Hello, 'bout a year or so ago found a method for calculating required SH. (WB*3-80-ambient)/2. WB was at the evap. intake; ambient was outside. Found at hvactalk.com; site is totally revamped and can't research it anymore. Any thoughts??? TIA, Barrie
Happy floodback :-)
BTW, I see NO WAY that theory can work. It makes NO allowance for system capacity, loads, what refer is in use, SHR, and a host of other things.
To start with, you're mixing apples and oranges right off the top - RA WB vs OA DB. The relationship between the two is very complex, very building-specific, very load-specific, very system specific, and can not be assumed in any simplistic formulae.
You make no allowance for envelope insulation or internal loads, even is we assume that you're trying to effectively derive 'load' from the ambient, which is impossible.
Then, we must assume you propose to vary SH as a means of reacting to load. But you don't have load data in the equatino ( you can not simply assume it from WB ! )
Unless you propose an electrically variable TXV or HGB, then this can not be a control strategy, only a charging strategy. As such, it don't work. In fact, as ANYTHING, it don't work.
Basically, you have the SH equivalent of '1 ton per 400 sq ft' in load calcs, if even that much.
You're fired.
< /Trump Moment >Barrie I have superheat charts for Trane, Heil, Carrier, Goodman. All but Goodman use ID wet bulb & outdoor dry bulb. Goodman uses ID & OD drybulb. I also have a program on my calculator for Carrier & Trane. Your formula matches my Trane program, at least on the first few tries. I will try to dig the formulas out of the calculator and post them. email me at sixfoot7 @ sccoast . net with a fax # & I will fax the charts to you. Remove spaces from email address. you can call me at
843-385-2220.Stretch
Yea, why not research 'superheat' and learn the correct proceedures?
kjpro
I'd be interested to see a post of those. Is that ALL they consider, OA DB and RA WB ? Nothing else ?
I understand how a manufacturer, for a piece of equipment they designed, can have such charts / formulae ** for that piece of equipment **, but I don't see how there can be 'one formula' for ALL equipment.
'scuse me but you might want to check the Rheem/RUUD charts..... ID dry bulb within a degree or 2 of comfort level, SP converted to ST vs actual ST should = a 15 - 20 degree deltaT Just keep the SP above 58 PSI or you will freeze.
RA 72 DB / 50 RH , WB = 60 , OA 95 ( hot normal day ) (60 * 3) = 180
180 - 80 - 95 = 5 5 / 2 = 2.5 degree SH ???? I don't think so !!!!! You're gonna have floodback !!!RA 72 DB / 80 RH , WB = 67 , OA 70 ( cool rainy day, lots of building leakage ) ( 67 * 3) = 201
201 - 80 - 70 = 51 51 / 2 = 25.5 SH ???? I don't think so !!!!! ( this implies running a MUCH warmer coil, which is the WORST thing to do in this condition ! )RA 72 DB / 30 RH , WB = 54 , OA 80 ( warm dry day ) ( 54 * 3) = 162
162 - 80 - 80 = 2 2 / 2 = 1 SH ???? I don't think so !!!!!
Take this same day, assume the power has been off and it's mid afternoon. So, the DB inside is up, but specific humidity hasn't changed, @ 58.6 grains ( same as it was in the above )
DB 90, grains 58.6, WB = 66 ( RH 28 % ) ( OA still 95, as above )
66 * 3 = 198 198 - 80 - 95 = 23 23 / 2 = 11.5 degrees of SH, under this max-load hot-startup extreme condition ??? Sorry, no way. That's like half or less of the SH you would expect.The charts I have are manufacturer data. Carrier is different than trane, york, heil etc. stese superheat charts work only for charging systems with pistons or cap tubes, not TXVs. Before you put them down, contact the manufacturer and ask for their charging charts. That is what these are, charts for verifying correct charge in COOLING. Won't work for heat. Carrier & Trane have slide rules with data on them. Others use tabular data that can be graphed The Trane data can be worked into a formula. What do you guys do when you service a unit? Pull all the refrigerant out every time and weigh in a new charge? If you do that on every tuneup - Ho Boy!!! Expensive and time consuming!
Stretch sixfoot7 @ sccoast . net remove spaces to email
Yep.
I wasn't putting them down, I was just saying there is no 'universal formula'. And the one posted here before doesn't work, IMO.
Hell no.
Unit charging chart, of course, if available. Otherwise, you gotta pick a SH that seems reasonable and do it.
TXV systems superheat is only a cursory check just to make sure the TXV is working. The correct charging method for a TXV system is sub-cooling.
ANd still others have a charging chart for heating mode
Not hardley
Superheat and sub-cooling is so much easier
Method for charging cap tube or piston metering device systems using suction superheat (Phew! What a title!)
Enter the chart (or formula) with indoor wet bulb and outdoor dry bulb temperatures. The formula or the intersection on the chart or will yield a superheat in degrees. Allow the system to run at least 10 minutes for conditions to stabilize. Measure the suction superheat at the suction service port using charging gauges and an accurate thermometer. If the measured superheat is above the value on the chart (or formula), add refrigerant. If the measured superheat is below the value on the chart (or formula), remove refrigerant. If the required superheat is below five degrees, add refrigerant if needed till the superheat is exactly five degrees, then stop. This is to prevent flood back. NOTE: in extreme conditions, required superheat will be zero degrees. See previous sentences.
