Question about heat “exchangers”
I work on mobile AC systems. Almost every AC textbook characterizes evaporators and condensors as heat exchangers. At first I accepted that without question. After all, they look like little radiators, right? But then as I learned more about the refrigeration cycle, I started to wonder if they wouldn’t better be described as heat “convertors” rather than heat exchangers.
On a heat exchanger, I expect to have a Delta T between the inlet and outlet. If I get a significant Delta T through an evaporator or condensor, I start looking for the problem that’s causing it. Heat exchangers create a change in the temperature of the medium. The job of a condensor or evaporator is to create a change in “state” of the medium (vapor to liquid/liquid to vapor. They convert BTU’s of sensible heat energy into BTU’s of latent heat energy with virtually no affect on the temperature of the medium pasing through them.
What I’m saying seems to be contrary to the textbooks. What am I missing?
New knowledge is priceless.
Used knowledge is even more valuable.
Replies
go play with a residential or commercial system. Lots of temp difference. Sensible to latent is still an exchange.
And check the difference in air temps. Change of state means that btus went somewhere.
Edited 4/30/2009 6:52 pm ET by rich1
There are different delta Ts involved here. In a HXR that is condensing or evaporating the delta T of concern is the saturation temp of the fluid boiling or condensing and the average temp of the other fluid (the fluid that is not changing state). The saturation temp varies with the fluid pressure. You can look it up in an HVAC or refrigeration book.
Bail me out. I'm too big to fail.
I guess what I'm contending is that there is a Delta T in the airflow of condeners and evaporators, but not in the refrigerant. I feel that this is a stumbling block to many AC apprentices because they expect the inlet and outlet temperature to be different. I have no experience with residential systems. However, I do know that a "properly" working mobile AC system will have high pressure entering and exiting the condensor, and low pressure entering and exiting the evaporator. Their inlets and outlets will feel virtually identical to the touch, indicating that there has been no significant change in the amount of sensible heat contained by the refrigerant. This is important to diagnostics due to the influence of superheat and the ease with which it can be detected. The pressure/temperature relationship dictates that one always follows the other. You cannot alter temperature while maintaining pressure. That is a basic law of physics.New knowledge is priceless.
Used knowledge is even more valuable.
You're over thinking this. There is a change of heat when a mass changes state
It would help to look at or at least envision an enthalpy-temperature diagram for a pure fluid. There are three things to consider. First, with no phase change (melting, evaporating), then yes, for a given substance (ok, "fluid," since we're talking refrigeration cycles), the temperature of the fluid must change if the fluid is receiving or giving up heat (technically "enthalpy").Next, while phase change is occurring (evaporating or condensing), if the pressure is constant then the temperature is constant. At a given temperature/pressure pair when phase change is happening, the heat content (BTU/lb) of the vapor is greater than that of the liquid in equilibrium with it. The absorption or release of heat energy is changing the amounts of vapor and liquid. When all of the liquid has been evaporated or all the vapor has been condensed, then the temperature of the fluid will change with further heat transfer.Finally, for any fluid, there is a temperature-pressure relation called the "vapor pressure curve" that defines the pressure exerted by the boiling pure fluid at any temperature. It is exponential in nature, curving up sharply as temperature increases.The vapor pressure curve extends from the "triple point" temperature (where there can be solid, liquid, and vapor phases all in equilibrium with each other, like ice, water, steam at 32 F), to the "critical temperature," where the pressure has become so high that the liquid and vapor phase properties are no longer distinguishable. [OK, perhaps this paragraph is more than you wanted to know, but read on.]To wind this up, as you heat up a liquid that is at a certain pressure, the temperature goes up until the vapor pressure exerted by that fluid against the walls of the container surrounding it reaches the pressure under which the fluid is being held. At that point, temperature stops rising and boiling commences, changing some of the liquid to vapor. While the boiling goes on, the temperature stays