I’m trying to understand the refrigeration cycle. Apparently, the refrigerant picks up latent heat at the evaporator (about 970 BTU’s) as it changes state from a liquid to a gas, and then releases that “same” heat at the condensor (about 970 BTU’s) as it changes state from a gas back into a liquid. This seems illogical to me.
I’ve learned that “latent” is a latin term that means “hidden”, and unlike sensible heat, it cannot be detected as measurable temperature. But energy cannot be created or destroyed, so it cannot be literally “hidden”. It has to exist in some form somewhere.
Can someone clear this up for me?
Thanks.
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Replies
It has to exist in some form somewhere
Exists in the state of the molecules. Correct term is difference in enthalpy at specific pressures and temperatures.
"latent' seems to be a trade vs. engineering term, maybe the 'techs' think it is 'hidden'.
For instance, changing R-12 gas into liquid releases about 67 BTU per pound, the difference in enthalpy. This can also apply to chemical reactions, ice melting to water, etc..
Google "enthalpy" and you will get a much more precise set of definitions.
That 970 BTU (presumably per pound) is likely for water/steam, which has an unusually high heat of vaporization ("latent heat"). In the engineering world, water also carries the synonym "Refrigerant 718."
Heat of vaporization varies from one compound to another, and even for one compound it varies with temperature, getting smaller as temperature increases. As explained in #2, the energy of molecules bouncing around randomly in the vapor phase is higher than when they are more constrained by the tighter packing in the liquid or constrained even more in the solid phase.
Also, the energy released when the refrigerant is condensed at some higher temperature (and pressure) is not only the energy absorbed when it evaporated at a cold temperature, low pressure, but also the energy of compression used to pump it up to the higher pressure. In a heat pump situation, the heat released at the higher temperature divided by the energy used to pump up the evaporated gas is the COP (coefficient of performance, usually 2-5).
Choice of refrigerant depends on the range of temperatures over which the process operates. There are all sorts of fluids that can work in one situation or another.
Anything else?
Edited 6/11/2008 1:36 pm ET by DickRussell
Your understanding of the term latent is correct. Latent energy can be absorbed or released from a substance with no change in temperature. Melting ice, for instance is 32 degF, as ice and as liquid. The difference is the "heat of fusion" or the "latent heat of fusion". The energy is there. When you boil water, you don't wonder where the heat from the burner went, you know it went into changing the water to steam, right?
The difference, as in temperature, is manifest in the energy of the water molecules. Where temperature is a measure of the mean velocity of the molecules, total energy or enthalpy is a measure of the overall energy of the molecules. The molecules in a liquid move in longer paths than those in a solid and have more energy. This is a simplistic and far removed from the classroom explanation, but the jist of it is here. How is it possible to freeze ice cream by adding salt to the ice? Salt raises the melting point of ice and causes the ice to melt more quickly. Melting ice absorbs a certain amount of energy per lb, if more melts, more energy is removed from the liquid mixture, causing it to freeze.
The part of the refrigeration cycle that doesn't make sense, especially only looking at the temperatures involved, is how you can you evaporate (boil) a substance at 45 degrees and condense the same substance back into a liquid at 115 degrees. The difference is the pressure. This why all conventional refrigeration systems have a compressor and an expansion valve/orifice. The lowest energy in the system is downstream of the expansion valve. Low temperature liquid (actually it is usually a mixture of liquid and gas refrigerant) at a very low pressure enters the evaoprator coil. This mixture has much less energy than the air blowing accross the coil. Energy is absorbed by the refrigerant and boiled. Still at a low pressure, this cold low energy gas is compressed by the compressor. Now it at the highest energy level (like 300 psi and 115 degrees F) of the system (hot gas is its term at this point) the refrigerant enters the condenser coil and because the outside air has a lot less energy than the hot gas, energy is given up by the refrigerant to the outside air and condenses back into a liquid. Since liquid takes up much less space than gas, the pressure drops very quickley as the gas condenses. This cool liquid is ready to be passed through the expansion valve for another trip.
Salt raises the melting point of ice and causes the ice to melt more quickly
Just not to confuse folks, but you have a typo there, salt water freezes at a LOWER temperature than fresh water. Thus the melted ice slush is colder to make ice cream than would be a fresh water slush.
