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zolar1
02-05-2002, 11:20 PM
I keep hearing about superheat this and superheat that.

What is it, how is it used, and how do I calculate it? (In SIMPLE easy to understand terms please)

What do I do with the resulting information? How much is too high? How much is too low?

All my past work was done with a guage and thermometer - if the pressure seems ok (I use the scale on the guage or chart versus the pressure indicated for typical refrigerants) and if it's cold enough for the aplication, then I leave it alone.

herefishy
03-05-2002, 01:05 AM
Zolar...

Go to the book store, and buy the text "Modern Refrigeration". It is usually readily available.

Or go to http://oberon.ark.com/~airekool/rb2.htm . There's a paragraph on Superheat. :)

Gary
03-05-2002, 02:29 AM
Not too surprisingly, I recommend...my books. :)

Dan
03-05-2002, 02:48 AM
Rather basic question coming from you, Zolar.:)

For refrigeration or air conditioning purposes superheat is the measurement of the temperature of the vapor versus the saturation properties of the vapor.

It is an important measurement in three areas of the refrigeration cycle:

1. At the outlet of the evaporator
2. At the inlet of the compressor
3. At the outlet of the compressor.

1: On systems that employ a restriction device for expansion, a small amount of superheat is desired to guarantee that all the liquid has boiled into vapor. The amount of superheat desired can change depending upon the application, but 10 deg F superheat is not a bad general target at the outlet of the evaporator. Low temperature refrigeration might call for a smaller superheat, whereas A/C could be okay with a larger superheat.

The thinking behind this is that the higher the superheat implies that you are wasting available evaporator surface.... think of it as tubes not filled with a boiling liquid.

2: At the compressor, superheat becomes important for a similar but also different reason. Let's say we have no superheat at the evaporator; this implies that liquid is overflowing the evaporator. That might be okay for the evaporator, but could mean death to the compressor. The minimum numbers you wish to see for superheat at the compressor are more like 25 deg F up to 40 deg F. Why? Because all systems are dynamic and if you read a low superheat at the compressor, it could happen that there are conditions where there is no superheat.... frosted coils, failed fans, etc. It is a safety margin at the minimum superheat spectrum.

There are also maximum inlet superheat considerations. Too high a superheat at the compressor inlet will significantly affect the superheat of the discharge of the compressor. This is where oil loses viscosity, slight amounts of moisture interact with the refrigerant and form acids, carbon deposits, copper plating, etc.

High superheats at the compressor are also relative to the duty of the compressor. Seeing ice on an end-bell of an ice cream compressor is a good thing. It is possible to have 60 deg F superheat under such a condition. No danger of liquid floodback, and decent cooling for the motor and minimisation of discharge superheat.

3. As I mentioned, high suction superheat produces a high discharge superheat. If you measure the temperature of the discharge line 6 inches away from the outlet of the compressor, you should mentally add from 50 to 75 deg F to that measurement to estimate the hottest temperature inside the compressor. The temperature the oil is exposed to.

Actually, this measurement is not so much "superheat" as it is temperature. The relativity between pressure and temperature is still, however, significant. For example, you could lower the discharge pressure and decrease the discharge temperature.

So maybe superheat is only important in 2 measurements: 1: Evaporator outlet, and 2: Compressor inlet. Superheat for each has a minimum and maximum ideal which splits attention toward making best use of the evaporator and protecting the compressor.

LOL! You asked for simple!

zolar1
03-05-2002, 03:23 AM
Thanks Herefishy, Gary, and expecially Dan.
What was confusing me was the difference between superheated steam (much higher temperature) and 'superheat' as defined by Dan. Although the 2 are similar in nature, the temperatures are quite far apart.
I have the book Modern Refrigeration already, but had difficulty understanding that partictular part. I guess it was because I kept thinking about steam tables and such (Hot) and refrigerant superheat (refrigerant being relatively cold). Therein lies my confusion. I have looked at various catalogs and haven't found sufficient test equipment regarding the measurement of superheat. Again, I learned much "old school" information (going strictly by gauge pressure and thermometers) which never included the direct measurement of superheat. Perhaps my mentor felt it best to trust gauges and charts rather than math...dunno?
continued.......

