new bee question

[sds5150]

I just did my first, leak fix, evac. and recharge on a ac unit for an office in my factory. (It's not really mine) My head is swelled, but I still don't feel like I did as well as I could of. I have some questions. The office is located on the first floor of the factory and the condensor unit is on the roof (2 stories up). I am unsure how to fill the unit properly. Down at the air handler/evaporator there is no actual metering valve. The high side line splits into about 6 smaller lines and they enter the evap. (capilary system, I assume) The condenser has a spec label that says I need 5 lbs 3 oz of R22. Does this take into account the 2 story length line set?? What if the installer cut the factory lines off to length? I filled the system with 3 lbs of 22 liquid on the high side, started the compressor and finished on the suction side with vapor to a total of 4 lbs 8 oz. and it is working well so far. 51 deg F. air at the duct entering the office. How should I properly fill this unit? Also, the only sercice valves are on the roof with the condenser. Thanks for the help!!

[Senior Tech]

Proper charging tecniques are one of the first things I would have thought you would have been trained in...assuming you are trained? Do you know what superheat and sub-cool are?

[NormChris]

Senior Tech is correct. You need to start by learning superheat and subcooling first. Only then will you be ready to use them to properly charge a system.

Start Here: http://www.hvac-talk.com/vbb/showthr...threadid=33829

The charge listed on a condensing unit is not what the system holds. You should not charge a split system by weight you need to charge by superheat and subcooling. But, first you have to know what they mean.

[Shophound]

Going by the info in your profile I take it you're a sparky trying to learn HVAC (HVAC student). Are you enrolled in a trade school or other form of instruction?

Your honesty is refreshing, regarding not feeling you did as well as you could have. Some guys having done what you just did would've quit with the big head, thinking they've conquered the world.
So, with that in mind, I'll address the questions you raise in your post.

First of all, you can bet your bottom dollar there's a metering device at the evaporator. You were probably expecting to see a thermostatic expansion valve (TXV) but instead saw a distributor and thought WTF? Where's the valve? You're likely looking at a piston metering device, otherwise known as a fixed orifice metering device, or sometimes referred to as a flowrater or check-flowrater.

Secondly, in the HVAC trade we do not "fill" refrigeration systems. We "charge" them. May seem like splitting hairs but going around talking about "filling" a system is likely to get you some odd stares from grizzly old senior techs. And the immediate stamp of NEWBIE, like the title of your post.

I invite you to slide down to the "For Your Information" and the "Tips and Tricks of the Trade" sections of this website and absorb as much as your brain can stand regarding the subjects of superheat and subcooling. Use the "search" feature of this site if you find yourself ravenously starving for more information on these two vitally important matters.

From what you've written, your understanding of the refrigeration cycle is green. Seasoned technicians charging a split system such as yours know the OEM's spec labels aren't gospel when determining the amount of charge to put into a system. You seem to basically understand that by quering about lineset length.
Also, when you were repairing the leak, did you flow nitrogen at a low pressure through the piping while brazing the leak? Did you install a new liquid line drier (removing any old driers in the process)? Did you do a pressurized leak test, then evacuate the system to 500 microns or less and then blank off the vacuum pump to observe the system for any abnormal rise in vacuum above 500 microns?

Basic things you must know to determine proper operation of this split system a/c and many others like it:

Proper airflow across evaporator coil. 400 CFM per 12,000 BTU. Evaporator coil, blower wheel, and filter must be clean before an accurate assessment of system performance can be obtained.

Clean condenser coil. Can't stress enough that it needs to be and stay clean if you want to nail the charge.

All non-condensables and moisture evacuated from the system after every time the system is open for repairs. Liquid line drier installed to remove moisture from the refrigerant and compressor oil.

A good understanding of pressure-temperature relationships. Carry a pressure/temperature chart in your wallet for the type of refrigerant you're using.

A good understanding of the vapor compression refrigeration cycle. Know the significance of superheat, subcooling, saturated vapor, saturated liquid, latent heat of vaporization and condensation. Know what the refrigerant is doing at each and every stage throughout the system.

Know how heat loads across the evap coil vary and what effect ambient temperatures have on condenser performance. Understand that although your system you just repaired now may be working well, on a hot day it may lay over and puke.

Understand the effect ductwork has on a system, and how supply or return air leaks in ductwork affect system performance.


And after all that, I've only scratched the surface. Seems overwhelming, yes? At first, of course. But if you get some good instruction into your head and the fortune of an experienced technician to show you a few things, you'll be light years ahead of where you are now.

[sds5150]

I am trained by aq few college classes only. I am EPA certified. There is no one in my facility to teach me hands on. I am trying to do the best I can by myself. I know that superheat is the sensible temp increase of the exiting evap. line. I believe a 10-12 deg F. superheat would be about right. Latent heat does not increase the temp of the refridgerant as it changes state. I thought that a system using a thermal expansion valve would be filled by super heat?? Also, if my evap. is downstairs and the service valves are on the roof how can you guess at the superheat? I know you guys think I'm some sort of idiot, but this is the only place I have to go with my questions. Thanks again for any "help"

[NormChris]

We do not think you are an idiot. Get that notion out of your head. We understand that at this point you are in need of a little education. In that regard you came to the right place.