constant, although the volume of the liquid/vapor mix must increase if the pressure is to remain the same. Finally, when all the liquid has been boiled off, the temperature of the vapor must rise if more heat is added.All this fits with the refrigeration cycle. When evaporated refrigerant, at some cold temperature and low pressure is compressed, the temperature rises dramatically. At any pressure in the compression path, the temperature is greater than the temperature that would match the pressure on the fluid's vapor pressure curve. This is "superheat."When the compressed and superheated refrigerant vapor enters the evaporator and air is blowing past the fins on the outside of the coils, the vapor refrigerant gives up heat. The temperature drops, as the superheat is removed until the temperature comes down to the vapor pressure curve at that pressure. Then the refrigerant begins condensing at constant temperature until it is all liquid again. At that point, further removal of heat drops the refrigerant temperature again, becoming "subcooled" below the temperature of the vapor pressure curve at the condensing pressure.As long as pressure is held on the liquid refrigerant, it stays liquid. Finally it is released across the expansion valve into the evaporator, where the pressure is held low by the suction of the compressor. The pressure in the evaporator is lower than the pressure corresponding to the fluid's temperature on the vapor pressure curve. The fluid can't stay all liquid at that temperature, so some of it flashes off to vapor. The enthalpy, per lb, of the vapor is greater than that of liquid at a given temperature, and the refrigerant has not yet absorbed any heat from whatever is to be cooled by the cycle (eg. frozen meat, air in a hot room). To keep the total heat content of the fluid the same, the temperature of the vapor/liquid mixture must drop dramatically. It "autorefrigerates." As the partly evaporated refrigerant passes through the evaporator coil, absorbing heat from the air side, the rest of the liquid evaporates. Then it's on to the compressor again to complete the cycle.On the air side, there is no phase change, unless dehumidification is going on at the same time. But then you have an air-water mixture, not a pure fluid. The air side of either the evaporator or condenser will always have a temperature change across it, everywhere along its path.
Heat is a not very exacting term and often used in ways that are not always clear.
Probably the most accurate phrase would be a "thermal energy" exchangers that they will cover anything that has been called an "heat" exchanger.
William the Geezer, the sequel to Billy the Kid - Shoe
the term is "enthalpy". Enthalpy = Internal energy + (pressure X specific volume)
the presence of condensation proves the "heat" is being "exchanged"
-Benjamin Franklin-
"the presence of condensation proves the "heat" is being exchanged"Technically that's true. Latent heat energy is being used to remove BTU's and convert humidity (water vapor) into condensation (water droplets. However, there is often a mistaken assumption that temperature is automatically altered in the process. If a cube of ice melts, does that make it a "heat exchanger"? Not in the conventional sense. There is no Delta T between the cube of ice and the water running off of it. How is that possible? Where did the heat go? The heat (more aptly defined as BTU's of latent or "hidden" heat) was converted to energy to cause the state of change. This is why I believe it is important to caution AC apprentices about viewing and diagnosing condensers and evaporators in the same way as other "heat exchangers". While the air passing through them will certainly absorb or release sensible heat, their job is to promote a state of change in the medium (the refrigerant), not a change of temperature...as in a car radiator for instance. Once the state of change has occurred in the refrigerant, any further delivery of BTU's will actually serve to DECREASE the efficiency of the system.New knowledge is priceless.
Used knowledge is even more valuable.
"Heat exchanger" is the correct term, because heat and temperature are different things. A phase change requires the transfer of heat, which is the function of the heat exchanger.Bail me out. I'm too big to fail.
You're exactly right. But the phase change requires BTU's of heat that can't be detected with any temperature measuring device. The average person assumes that anything having to do with heat can be felt, including young apprentices attempting to learn HVAC. But the latent heat necessary to provoke a change of state will not be seen on any thermometer or pressure gauge. It often takes a good technician many years to grasp this. Some never do.New knowledge is priceless.
