View Full Version : Superheat

03-03-2004, 11:14 PM
Bernoulli Law of Velocity: states that with an increase of velocity there will be a decrease in pressure. In the center of a refrigerant pipe with an increase of velocity, the vapor pressures drop. Vapor pressure is less in the center of the refrigerant pipe because of a higher velocity. At the same time, there is a higher pressure and lower velocity on the walls of the refrigerant piping due to friction loss. Liquid may be in the center of the refrigerant piping at a lower pressure and a higher velocity. With the pressure drop in the center of the refrigerant pipe, liquid can be present until .5F to 2F of superheat is attained.

Superheat is the measurement of how full the evaporator is of liquid refrigerant. High superheat means the evaporator is empty. Low superheat means the evaporator is full. Superheat should never fall below 4 degrees or a compressor failure will occur. Never add refrigerant when the compressor is at full load amp (FLA) or running load amp (RLA). High superheat at (FLA) or (RLA) means high heat load on the evaporator or condenser.


Prof Sporlan
04-03-2004, 01:40 AM
If we assume an R-22 a/c unit operating at a 45F evaporator and 110F liquid entering the expansion device, we can determine refrigerant quality entering the evaporator as follows:

hf (110F) = 42.7 Btu/lb
hf (45F) = 23.1 Btu/lb
difference: 19.6 Btu/lb lost by liquid cooling

hfg (45F) = 85.5 Btu/lb

quality = 19.6 / 85.5 = .23 or 23 percent

This means 23 percent of the refrigerant mass entering the evaporator is in the form of saturated vapor. 77 percent is saturated liquid.

But vapor takes up more volume than liquid. The volume percentage (void fraction) taken by the vapor can be estimated as follows:

void fraction = 1 / (1 + [(1 - quality) * rhov] / (quality * rhol))

where: rhov = saturated vapor density = 1.66 lb/ft3
rhol = saturated liquid density = 69.5 lb/ft3

void fraction = 0.93 or 93 percent

Or 93 percent of the volume at the beginning of the evaporator is simply filled with vapor. If we assume saturated vapor at the outlet of the coil, and proper refrigerant velocities, i.e., no liquid trapping in the coil, we can assume vapor takes up an average of about 96 percent of the volume for this evaporator

So a properly operating DX evaporator in this case is essentially empty... :D

Prof Sporlan
04-03-2004, 01:50 AM
liquid can be present until .5F to 2F of superheat is attained.
Surface tension of the liquid refrigerant also plays a role. Placing a sight glass at the outlet of an evaporator coil and varying the superheat controlled by the TEV can be quite interesting! It has been the Prof's experience with R-22 residential units that one will typically see minute refrigerant droplets in the suction gas up to about 7F superheat.

04-03-2004, 02:00 AM
Thats why on an air conditioning system 15 degrees is a reasonable number but what about refrigeration where the superheat is much lower? does the density make the difference :rolleyes: :confused:

Prof Sporlan
04-03-2004, 03:14 AM
No one minds the minute droplets that buzz thru low superheated gas in the suction line. They don't represent enough refrigerant mass that can harm the compressor... assuming they make it to the compressor. But they make interesting discussion as to how liquid droplets can exist in superheated vapor.

One needs to worry about "slugs", however... :)

The Prof suggests TEVs not be set lower than about 4F superheat on low temp coils. Trying to control lower superheats can strain the precision of pressure gauges, thermocouples, and the ability of the TEV to control steady enough. :)

04-03-2004, 04:04 AM
SO you would agree that the vapor phase refrigerant continues to move through various suction line components, picking up 10 F to 20 F of additional superheat and by the time the molecules enter the compressor and refrigerant passing around the electric motor windings and rotor of a hermetic or semi-hermetic compressor they should pick up an additional 30 F to 60 F of superheat as watts are converted to torque then Btu, so that in the short time that it takes the refrigerant to enter the cylinders and be discharged at the top of the piston stroke, the refrigerant may pick up 50 F to 90 F more superheat


04-03-2004, 11:32 PM

05-03-2004, 12:14 AM
Imagine that we place a container of refrigerant on a block of ice, and put a heating pad on top. We now have superheated vapor, subcooled liquid, and saturated surface, all in one place. In a dynamic (energized) state, the static (non-energized) gas laws do not apply.

Saturation exists where liquid and vapor are in direct contact. At the bubble/droplet surface. It is theoretically possible in a running system to have a vapor bubble filled with superheated vapor, saturated at its surface, and surrounded by subcooled liquid. By the same token, it is possible to have a liquid droplet that is subcooled inside, saturated at its surface, and surrounded by superheated vapor.

What we see in the sightglass in more likely to be saturated vapor surrounded by subcooled liquid. Likewise in the suction line, most likely the liquid droplets are saturated inside and surrounded by superheated vapor.

As it happens, I have tested this very phenomenon extensively and found that (under standard conditions) vapor in the liquid line disappears at 10-15F subcooling, and that liquid in the suction line disappears at 5-10F superheat.

I will happily leave it to others to devise a formula for this.

05-03-2004, 12:23 AM
Originally posted by shogun7

What the prof said was exactly what I said but I made it simple .

