Charging a system

Glenn Harrison_2

If it's a cap tube or piston metering device, the best way is to measure your outdoor dry bulb temp, and your indoor wet bulb temp, and plot them on a charging chart or slide rule which you can get from Carrier or Trane, and it will tell you exactly what your superheat should be. You should then sheck you subcooling to make sure it is not to high, indicating other problems.

If you have a TXV, then the only proper way is to use subcooling. You have to get the proper subcooling from the manufacturer for each particular size unit.

I will tell you that if your working on Lennox units, they will tell you to use Subcooling on a piston type unit, and they leave a chart in the unit as to what the SC should be. However, this method will sometimes allow the superheat to get to low and cause liquid to enter the compressor, so you should always double check the superheat using the method described above.

I have also attached an article on A/C charging, and an electronic version of the superheat chart I mentioned above

Bob Sweet

When charging a system

Is it best to use super heat, supply and return split, sub cooling or what Thanks.

clammy

charging

I always go by superheat and the manufactors guidelines if they want sub cooling then go by that .I always check the temp difference across the evaporator and do a wet bulb entering reading and then check the manufactors charging chart also check what type of metering device the evap is using metjet or txv .I usually always go with superheat at the suction port but i believe it really should be checked at the txv,s sensing bulb unless it's a metering orfice.On service calls if it seems to be running high head check the cleaness of your condenser coils and check your sub cooling for over charging you will usually find to much sub cooling when over charged . peace and good luck clammy

mickey_2

charging

superheat and subcooling are the best way to charge, the problem is that on existing systems the manufactures info is not always onsite. I give all my techs a slide chart that York puts out that gives the necessary subcooling and superheat based on the wb and db temps at the time of the service call. If they take the time to take the measurments and find the metering type it works every time. I also require all that info on the service ticket so I know it's being done. You would be supprised how many less callbacks we get on charging calls now. Mick

Eugene Silberstein 3

System Charging Methods

AS mentioned by your colleagues, the metering device being used determines the method or methods that should be used to charge a system.

If the system is using a fixed bore meterng device (such as a capillary tube) or an automatic expansion valve you should use the evaporator superheat. Typically, an evaporator superheat between 8 and 12 degrees is acceptable, although different manufacturers have their own acceptable guidelines.

If the sysstem is operating with a thermostatic expansion valve, subcooling is the way to go. Typially, system subcooling should be between 15 and 20 degrees.

As a review:

Evaporator superheat = Evaporator outlet temperature - Evaporator saturation temperature

Condenser subcooling = Condenser saturation temperature - condenser outlet temperature

Some Cool Tips on Charging:

High superheat is an indication of an undercharge
Low superheat is an indication of an overcharge
Low subcooling is an indication of an undercharge
High subcooling ia an indication of an overcharge

High superheat AND low subcooling is an indication of an undercharge

Low superheat AND high subcooling is an indication of an overcharge

High superheat AND high subcooling is an indication of a liquid line restriction

Low superheat and low subcooling is an indication of an overfeeding metering device

NOTE: These are just guidelines and INDICATIONS OF A SYSTEM PROBLEM, since other system problems may result in the appearance of these symtoms.

Hope this helps.

don_144

If I may add

that the sc/sh temps can only be dail in if coils are clean and you have proper airflow across the coils.

Checking TESP I think should be the first place one would start or should start with all the poor distribution system
out there.

Eugene Silberstein 3

Most Definitely!

Very true, Don. Proper airflow through both the evaporator and condenser coil must be confirmed before adding.removing refrigerant from the system.

All to often, the first thing a field technician does is grab the refrigerant tank from the truck and pump refrigerant into the system. Very bad thing to do.

Consider this. A technician gets to the job, gauges up and finds both the head and suctin pressures to be low. without checking anything else, he runs to the truck and dumps refrigerant into the system in an attempt to get the system pressures up. After succeeding (more or less) to get the pressures up, further checking indicates that the air filter is clogged with dirt. Upon replacing the air filter, the operating pressures are now too high. The excess refrigerant must now be recovered from the system.

On the flip side, consider the scenario where the condenser coil is dirty. In this case, the operating pressures will be high (except in the case where there is an aujtomatic expansion valve). Here, the technician goes through the trouble of removing the excess refrigerant from the system. After all is said and done and it is eventually discovered that the condenser coil is dirty, the refrigerant must then be added back to the system to bring the system pressures back up.

Airflow through the evaporator coil can be affected by a number of factors including: closed supply registers, dirty air filters, blocked supply or return grilles, loose duct liner, defective blower motor, dirty blower wheel, damaged belts, loose belts, improper pulley adjustment, dirty evaporator coil, system undercharge (frosted coil).

Thanks, Don. It can't be stressed enough! Verify airflow through the coils first!

don_5

You know

that story about grabing the guages hits home,I was one of them guys many years ago.

Welcome to the wall professor,and thanks for sharing your knowledge.

Eugene Silberstein 3

Thanks Don

I was one of them as well. And the reason I was one was that the person who originally taught me the industry was one as well and so on and so on and so on.

Thanks for your support.