zolar1
03-05-2002, 03:31 AM
I am more able to relate superheat with what I'm reading on the gauges and thermometers. The 10 deg F superheat on an evaporator directly corresponds with what I call 'Balancing' the evaporator. I try for 5-8 deg F "superheat" as it's called. So my methods were sound, it's just that I didn't equate the terminology versus the methodology. Or perhaps it would be the question of "IQ" tests whereby caucasian kids were asked what a "duce-and-a-quarter" was and the african-american kids knew what it was right away (it actually turned out to be a Buick Electra 225 automobile).
It seems a slight difference between what I was taught and the theoretical side of things - but I'm learning.

zolar1
03-05-2002, 03:40 AM
Gary, I looked at the list you have to offer, but my wife REFUSES to let me sell a kidney on ebay to get the books. I have to earn lots more money before I can afford to buy much else. I'm still paying off nearly $4000 borrowed to buy basic refrigeration tools.

Herefishy, I looked at the url you posted, and it is indeed easy to understand. I already knew the basic configuration of a refrigeration system. It's the theory I needed to learn to complete the 2nd phase of my quest. The third phase would be experience - of which I need more.

Remember - even Albert Einstein didn't know everything! LOL

Again, thanks for all the help. Dan did forget to mention the procedure and tools needed to measure superheat. Do I tape my thermometers on the lines?

zolar1
03-05-2002, 03:55 AM
I was looking at the refrigeration basic's site again, and in their diagrams, I didn't see a drip loop or accumulator. The drip loop would be cheap, but maybe the accumulator wasn't needed for their examples.

Regarding superheat, by the addition of an extra heat exchanger, aren't your readings going to be amiss??
I wish I knew how to post a pic/drawing here. I could post a modification I made to a refrigerator and see what you guys think.
It really subcooled the liquid, and the compressor was barely running about no load amps when I got done. Equilavent to a 100watt light bulb.

zolar1
03-05-2002, 03:57 AM
PS just so you all don't think I'm a ditz or something, I attended the Naval Nuclear Power School, Orlando Florida while in the US Navy. Calculating the speed of a neutron seemed simple compared to calculating superheat - LOLOL

Prof Sporlan
03-05-2002, 04:41 AM
Calculating the speed of a neutron seemed simple compared to calculating superheat

The Prof is still trying to understand how a neutron can be made up of two down quarks, one up quark, and each having a different color..... red, green, and blue. Did the Naval Nuclear Power School teach elementary particle physics? :p

zolar1
03-05-2002, 02:17 PM
LOL...you sound like the M & M guys in the commercial! LOLOL
I wish I could answer that question, but I'm not sure if I would get into trouble or not - I have a baby on the way and don't want to mess that up, sorry.

But you can rest assured, everything that is needed for the job was taught.

Dan
04-05-2002, 12:58 AM
The Prof is still trying to understand how a neutron can be made
up of two down quarks, one up quark, and each having a different
color..... red, green, and blue. Did the Naval Nuclear Power
School teach elementary particle physics?

No. If the navy taught it, it would have been red, white, and blue. Funny though. I was in charge of servicing the commissary before they shut it down. Guess what my biggest problem was?

High suction superheat at the compressor units!LOL.

Dan
04-05-2002, 01:10 AM
Regarding superheat, by the addition of an extra heat exchanger,
aren't your readings going to be amiss??

Evaporator superheat should be read before a suction to liquid heat exchanger. If you want to really "milk" the evaporator surface, you could read the superheat after the heat exchanger, but I wouldn't recommend that unless you installed another heat exchanger after it. I am trying to recall something that Marc suggested on his website that I really liked.

Gary
04-05-2002, 01:48 AM
That would be the two heat exchangers with the TEV bulb mounted in between them, Dan

aaron crimmins
04-05-2002, 03:30 AM
if you want a good tool for reading line and surface temps get a supco pt100, also it only costs $50 u.s. , i also have a question about superheat. why would you ever read a colder suction line temp at the compressor then at the bulb location? this isn't a quiz i ran into this today and it threw me for a loop, it should be warmer. i double checked it with three other thermometers any ideas?