Indoor wet bulb converts to enthalpy, or total heat in the air. As wet bulb increases, load increases and superheat goes up. Outdoor dry bulb determines head pressure to a large extent. As outdoor dry bulb increases, head pressure goes up & pushes more refrigerant through fixed restrictor; superheat goes down. These conditions create relationships that interact in predictable ways. This information can be used by the manufacturers to create charging charts and tables.
I have been using this method since I first heard about it at a Trane training class. It really works! I developed the formula by reverse engineering the graph Trane provided.
Note: W=79 minus Indoor Wet Bulb Temperature
Formula for 10 SEER & Below Trane: (-1*(.4+(W*0.0178))* OD Dry Bulb) + 72 = suction superheat
Formula for 11 SEER & Above Trane: (-1*(.3166+(W*0.016))* OD Dry Bulb) + 62.5 = suction superheat
Formula for Trane Packaged Units: (-1*(.4+(W*0.0176))* OD Dry Bulb) + 71 = suction superheat
I will try to dig out the Carrier formula. It is not as accurate, but is mostly within 1 degree of Carrier's tables. I have these formulas programmed into my graphing calculators to make checking charge easier. Note that I have to use charts or tables for Rheem/Ruud, York, Goodman & Heil. I have no formulas for those brands. I don't know how to post the charts, I can only FAX them & some of them are slightly smudged.
Stretch
RA 72 DB / 50 RH , WB = 60 , OA 95 ( hot normal day ) (60 * 3) = 180
180 - 80 - 95 = 5 5 / 2 = 2.5 degree SH ???? I don't think so !!!!! You're gonna have floodback !!!Trane chart says 2 degrees - go fight with Trane, it is their charging chart. Under these conditions a small amount of liquid will not hurt, the hot motor windings will evaporate it!
RA 72 DB / 80 RH , WB = 67 , OA 70 ( cool rainy day, lots of building leakage ) ( 67 * 3) = 201
201 - 80 - 70 = 51 51 / 2 = 25.5 SH ???? I don't think so !!!!! ( this implies running a MUCH warmer coil, which is the WORST thing to do in this condition !With an old Trane system, the superheat should be 29 degrees, with a New 11 SEER Trane system, the superheat should be 22 degrees, with a Trane packaged system, the superheat should be 28 degrees. depending on conditions, the superheat will vary from zero degrees to over 35 degrees. that is actual systems with piston metering device. If you want constant superheat, install a TXV. That is why a TXV is better than a piston. For efficiency, dehumidification, comfort!
RA 72 DB / 30 RH , WB = 54 , OA 80 ( warm dry day ) ( 54 * 3) = 162
162 - 80 - 80 = 2 2 / 2 = 1 SH ???? I don't think so !!!!!Old Trane & Packaged Trane = 4 degrees, 11 SEER Trane = zero degrees.
Stop thinking and start measuring. Get a trane charging slide rule and a Carrier charging slide rule and see what the equipment manufacturers say. Or start using TXV on all your jobs and charge by subcooling. This is why I use a lot of Lennox! They use TXVs on almost everything split.
stretch
WAY!
Trane charts say 10 to 15 degrees, depending on type of system. Don't go by rules of thumb, you will get a brown thumb. Get Trane & Carrier charging charts and check those conditions again!
Stretch
Take this same day, assume the power has been off and it's mid afternoon. So, the DB inside is up, but specific humidity hasn't changed, @ 58.6 grains ( same as it was in the above )
DB 90, grains 58.6, WB = 66 ( RH 28 % ) ( OA still 95, as above )
66 * 3 = 198WAY!
Trane charts say 10 to 15 degrees depending on type of system. Of course other manufacturers will have slightly different superheats. Get their charging charts instead of using a rule of thunb. Tour thubm might get smashed.
stretch
198 - 80 - 95 = 2323 / 2 = 11.5 degrees of SH, under this max-load hot-startup extreme condition ??? Sorry, no way. That's like half or less of the SH you would expect.
Buy Gary Lloyds books and through all those charts away.
Rich
If you have them in a computer format, email them to me, I can online put them :-)
Writing I may not know good, but online putting I am well at :-)
I printed them using a DOS CAD program called Generic CADD. It does not work on my new machines and printers. I will try to send you a scanned JPEG.
Stretch
Tks, I look forward to them. Yeh, G-CADD was cool, in its day, for sure :-) I think it's still around, but costs an arm and a ball, and won't read old files anyway, probably.
I still can't swallow that 'charge a system to 1 SH when the return air is 90' one. I think there have got to be limits stated on the charts having to do with 'when system is settled down to normal operating range.'
IMO, if you crank a 1 SH when the return is 90, you will guaranteed flood from over charge when it's 72.
Damn ! You just struck it rich !
That's now been replicated on at least 100,000 computers !!! Someone owes you $ 1,000,000 !!!!
Do you take Paypal ?????
Oh, what the hell - here's another $ 1,000,000 for you !!!!!
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