Kal Row

does jim have a chart for puron??

soot_seeker_2

suction line temp curve..

is the suction line temp curve in figure 2 only useful for non-txv?

if you couldn't easily get into a tight attic space to inspect the handler, is there any obvious way to tell txv from non-txv?

ss

enthalpy

hello professor!
Do these guidelines on superheat and subcooling hold true in refrigeration. I know the refrigeration cycle is the refrigeration cycle, but can you still check your charge the same way.
Also, you have told us the delta T of the coil is 35 by design in a A/C. What is it for a med temp box that is to maintain 38-42 (dairy,display ect). Refrigerant is 409A a blend, I know that has bubble and dew points, can you ball park operating temps and pressures.
Going to attempt to check one this weekend any advice would be helpful.

Glenn Harrison_2

Unfortunately...

I don't know, and I don't remember where I got that file from. I'm thinking I got it at HVAC-talk.com in one of the threads, but I'm not sure. Sorry.

Glenn Harrison_2

I would have to say...

yes it is only good for a cap-tube or piston metering device system. In general, a TXV will maintain superheat as you continue to add refrigerant.

Unfortunately, I don't know of an easy way to determine if a TXV exists or not. I usually wind up finding out the hard way.

Kal Row

you got it from jim wheeler..

and this was his response:

No, sorry. You can find one at most wholesalers under the listing R-410A


pity, cause his chart is soooo conveanient

Eugene Silberstein 3

Missed the post, sorry

Enthalpygal,

Sorry, I missed the post. Typically, the delts-t of the coil for refrigeration is in the range of 20 degrees. Depending on the products being refrierated, the humidity may also play an important role. If the product is sealed (bottles and cans), humidity is not an issue. However, if the product has specific humidity requirements, such as fresh produce, RH becomes an issue. The higher the required humidity, the lower the delta-t betwen the evaporator saturation temperature and the box temperature.

So, the delta-t for refrigeration applications can range from 7 degrees to 20 degrees.

For your R-409 a example, the delta-t should be about 15 degrees. (40 degrees - 15 degrees = 25 degrees)

michael terry

I have seen

dirty rooftop condensers. with air handlers equipped with
txv valves .running 300 psig of head (r-22) forcing enough
refrigerant thru the valve,as to make the valves bulb respond by closing. thus increasing head pressure and sub cooling.AS I understand valves are rated for abt 100 psi differintial.so txv valves have difficulty controlling refrigerant flow at higher head pressures.so long story short,clean those condenser coils.Ive seen 300high side with 50 on the low side,after cleaning 225 hs with 65 on the low side.still wasnt able to get all the cotton dust out of the evap coil. but food for thought.


mike

will smith_4

charging systems

Lots of ways to charge a unit, lots of ways to go wrong. I've been working on commercial systems for 15 years now, and the one thing I've learned is that if you follow only one way, you will get burned. RTUs are easy-change a compressor, weigh the charge in. (most charge weights are listed) Fire it up, check delta t's. What if the coils are dirty? Well, clean 'em. What if the duct wasn't sized right? What if they ran flex with no thought of proper cfm? (happens ALL the time here in Illinois-you don't need to pass anything other than a check to get into HVAC in Illinois except in a few locales)Split systems- are they piped right?
Long story short: What has Dan always said? The cause is rarely in the same room as the problem. In our trade, you can't assume the guy before you knew what he was doing. Don't try to " crank up the pressure" without looking at what the system is connected to. The low bid gets the job far too often-then we get to deliver the happy news:there is always a result!

will smith_4

charging systems

Lots of ways to charge a unit, lots of ways to go wrong. I've been working on commercial systems for 15 years now, and the one thing I've learned is that if you follow only one way, you will get burned. RTUs are easy-change a compressor, weigh the charge in. (most charge weights are listed) Fire it up, check delta t's. What if the coils are dirty? Well, clean 'em. What if the duct wasn't sized right? What if they ran flex with no thought of proper cfm? (happens ALL the time here in Illinois-you don't need to pass anything other than a check to get into HVAC in Illinois except in a few locales)Split systems- are they piped right?
Long story short: What has Dan always said? The cause is rarely in the same room as the problem. In our trade, you can't assume the guy before you knew what he was doing. Don't try to " crank up the pressure" without looking at what the system is connected to. The low bid gets the job far too often-then we get to deliver the happy news:there is always a result!

Eugene Silberstein 3

Airflow, airflow, airflow

Always, always, always verify airflow before adjusting the charge!

Eugene Silberstein 3

Superheat: Too much of a Good Thing can be Bad!

I have received a few e-mails about superheat and its affects on system operation, so I bumped up this thread which contains a bunch of stuff on superheat, airflow and related issues.

I must restress that the purpose of superheat is to protect the compressor from having liquid refrigerant retujrn to it. If compressors could pump liquid, system efficiencies could be greatly increased by having the evaporator operate completely filled with liquid.

As the refrigerant in the evaporator boils a latent heat transfer is taking place, which is a change of sate with no change in temperature. During latent heat transfers in the evaporator, many btus are transferred from the medium being cooled to the refrigerant.

Once all of the liquid boils of in the evaporator, the refrigerant is still much cooler than the medium being cooled, so the vapor refrigerant can still absorb btus. However, adding heat to the vapor in the evaporator will raise the temperature of the refrigerant. This is referred to as a sensible heat transfer. Sensible heat transfers facilitate the movement of substantially fewer btus than latent heat transfers.

Therefore, if an evaporator operates with high superheat, the compressor will be in less danger of having liquid return to it, but the amount of heat that the evaporatpr absorbs will also be reduced.