Dan
04-05-2002, 03:43 AM
i also have a question
about superheat. why would you ever read a colder suction line
temp at the compressor then at the bulb location? this isn't a
quiz i ran into this today and it threw me for a loop, it should be
warmer. i double checked it with three other thermometers any
ideas?


Let's try this scenario: You are overfeeding the evaporator and misreading that you have superheat. There is pressure drop in the suction line so the saturation temperature of the refrigerant lowers and you get a colder suction return temperature.

Gary
04-05-2002, 04:00 AM
That makes sense, Dan. If there were a restriction in the suction line, it would become even more pronounced. And it would imply liquid at the restriction.

What was the superheat at the bulb, Aaron? And the temps at the coil outlet and compressor inlet?

aaron crimmins
04-05-2002, 04:17 AM
two tev are feeding this unit, it is a split row evaporator i have 12f superheat at one tev and 15f at the other. i am taking my suction pressure at the compressor and my line temps at the bulbs, but i'm showing 4f at the compressor. only thing in between the evap. and the compressor is a accumalator, about a 25 foot pipe run on the same level. i have to go back next week to look at it closer, units cooling good, i was just doing a p.m. check and the suction line felt damn cold. seems really goofy

Gary
04-05-2002, 05:08 AM
What temps do you have at the bulbs?

aaron crimmins
04-05-2002, 05:26 AM
r-22... 60psi suction, 38f suction line temp at compressor, 46f at one bulb,49f at the other.

Gary
04-05-2002, 05:38 AM
Let's imagine that the pressure in the coils is actually in the neighborhood of 80psig and full of liquid, that there is a restriction in the accumulator, dropping the pressure to 60psig. I'm wondering what the subcooling is.

zolar1
04-05-2002, 06:45 AM
Shouldn't the 2 TEV's be set at the same superheat?

aaron crimmins
05-05-2002, 02:46 AM
hmmmm. i'll check the subcool. also what is the pressure drop for a typical accumalator?

Prof Sporlan
05-05-2002, 03:12 PM
r-22... 60psi suction, 38f suction line temp at compressor, 46f at one bulb,49f at the other.
Interesting....


Let's imagine that the pressure in the coils is actually in the neighborhood of 80psig and full of liquid, that there is a restriction in the accumulator, dropping the pressure to 60psig. I'm wondering what the subcooling is.
A very noticeable temperature drop across the accumulator should be apparent if this were the case.

If the temperature drop were gradual over the length of the entire suction line, it might suggest a very undersized suction line.

Another possibility: are you sure you have R-22, and only R-22, in the system?

At the moment, Gary's idea of a restriction seems to be the likely culprit. Find the temperature drop.... :)

walters
06-05-2002, 06:34 PM
Hello Family,
I am kind new in this business and like to learn a bit by reading your questions and answers - great source, thanks the technology!
I've been learning about superheat (slowly...) and discovered a great contribution at
http://www.achrnews.com/CDA/ArticleInformation/features/BNP__Features__Item/0,1338,8033,00.html
It's Superheat charging curves for technicians, written by master John Tomczyk.
Imesure superheat with my Fieldmaster and a special thermo-clamp attached, and of course, I mesure the presure with my manifold.
If I get enough money for my kidneys, I'll buy the SH-31N Superheat and Subcooling Gauge from http://jbind.com/
(some 220 dollars, don't tell my wife...)
Y'all have a great day - walter.

Dan
07-05-2002, 12:55 AM
r-22... 60psi suction, 38f suction line temp at compressor, 46f at
one bulb,49f at the other.

One thing is likely, liquid overfeed on a 38 deg F suction line... you only have 4 deg F superheat by your measurements and that could likely be the insulation effect of the pipe. I defer to the previous questions regarding the confusion of the readings.

As far as pressure drop through the accumulator it should be 2 psig or less.

Gary
07-05-2002, 01:20 AM
Hey... when did John become a master?... lol

zolar1
07-05-2002, 04:38 AM
Walters, I hear ebay has a good sale in kidneys...lol

zolar1
07-05-2002, 04:49 AM
Which would be the best choice of the two:

An electronic sight glass -OR-
a clamp on thermometer?

