.cond:fan not working if air cooled
3.cond;coil clogged up with dirt
4.ambient temp.very high
-
23-05-2010, 04:45 PM #77
-
25-05-2010, 03:43 PM #78
Similar Threads
-
Trouble shooting tx valves
By TECH TIM in forum Trouble ShootingReplies: 11Last Post: 01-02-2011, 05:13 PM -
fridge/freezer trouble shooting
By Martin Bickle in forum Trouble ShootingReplies: 0Last Post: 01-03-2010, 03:24 PM -
Need help trouble shooting mini split ac error code "e4"
By grindrx in forum Air ConditioningReplies: 5Last Post: 23-07-2009, 08:57 PM -
books for Trane trouble shooting
By RobertK in forum Refrigeration BooksReplies: 11Last Post: 15-07-2009, 12:17 PM -
Trouble Shooting
By Vincent Yu in forum FundamentalsReplies: 20Last Post: 22-11-2005, 02:04 PM