Used knowledge is even more valuable.
ummm.......you ever use a sling psychometer?
"ummm.......you ever use a sling psychometer?"I don't even know what that is.New knowledge is priceless.
Used knowledge is even more valuable.
"But the latent heat necessary to provoke a change of state will not be seen on any thermometer or pressure gauge. It often takes a good technician many years to grasp this. Some never do."How can one be a good technician without understanding the basic process of the device?BruceT
"How can one be a good technician without understanding the basic process of the device?"I've known a lot of good techs over the years that didn't truly understand the basics. Some were just gifted; some were just lucky. I've never been either. The only thing I really have going for me is experience. I'm not against textbooks. I happen to be a teacher. I just get frustrated with the people who write the textbooks. Sometimes they know just enough to mislead a student who is trying to learn his craft and may one day have to use those skills to put groceries on his table. Maybe I started this thread just to vent off a little. Sorry, guys. I didn't mean to create a controversy.New knowledge is priceless.
Used knowledge is even more valuable.
No offense, but you can't call yourself an AC tech without knowing how to use a psychometric chart or a sling psychrometer.
It deals with latent and sensible heat.
"No offense, but you can't call yourself an AC tech without knowing how to use a psychometric chart or a sling psychrometer."Why don't we stick to learning from each other, and having some fun doing it, instead of being on watch looking for HVAC tech impersonators.Bail me out. I'm too big to fail.
I agree. But what bothered me was the implication that he is teaching others about AC. And if you don't know pyschometrics you don't know AC.
I see it as an electrician who know volts but doesn't know amps.
I honestly hope I didn't offend.
Having said that, I don't do AC. Up here, it is a seperate trade. We don't have HVAC as a trade. I'm a plumber/gasfitter. We do heating.
I know the basics of AC, but I try to stay away from it, too much other work to do.
Edited 5/3/2009 4:11 pm ET by rich1
Not to mention that you need to know what the temperature difference is in the medium being heated or cooled, ie; water or air. That is where the real part of the heat exchanger comes into play.
The unit with orifice and/or compressor maybe could be considered a "converter", but the finned gizmo, minus orifice, compressor, etc, is just a heat exchanger -- it relies purely on delta T to move heat from one side to the other.
here is a good primer on psychrometrics and AC.
http://www.air-conditioner-selection.com/psychrometric-chart-air-conditioner-sizing.html
Thanks for the link. I had to read through it twice to grasp some of it. However, it was helpful and I appreciate it. I'm not offended by your comments. Most people assume that instructors know everything about what they teach. Few of us do. That's why many of us frequent forums like these.Please bear in mind that I work with mobile AC, not residential AC. Maybe there are some stark differences involved. I've never seen any textbooks on mobile AC that explores psychometric AC charts. If I'm less than credible in your eyes for not being familiar with that tool, I must have a lot of company. Anyway, I sense that I've worn out my welcome with this thread. I regret that. You guys always teach me something new. Thanks for allowing me to offer my two-cents worth. And thanks for sharing yours. I'll try to spend more time reading posts and less time submitting them.New knowledge is priceless.
Used knowledge is even more valuable.
Nope, you ain't worn out your welcome.
All AC or refrig is affected by humidity. AC in a car is a little different in that if you feel hot you just crank down the temp some more. Who cares what the actual temp is as long as you feel cold air blasting out the vents.
A stationary system needs to be sized for design conditions which vary by location. Oversizing wastes a huge amount of energy.
A car AC is built for the worst conditions in North America, but most will never see that.
So being grossly oversized and being checked and tuned in a comfortable shop, I'm not suprised you would find a lot of temp difference. Be interesting to put sensors on and sit in a traffic jam on a 100+ day.
BTW, you do deal with latent and sensible heat all the time. Ever notice that puddle of water under the car after you have been travelling with the AC full blast. Or better yet, found the carpet wet after the drain line gets plugged?