Gary
07-05-2002, 12:36 PM
If you can read subcooling, you don't need a sightglass, electronic or otherwise.

walters
07-05-2002, 12:48 PM
As mentioned, I am a beginner myself and have to learn a lot, I do this with my computer on the Internet. I have no experience with a sight glass; but I had already a Fieldpiece SC66 meter which comes with a temp. probe. I discovered the clamp a while later in a supply store, it is actually a simple metal clamp with two plastic pieces attached (with glue). One of the pieces has a hole with the temp sensor mounted (again with glue), I think everyone could do this at home!
If you'd like, I e-mail you a pic to let you see how this was made.
I use it already for a couple of weeks, it really helps a lot. I want to know exactly what I am doing, this method gives me some confidence.

Joke: Yesterday I did not even know how to spell ingeneer, today I am already one...
lol - Walter S.

zolar1
07-05-2002, 02:54 PM
OK...skewl time (he he)

How do you read subcooling and with what tool(s)?

<Just not used to all this fancy terminology>

zolar1
07-05-2002, 02:56 PM
Sure Walters, you may send me an email. The address is in my signature below.

Gary
07-05-2002, 10:24 PM
OK...skewl time (he he)

How do you read subcooling and with what tool(s)?


Your first step should be to go to my website and look for a book called "TECH Method Lesson Series". :)

Just as superheat is temperature above saturation, subcooling is temperature below saturation.

In other words, subcooling is the difference between SCT (saturated condensing temp) and liquid line temp.

walters
08-05-2002, 12:32 PM
Originally posted by zolar1
Sure Walters, you may send me an email. The address is in my signature below.
OK zolar1 - give me a few days, I'll put it up on my Website. Also - I need to take some pics first, but I am terribly busy right now.
Also, no question, I'll buy the "TECH Method Lesson Series", I just need a bit more time and money to get there.
See you soon - Walter S.:)

Gary
08-05-2002, 02:25 PM
If anyone would like to see what my customers think of my books, go to the alt.hvac newsgroup and look for a thread titled "Who is using this?"

Just to whet your appetite, here is a quote from a contractor in Columbus OH:


Give me a kid out of hvac school, this book, 30 minutes to talk to him and a week of him using this and he will smoke your ass.
This get your attention?

This is like taking 25 years experience of someone good and dropping it in your lap.

walters
09-05-2002, 03:50 PM
Here are some pics on how I setup my superheat calculation for charging a unit. Hope it helps a bit...
http://Appliance-Repair-Shop.com/subc.html
walter

sturrup
09-05-2002, 07:28 PM
:) Super heat is that heat energy (usually expressed in degrees) above saturation. Super heat value may be used for various troubleshooting applications, however, I use super heat most frequently to determine evaporator efficiency, and TXV operational parameters. (ie how well TXV is working according to OEM specs).

There is an excellent article (Form 10-135) on SPORLAN's website that spells out in detail how knowledge of super heat (and subcooling) may be used to thoroughly analyze a refrigeration or air conditioning system.

Hope this information helps.

William Sturrup
Senior Field Supervisor
Bahamas Air Conditioning Institute
Nassau Bahamas

frank
09-05-2002, 09:11 PM
Hi William

Nice to see new "faces" on the forum. We visited the lovely island of Nassau a couple of years ago - having a job you love on an island like that is only a dream for most of us. How do you cope with the extreme ambient temperatures making your equipment run at maximum capacity all the time?

Most of our kit in the UK seems to run well beolw par due to the low ambients yet we still design for the worst situation - normally 28deg C at high summer! I BET THATS JUST A CLOUDY DAY TO YOU!

When the ambient rises here we seem to get overloaded with breakdown calls as the systems start to do some REAL work.

Frank

walters
10-05-2002, 02:37 AM
thanks william,
I specially like the link to the site...
Well - I finally found it - here we go:
http://sporlan.com/10-135.htm
I think it's a pretty good contribution!
Have nice day - walter.