BTW, some may not know that 0F is the freezing point of a saturated salt solution (unfortunately a kind of random salt solution including other than NaCL that Gabe* used) For a full story, the below was my first google hit that has most of the story.
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/zero-fahrenheit.shtml
*edit, so I dont confuse - Gabe is Gabriel Fahrenheit, not the Gabe of BT. Is he still lost in the Bermuda triangle??
Edited 6/11/2008 3:31 pm ET by junkhound
Good addition to the discussion. However, just getting picky here (yah, I know, being a nerd), two points""Salt raises the melting point of ice and causes the ice to melt more quickly." -- The freezing point is depressed by addition of salt. However, assuming the bucket is wrapped to minimize overall heat loss, so that the total energy content is largely unchanged, there are two offsetting effects: some of the ice melts, but the heat absorbed to melt it comes from the mix itself, resulting in the temperature of the mix dropping. You get less solid, but more liquid, and the equilibrium temperature is lower so total enthalpy is maintained (a heat balance). The salt/ice/water mix becomes sufficiently cold that heat can flow out of the cream mix, forming tiny ice crystals and becoming ice cream."Now it at the highest energy level .... the refrigerant enters the condenser coil and .... condenses back into a liquid. Since liquid takes up much less space than gas, the pressure drops very quickly as the gas condenses." -- The pressure doesn't drop much in the condensing coil, just a little from friction loss. Pressure is largely held all the way through to the expansion valve. The compressor keeps stuffing hot gas into the coil behind the condensing refrigerant as its volume shrinks, to maintain the high side pressure. If the pressure of the condensed and subcooled liquid were to drop substantially upstream of the expansion valve, it would start to boil off again and autorefrigerate, in the wrong place.At the risk of giving the OP even more info than he perhaps asked for, every fluid has a unique vapor pressure curve, showing the pressure exerted by the fluid at any temperature. For example, at 32 F, water exerts 0.09 psi absolute pressure; at 212 F it exerts 14.7 psi (one normal atmosphere). Actually, just having the vapor pressure curve for water you can calculate the whole psychrometric chart or convert between T, RH, dewpoint, etc. at any atmospheric pressure. But that's off track.A refrigeration cycle operates between two points on the vapor pressure curve of the refrigerant. Vaporization occurs at a low T/P point, as said before. Compression of a vapor results in raising its temperature dramatically. Feel the bottom of a tire pump as someone works it like mad; it gets hot. The discharge pressure required is where the corresponding temperature on the vapor pressure curve of the fluid is sufficiently above that of the heat sink that accepts the rejected heat (outside air temp in the case of an A/C). The compression step always results in superheat, so that some gas cooling must occur in the condenser, down to the temperature on the vapor pressure curve at that pressure, before any condensing can occur.OK, someone else's turn.
You guys are making my head hurt. I'm very impressed with your knowledge of this.
OK, if a BTU is the amount of heat energy needed to raise 1 pound of water 1 degree in temperature at sea level, and it takes 970 BTU's to convert that same pound of liquid water to a vapor, how can water exist as either a liquid or a vapor at 212 degrees F?
New knowledge is priceless.
Used knowledge is even more valuable.
Say the 1 lb of water is at 211 F. It takes 1 btu to raise the temp to 212. But you'll have to add another 970 btu to make it into steam, and the temp will still be 212. Until you add that 970 btu, what you'll have is 1 lb of a mixture of water and vapor at 212.
Think about what happens when you sweat, or put on a wet shirt. It cools you off. Why? The liquid is transforming to a gas, and absorbing heat in the process.
"Think about what happens when you sweat, or put on a wet shirt. It cools you off. Why? The liquid is transforming to a gas, and absorbing heat in the process. "Again, not to pick nits, but to help clarify:
Heat is the average velocity of molecules. The higher the velocity, the higher the temp. When you sweat, the more energetic molecules escape, lowering the average velocity, lowering temp. The heat isn't absorbed, just carried away by the higher velocity molecules. Turn a fan on and more molecules of water at higher velocity containing more energy, more heat, are transferred from your skin to the air, resulting in cooling. Same principle in AC and heat pumps.
Hidden ... from sensible heat measurement ... i.e. a thermometer which measures temperature of a fluid or solid. The energy is in the evaporated fluid that is released when it condenses back to a liquid.
You got the concept right ... don't worry about the rest ... that's for the engineers.