Once again, as you were advised you need to read up in the For Your Interest Area at this site. You are actually on the right track with your understanding of superheat but you need someone to show you how to actually apply what you are learning to the real world.

I encourage you to hang in there. But, it would be best if you had some on site help to guide you.

[Shophound]

I know you guys think I'm some sort of idiot, but this is the only place I have to go with my questions.

To the contrary. I wouldn't have taken the time to post what I did earlier had I held in my mind you to be an "idiot".

You're green, yes, but an idiot? Not my place to make that call. Idiots manifest themselves over time. In my experience, idiocy arises out of a combination of arrogance and ignorance. Your original post showed a bit of humility which led me to believe your ignorance could be fixed. Ignorance is never a bad thing as long as the humility is there that will allow the ignorance to be diminished.

TXV systems are charged (not "filled", my friend!) by subcooling and fixed orifice systems are charged by superheat.

[sds5150]

First off thanks for your replys. I expect some crap for being new, but I also would like some good help and I seem to have gotten some. Thanks to all, especially Shop Hound. (I am enrolled at the local college to get my HVAC Certificate and Associates in Industrial Electricity.)

I did not have to braze, the leak was in a loose fitting. That brings me to another question. Shophound, I did not do a pressurized test, because I don't have the equipment for that yet. What I did di was bring the system down to 380 microns (because it didn't seem to want to go any lower, moisture??) Then I valved off the pump for about 10-15 minutes. The system only increased 20 microns. From what I've read I should have hit 200 microns. Any input on all that?

Thanks for the "Charge" info. That will stick in my brain!
I have a temp/pressure chart. If the condenser fan has a high side pressure switch, the temp/pressure drops when it is on. How is the actual high side pressure determined in such a situation?

I have cleaned many condensers so far! This I can handle! I will visit the "Tricks of the trade". Thank you.

[jtricor]

Shophound I think if you keep filling him with all this info he will get charged, no really I think you and NC did a good job at not ripping this guy to pieces, but still think he should try to find some hands on training of some sort at least on the safety side.

[Senior Tech]

Crap...you ain't seen any yet, wait'l dice gets here...
In all seriousness read all you can in the "for your interest section...wealthy and wise men got their beginnings there.

[sds5150]

Bring it on!! As long as I get some valid direction, I can take all you can dish (I think?) This is the most fun I've had in a post!

[Shophound]

Originally posted by sds5150
First off thanks for your replys. I expect some crap for being new, but I also would like some good help and I seem to have gotten some. Thanks to all, especially Shop Hound. (I am enrolled at the local college to get my HVAC Certificate and Associates in Industrial Electricity.)

I did not have to braze, the leak was in a loose fitting. That brings me to another question. Shophound, I did not do a pressurized test, because I don't have the equipment for that yet. What I did di was bring the system down to 380 microns (because it didn't seem to want to go any lower, moisture??) Then I valved off the pump for about 10-15 minutes. The system only increased 20 microns. From what I've read I should have hit 200 microns. Any input on all that?

Thanks for the "Charge" info. That will stick in my brain!
I have a temp/pressure chart. If the condenser fan has a high side pressure switch, the temp/pressure drops when it is on. How is the actual high side pressure determined in such a situation?

I have cleaned many condensers so far! This I can handle! I will visit the "Tricks of the trade". Thank you.

If you pulled the system down to 380 microns, blanked off the vacuum pump, and saw only a 20 micron increase over 15 minutes, you're doing good. A significant amount of moisture in the system would've given you a much faster rise in pressure.

I'm not sure I understand your question regarding the temp/pressure switch. Typically a high side pressure switch operates on pressure only, not both pressure and temp.
What you may be asking is that your particular condenser fan cycles on a high pressure switch, which at first glance would tell me it's a form of head pressure control for low ambient operation. It allows the a/c to continue running normally when the outdoor temperature is low.
Usually, if the fan is cycled with this switch, and the switch is adjustable, you pick the minimum pressure you want the condenser to run down to before the fan cuts out, and then a maximum pressure the condenser builds up to before the fan cuts back on. Just throwing out a range for R22, 190-225 psig, but you'd have to tweak it for the system you're on to get the best performance.

Even so it's not the best way to regulate head pressure, but it works.

[Shophound]

Originally posted by jtricor
Shophound I think if you keep filling him with all this info he will get charged, no really I think you and NC did a good job at not ripping this guy to pieces, but still think he should try to find some hands on training of some sort at least on the safety side.

I agree he could benefit loads by having an on-site mentor, but not everyone has that luxury. So, if he's going to come to a website like this to get info, he should get the best info possible through this limited medium.

And to sds5150, above all, BE CAREFUL! Don't ever try to be a "hero" with this stuff if you know you're in over your head.