Prof Sporlan
05-03-2004, 12:54 AM
[quality kg * Vap m/kg]/[quality kg * Vap m/kg + (1-quality kg) * Liq m/kg] * 100/1
Marc, it would appear you have it correct. Translating to the Prof's terms:

(x / rhov) /((x / rhov) + (1 - x) / rhol)) = 1 / (1 + (1 - x) / x * rhov / rhol) :)

05-03-2004, 02:28 AM
We all know that the void fraction wave is dependent on the liquid superficial velocity and the pipe diameter which are two main factors that affect the evolution of the flow regime and the characteristics of void fraction. In addition, the propagating velocity value of void fraction wave is higher in big diameter pipe than that of small diameter pipe. So tell me how did you arrive at your numbers? Did you consider this?

05-03-2004, 09:28 PM
Marc truly old man you really have to calm down! You can audit all you want. But If I truly dont feel that I started this I wouldn't carry on with this nonsense. Ill make a deal with you if you keep your personnel opinions about me to your self I will do the same. I certainly never intended to get in this kink of situation. What do you say? How about a truce?

06-03-2004, 12:09 AM
Why mention Bernoulli?
Bucause he's a friend of mine??
Jeezz Your putting me to sleep again. I don't give a sh.t about all that Just answer my question the ball is in your court Thats all need be said YEA or NAY?

08-03-2004, 08:37 PM
Oh did you say something? Heh! life holds no end of droll amusements for those observent enough as they occur Marc.
Opps! oh hell I went and said it Great...now I'll have to kneel, face the corner and recite appologetic expiations for the next month! Nontheless one can hardley think of a richer jest then to have Adolf race as fast as he can to the finish line with his bambozalment used to contort the simplistic origional statement. In fact, Adolf is so smart that he can relay to us just how many Angels can fit on the head of a pin,what their bra sizes are , which ice cream they favor, and whether they ever had illicit contacts with Paris Hilton! One gets the feeling that he has an ax to grind. As I rather enjoy this I have decided to treat you as a non entity. So hasta lavista, goodbye, so long, adios! you and I have nothing more to say to each other.. cuz!

" the evaporator refrigerant mass flow could be kept as original except now the suction velocity pressure would have increased"
no sh.t harry james.

08-03-2004, 09:22 PM
Thanks Marc, I had my youngest grandson with us all weekend and I didn't want you to distract me (upset) and I needed some time to recharge my batteries>You must remember I have sons as old as you...I think? OOPs there I go again I just slipped the cluth!

08-03-2004, 09:26 PM
Quite so, but then I think you may have the edge!
PS whats with the Adolf?

11-03-2004, 07:39 PM
Thats the American verson and you look different then I pictured you!
Roger the toger

11-05-2004, 08:34 PM
Hi Marc & Shogun

You guys are entertaining!!!! And Marc I really am south african and i'd love to see you repairing a fridge in that gear!!! PS I'm sorry you and the prof stopped giving me a lesson on superheat to engage in your alternative entertainment. I was really trying to make sense of the formulas you were dishing out, but please if possible give us an example, that tends to help a lot, for the newbies.( Thats Me)

11-05-2004, 08:55 PM
Jy is 'n boer dan ne. Thing are actually quite good here, only stuggling to find good fridge mech' s. the crime has really gotten better ( i think) especially in jhb. but it is still the land of milk and honey. I be you miss it!!! Sunshine and all. Just out of interest you had me in tears reading earlier, i too am struggling for time but this forum ......and yours are very informative but you just blow me away with those formulas. where can i start getting up to speed with that type of info? There if F*All in Africa about refrigeration

11-05-2004, 09:10 PM
To put it in a nutshell .........Bring drink to Les and Heidi' s house and put your name on your cD or we are going to steal it. Let us know if you are coming so we can make food for you

From your Teacher

11-05-2004, 09:22 PM
you just blow me away with those formulas. where can i start getting up to speed with that type of info? There if F*All in Africa about refrigeration
Where can we go online maybe for additional info?

12-05-2004, 05:11 PM
I have my N3 but never went for the n4 as i thought is wasn't worth it, do you think it is?

I have tried to get that book but no luck online and no stores here have it, obviously the si version.

12-05-2004, 06:42 PM
Funny, but the South African was almost completely understandable by me.
It seems to be an old female teacher of you. Is this correct?
Is a spitbraai a BBQ?
They never use foreign words or deform existing English words for something new. They invent a new word for a new item.
It's a much richer language then Dutch or Flemish.


13-05-2004, 06:32 AM
Afrikaans is actually a form of dutch a lot of dutch people say that it is old dutch language so it would be very rich, also the afrikaans speaking community are very traditional to a large degree.

23-08-2004, 05:54 PM
Neat stuff here. Never looked at it the way Prof does, but then he is the Prof. Full by one measurement and empty by another measurement. All depends on your parameters and angle of attck.

I do wonder about large diameter pipe effects though (even though it was mentioned in a tungue and check manner). Will the increased velocity at the center of the pipe have any effect. Seems like it would be able to support more liquid entrainment. Larger diameter pipe would have more surface area and lower surface velocity. Maybe the liquid would stick to the side giving less liquid entrainment. I vote the later since they use smaller piping for oil return. (Not quite the same, but thats still my vote.)