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Thread: causes and affects.
11-22-2012, 02:57 PM #1New Guest
- Join Date
- Nov 2012
causes and affects.
New to the Industry and this site. First year apprentice any help will be greatly appreciated. " What are all the factors that can affect superheat and why". Same for subcooling. thanks in advaced. happy thanksgiving everyone.
11-22-2012, 06:01 PM #2Professional Member
- Join Date
- Nov 2006
What do you think are factors? Try to think your way thru it. You will learn more and understand it better If you put some effort of your own into it. Take a guess...
11-22-2012, 06:08 PM #3
More important, IMO, is understanding the WHY that various things affect superheat.
The important thing to remember is how HEAT affects superHEAT.
Summing it up simply. More HEAT, more superHEAT. Less HEAT, less superHEAT.
11-22-2012, 07:30 PM #4
..and while you are thinking about those ideas, let me point you toward more accurate communication skills, which are essential in this business.
The expression you wish to ask about is "causes and effects," not "affects." Here is a page that will help you nail it down. It is one of those words that means something different, with one letter being changed.
Insure and ensure also have different meanings. When you insure something, you are buying or selling insurance. When you ensure something, you are making certain of it.
11-22-2012, 08:23 PM #5Regular Guest
- Join Date
- Nov 2011
You have to evaluate four numbers generally, to come up with a useful analysis. And to do that, you have to understand the "cause and effect" relationship between the four numbers. And to do that, you have to understand the most basic fundamental of vapor compression refrigeration, which is the boiling point concept. The secondary fundamental is heat transfer.
That's what mechanical refrigeration is all about. Heat energy is transferring either into, or out of, the refrigerant, causing it to either boil or condense. The superheat and subcooling measurements provide some insight as to what degree of heat transfer is taking place.
If you can find a good 4 hour seminar on diagnosing high temp application refrigerant systems, chances are the instructor will spend 3 of the 4 hours reviewing fundamentals, working up to boiling point and heat transfer, as they apply to vapor compression refrigeration. He knows, once you understand that, the analysis is easy.
Somebody will answer your question, and if not, I'll come back and do so. But like I said, you'll fare better, reviewing one of your books, or go to this link for a good review (after the "measuring" discussions).
11-22-2012, 09:45 PM #6
Force and load set the target superheat.
Force is the pressure of the high side, forcing the refrigerant into the fix orifice so it can be measured by the outside air temperature at the condenser. Load is the total heat of the air entering the evaporator and can be measured by the wet bulb temperature. Wet bulb temperature is an indication of the total enthalpy of the air not the dry bulb. Dry bulb is the intensity of the heat. When outside air temperature changes or the heat (enthalpy) of entering air to the evaporator change, the superheat changes. With a 85 degrees outside dry bulb and a 60 degrees wet bulb return air the superheat on a fix orifice is 6 degrees but when the return air wet bulb go up to 78 degrees wet bulb the superheat is now 30 degrees. It all depends on the total heat of the air.
Any time superheat on a fix orifice is charged to the technician need check the evaporator TEET of the air differential air temperature. Temperature difference of an evaporator coil will vary with the total heat of the air entering the evaporator and the load on the condenser. This temperature will vary from 10º F to 30º F depending on total heat of the air or the volume of the air entering the evaporator. If the air going into the evaporator is 80 degrees dry bulb and 79 degrees wet bulb the temperature of the supply air is 72 degrees, If the air is 80 dry bulb and a 55 degrees wet bulb the supply air temperature is 56 degrees. Both need to be check. With air flow at normal operating conditions the temperature difference should be 16ºF to 22ºF depends on the design of the system.
Force and load set the Subcooling
A TXV is charged to the subcooling of the refrigerant, check the superheat, but charge to the subcooling.
Low subcooling means that a condenser is almost empty. High subcooling means that a condenser is over field of liquid. Over charging a system with refrigerant increases pressure due to the liquid filling of a condenser and shows up as high subcooling. To move the refrigerant from condenser to the liquid line, it must be pushed down the liquid line to a metering device. If a pressure drop occurs in the liquid line and the refrigerant has no subcooling, the refrigerant will start to re-vaporize (change state from a liquid to a vapor creating bubbles, flash gas) before reaching the metering device.
An A/C with a Thermostatic Expansion Valve (TXV) is charged to the subcooling of the liquid line leaving the condenser because the superheat is fixed by the TXV. The superheat is specified by manufacturer and is normal between 10 to 14 degrees +- 2ºF in most residential air conditioning systems. Subcooling is the amount of liquid held back in the condenser. This allows the liquid to give up more heat, below saturated pressure- temperature. For every one degree of subcooling at the same condensing+ pressure, capacity will increase .5 percent. Increasing subcooling with an increase of discharge pressure and compression ratio, decrease capacity. Add 5 degrees of subcooling for every 30 feet of liquid line lift to prevent the formation of bubbles, flash gas due to the pressure drop.
11-23-2012, 09:43 AM #7Regular Guest
- Join Date
- Nov 2011
The concepts got easier for me when I began thinking of the measured values as "liquid volume indicators". Excess liquid volume in the evaporator results in low superheat, deficit liquid volume results in high superheat. Excess liquid in the condenser produces high subcooling and deficit liquid results in low subcooling. Then it's a matter of knowing/understanding what can effect changes in liquid volume. And (while taking the type metering device into consideration), those factors are: changes in rates of heat transfer, changes in refrigerant charge and changes in refrigerant metering.
Most of the time, it's about "flow": airflow (heat transfer) and refrigerant flow (rate of metering). If the "flows" are right, the system runs as designed. When the "flows" get screwed up, the system doesn't run as designed.
(And relative to the grammer:
verb (used with object)
"to produce as an effect; bring about; accomplish; make happen: The new machines finally effected the transition to computerized accounting last spring."
11-25-2012, 08:14 AM #8
Trust me, I realize that you young guys have been shortchanged in your public school education for the past 30+ years, so it is incumbent upon you as smart individuals to complete your education, so that you become educated men who are capable of communicating your ideas in a clear and articulate manner.
I would start by reading at least three classic books a year. Those authors were experts in communication, and while you may not ever write in the way they did, you can export ideas about communication in English from that writing style, and incorporate them into modern writing, as you might find in professionally written articles in the Wall Street Journal.
11-27-2012, 05:42 AM #9Contractor locator map
How many times must one fix something before it is fixed?