[needlevalve]

Being green is what alot of this talent enjoys. They have learned, sometimes once often twice. Hang-in there.
Cover your basics terms so these guy can understand your point of reference. Suction line, Hot gas line, liquid line, compressor, evaporator, condensor, txv, oriface, cap. tubes, charging liquid, charging vapor, superheat, saturated vapor(suction pressure) saturated liquid(discharge pressure)subcool, etc... so far so good

[sds5150]

Shophound,

You are correct about the high side pressure switch being for low amb. temps. I realize that the switch works off of system pressure not temp. My question involves using my temperature/pressure chart. If I look up what high side pressure should be on the chart and watch the actual pressure, how do I determine what the actual high side pressure is? I watch the gauge, when the fan is off pressure climbs (because the internal temp is increasing), and when the fan turns on it drops (because of the temp drop)The system does not and will now keep a constant pressure. Other than knowing I have a good enough charge to actuate the fan and cause a differential in the system for refridg. flow, what can viewing high side pressures in this situation do for me? Thanks

From advice of earlier posts I set my high side switch to cut in at 275 and cut out at 225. (50 psi diff)

[Shophound]

Originally posted by sds5150
My question involves using my temperature/pressure chart. If I look up what high side pressure should be on the chart and watch the actual pressure, how do I determine what the actual high side pressure is? I watch the gauge, when the fan is off pressure climbs (because the internal temp is increasing), and when the fan turns on it drops (because of the temp drop)The system does not and will now keep a constant pressure. Other than knowing I have a good enough charge to actuate the fan and cause a differential in the system for refridg. flow, what can viewing high side pressures in this situation do for me? Thanks

From advice of earlier posts I set my high side switch to cut in at 275 and cut out at 225. (50 psi diff)

First of all, you need to understand why a condenser fan is cycled during low ambient operation. It is to keep head pressure from falling too low, which you may already suspect. However, ask yourself why the head pressure should not drop too low during low ambient conditions.

The answer is that for best operation of the metering device (MD) and evaporator, a sufficient head pressure must be maintained on the liquid side of the MD (more precisely, a sufficient pressure differential between the high pressure side and the low pressure side). If this pressure drops too low, the mass flow rate of refrigerant falls off to where it can't pick up enough heat for a given heat load placed across the evap coil.

In my opinion, condenser fan cycling with a pressure switch is the least adequate method to maintain head pressure under low ambient conditions. An electronic device that varies the speed of the fan motor according to either head pressure or the temperature of a given return bend on the condenser coil works much better. This method avoids the wide swings in pressure common with fan cycling. Your TXV or FOMD isn't seeing continual variation in pressure differential, just a steady head pressure maintained by the fan being slowed or sped up to maintain pressure.

In your case, however, you're stuck with this method, and sounds to me like you're asking how to determine the proper head pressure when it's constantly swinging up and down. Answer: you don't.

Rather, this is where a working knowledge of superheat and subcooling enters the picture. If you're lucky, the manufacturer (OEM) of your split system left a superheat charging chart somewhere in the unit. If you're unlucky, the OEM never included one or it's been long lost. In that case, Jim Wheeler's charging chart in the FIY section of this site isn't a bad place to start. It's not perfect, but it'll get you closer to OEM spec than using the infamous "beer can cold" method of charging a system.

Even so, when your condenser fan cycles and your head pressure fluctuates, everything else in the system is going to fluctuate as well. Head pressure, suction pressure, subcooling superheat, mass flow rate, amp draw...everything!
This is why most OEM's recommend that systems with low ambient kits installed (fan cycling or speed control) be charged when all fans are fully engaged and ambient temp is above a certain point. In other words you'll have far better luck determining where this system should be at, charge-wise, when it is operating closer to normal design conditions rather than low ambient conditions.

One thing to note as you study charging fixed orifice metering device systems by superheat. As the outdoor ambient temperature increases, the mass flow rate through the evaporator also increases, which initially would result in higher evap (suction) pressure. However, as interior heat load diminishes as the unit continues running, the evap pressure will drop and superheat will drop along with it.
A dirty condenser coil can artificially raise the head pressure, which could lead to a situation that's been posted about several times lately...high head pressure/low superheat.

Main reason I threw in the above is to get you thinking about the importance of understanding superheat and subcooling readings. You CANNOT charge a system by suction and head pressures alone. If you persist on doing that, you'll never get it right and you could risk the longivity of the system. Now is the time to get a good understanding of the refrigeration cycle down pat, before you pick up a bunch of bad habits that takes years and a lot of arguing with other techs to undo.

[john dalton]

Dear Sds5150,

I see you have been given good and sound information from this forum’s own “ HVAC online mentors”. This information will surely increase your knowledge of the HVAC/R industry. They have also challenged you to continue to increase your HVAC/R information gathering by directing you to several areas of this forum.

The preverbal ball is now in your court my friend. If you’re earnestly seeking to better yourself in your apparent chosen craft, you would be best served to do two things at this point. First, saturate yourself in the information areas of this forum, as you have been directed, and second, and most important, “apply” the knowledge you have acquired. Remember the old saying:

Knowledge is the mere accumulation of facts and information, where as wisdom is the application of them.

Be wise my friend, and good hunting.

Keep posting…………………….

Respectfully Submitted,
John J. Dalton