superheat

snooked321

I asked this question in another forum.... I am not agreeing with another tech on this..This is for asystemwith a piston. I find my target superheat.Lets just say 8 degrees.I get my low side press. at the port at the cond. I check line temp at the oulet of the evap. Now he is checking it at the suction line at the cond., I say that is going to overcarge because by checking it there would make your whole suction line part of your evap. Now if you were going to check a txv you would get your line temp. at the bulb location which is at the evap. outlet. Both txv and pistons are metering devices and are mesuring refrig. through the evap. not the evap. and the suction line. Before this starts up i know its important to check it at the cond. also and look for around 20 deg. there for proper cooling of the comp. Some input and exactly how you all check this would be greatly appreciated.

DuMass

Well, you said target superheat right?
Don’t target superheat charts specify the suction temp at the condenser inlet, along with indoor wet bulb and condenser inlet air temp?

beenthere

A 50' 3/4" insulated vapor line. On 95° day, could gain as much as 750 BTUs an hour.

On a 2 ton system. this could be a vapor line temp increase of .3°F(2 ton unit at 24,000 BTU TC, with a SHR of .7, for a SC of 16,800BTUs). Your compound gauges have an accuracy rating of +/-3PSIG. Your temp probe will have its own +/- tolerance. Between them. A .3° line temp difference between the coil manifold and the line at the condenser service vale is of no consequence.

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snooked321

Got ya! I will be checking the suct. line temp. at the cond. for now on. Thank you for the responses

beenthere

No problem.

Now for the real tough question.

Where are you reading your wetbulb temp at to determine what your target SH is.

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snooked321

I have been checking it as close to the return grill as possible. That is right isn't it?

beenthere

Actually. On many systems. it should be checked at the furnace/air handlers return itself.

If the furnace/air handler isn't in conditioned space. then part of the return isn't in conditioned space(Some will be in conditioned space, but the return still runs through unconditioned space.. And with return duct leakage. The return wetbulb isn't the same as the wetbulb at the return grille. So if its taken at the return grille, you end up with a lower wetbulb, then the unit actually has being returned to it. And can end up with an over charge. How much depends on how much duct leakage there is. And the outdoor conditions of the day your checking it.

EG: On a 90° outdoor temp day. You could easily get a wetbulb reading of 64 at the return grille. And a wetbulb reading of 66 at the return of the furnace/air handler. That would be a 4° difference in target SH.if the difference was a 4° difference between wetbulb readings(68 instead of 64), it would be a difference of 7°.

If you don't have a digital meter for wetbulb. You can't take a reading in the duct/at the units return. And before the digitals came out. We all pretty much had to take it at the return grille.
And then see if the coil temp difference went up what it should have or not.

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snooked321

snooked321

OK I had to see if the pic would show... lol.. How do i check WBT at the return instead of in front of the return grill with this?

beenthere

You need to drill either a 3/8, or 1/2" hole in either the return plenum, or the filter door.

Then stick the probe end of you digital in it. I use knock out plugs to cover the hole when I'm done.

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snooked321

Beenthere,
After getting your WBT,condensor inlet temp. and suction line temp. What are you using to get your target superheat. Also procedure do you go by when you get a system up and running and the conditioned space is still warm and cooling down? If I find my SH when the space is warm how far off am I? Anybody with some input please chime in here.

snooked321

I am using a slide chart but reading here it seems i could be off on this. I would just like to know how far off i am and if there is a better way to get closer.

beenthere

Usually a SH slide chart. The units chart if it has 1.

The charts are all pretty much based on 400CFM per ton through the evap coil. Since there may be less or more. The charts all include a +/- of 5°. I think some might only be 3°.

When you have a house that is at say 80° with a 72°wetbulb(68%RH), at 95° outdoor temp. A slide chart will give you a target SH of 22°.

Once the indoor drybulb temp and wetbulb drop. if you take another wetbulb reading. You might come up with a76° drybulb temp and 66° wetbulb(59%RH). With the same outdoor temp. The new target SH would be 10°. And when you check your SH. You'll find it dropped to about that.

As the heat energy in the air decreases. So will the SH.

The reason that the target SH is so high in my example. Is because the indoor air contained a high amount of heat energy. So the refrigerant in vapor form was able to absorb a large amount of it. Which means that the vapor temp will be high Giving you a high SH.

The higher the heat energy in the air. the more heat the vapor absorbs.

Normally. Most of the heat the refrigerant absorbs. Is done through the latent change of state, from liquid refrigerant to vapor.

Under high indoor load conditions. The vapor itself absorbs more heat. Which is a sensible heat transfer. And shows up as a high SH.

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snooked321

Thank you so much,this is a big help, but you brought up another question when you mentioned the proper cfm through the system. Is there a way to get actual CFM from static pressure readings on a system without a anemometer? Or even close to what the system should have?

beenthere

A manometer will give you the static pressure the blower is working against.
What manometer do you use for checking gas pressure on furnaces.

But you need the fan chart to know what the actual CFM is.

If you get a pitot tube. You can measure velocity pressure, and convert that to CFM, when you know the sq ft area of the duct.

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snooked321

I never dealt with much gas in florida before moving north and since then have taken a gas furnace course and obtained a position with a company that understands this. I have no problems with the controls. I took the class as a refresher. I do own a magnahelic and understand how to get my static pressure readings. This website has helped me learn the error of my ways in this industry. Thanks Were can I get fan charts? If i ask at work, I am sure i will get, we dont have time for that, if we did all that we would only get 3 calls a day done and the business wouldn't make any money! So... I can accomplish alot on my own and know i am doing things the right way and make happy customers. And sooner or later they will get a grip on things on how quick the right thing can be done in a timely manner?? Right?

snooked321

When I interveiwed i felt the service manager didn't understand half what i was discussing with him or didn't care? I will do what has to be done to make my customers satisfied and at the same time the company i am employed for. I am confident in my skills and at the same time see the shortcomings in the techs. beside me that are taking many shortcuts ( which seems to be the norm).

beenthere

You can get the fan charts from the install manuals.
unfortunately. No one has ever made a full catalog of old units data charts.

One thing that you can semi rely on. is that when you think you have a low air flow problem. And you read .8" or more static pressure. Your are correct, you have low air flow.

Then from the static pressure readings,just see weather its the return, supply or both that need addressed. And can let the customer know from there. What needs to be corrected.

Sell a few duct renovations/moderations. And the company will be happy that you check static pressures.

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cool_air

The way I've learn is by checking the superheat at the compressor but I do check it at the outlet suction line on the evap. the latenent load on the evp coil and hot pull downs will drive up the readings. Reading Hi superheat. as for as subcool goes I check it at both places to make sure that there is a column of liqued ref. interning the metering device. I also check the T D across the conderser. I use superheat subcool charging cal.Digital Fluke temp meters electronic scales Wt bulb dry bulb. Works great.If you really want to check how the system is working get a wetbulb/drybulb at the return and the supplys (averages) then plot it on a psch. chart. find the state points connect them all the way to the left to get the enthalpy BTUS per pound of dry air This will give you the tonage of capacty of the unit is doing. I use a digitial sling sycromter.Ps make sure that your getting the right air flow per ton if not your screwing up!!! Wrong air flow can charge correctly. Don't mean to keepgoing on hope this helps.