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-   -   TXV question (http://www.diychatroom.com/f17/txv-question-119379/)

 dlloyd 10-06-2011 02:48 PM

TXV question

In a heat pump heating mode, the TXV allows the warm, high pressure refrigerant to lower the temperature and the pressure because of the Gas Law: P/T= constant. (in almost a straight line relationship for R410A). But what happens to the heat energy? When the temperature goes down, it is losing heat energy. Where does it go? I think that it is transformed into mechanical energy to operate the TXV. But I am not sure. It has to go somewhere. It cannot be used up-Laws of Thermodynamics. Any thoughts?

 JJboy 10-06-2011 04:02 PM

http://www.ftexploring.com/energy/2nd_Law.html

It's not losing heat energy its spread out.

 beenthere 10-06-2011 06:16 PM

A pound of vapor can hold more heat energy then a pound of liquid at the same pressure.

 Master of Cold 10-06-2011 07:51 PM

The heat went to the condenser, and was expelled.

 dlloyd 10-07-2011 07:54 AM

TXV Heat

Thanks for your comments but we must go deeper. I was not sure of the answer of where the heat goes as the refrigerant enters the TXV but now I am.

"Heat went to the condenser" No, the refrigerant has already gone past the condenser and still has 118 degrees F of heat energy.

"Heat is spreading out" Where?

"Heat goes from warmer to colder body" Yes, that is the 2d Law of Thermodynamics but where is it going? The temperature of the refrigerant is colder after the TXV.

You guys are partially right. Some of the heat energy is flowing to the colder metal of the TXV and then into the cooler air.

But look at that TXV—It has no electrical power but it has moving parts.
It is thermostaticly controlled with a bulb that picks up heat and transmits it to a diaphram that actually moves to change the orifice size. That takes energy. That energy comes from the heat energy in the refrigerant. I rest my case.

Now, I will give you a harder geothermal heat pump question. The compressor raises the pressure of the gas and the temperature increases almost proportionatly. But the temperature of a substance cannot increase unless heat energy is introduced. So, where does that heat energy come from?

 Technow 10-07-2011 08:32 AM

Heat content has nothing the do with temperature only, but the actual state of the substance.

There is negligible heat transfer across the TXV . The enthalpy is the same on both sides, the mass flowrate is the same thru it....no work is done. The total heat energy on one side of the txv is the same as the other....only the 'state' has changed and this state allows it to absorb heat from the outdoors thru the condenser.

 hvac benny 10-07-2011 08:34 AM

I think you may want to go back and review gas laws and thermodynamics. Hint: Gas lawS.

 Technow 10-07-2011 08:35 AM

Quote:
 Originally Posted by dlloyd (Post 743722) Thanks for your comments but we must go deeper. I was not sure of the answer of where the heat goes as the refrigerant enters the TXV but now I am. "Heat went to the condenser" No, the refrigerant has already gone past the condenser and still has 118 degrees F of heat energy. "Heat is spreading out" Where? "Heat goes from warmer to colder body" Yes, that is the 2d Law of Thermodynamics but where is it going? The temperature of the refrigerant is colder after the TXV. You guys are partially right. Some of the heat energy is flowing to the colder metal of the TXV and then into the cooler air. But look at that TXV—It has no electrical power but it has moving parts. It is thermostaticly controlled with a bulb that picks up heat and transmits it to a diaphram that actually moves to change the orifice size. That takes energy. That energy comes from the heat energy in the refrigerant. I rest my case. Now, I will give you a harder geothermal heat pump question. The compressor raises the pressure of the gas and the temperature increases almost proportionatly. But the temperature of a substance cannot increase unless heat energy is introduced. So, where does that heat energy come from?
Heat of compression

 HVACDave 10-07-2011 10:11 AM

As the liquid refrigerant goes through the expansion valve it is lowered in pressure on the evaporator side which creates a new saturation temerature. As this pressure drop occurs some of the liquid refrigerant has to boil off to lower the temperature of the remaining liquid entering the evaporator.

The heat energy is still tied up in the vaporized portion of the refrigerant that is now going into the evaporator as a liquid/vapor mixture. The vaporized portion of the liquid refrigerant is called flash gas, and typically uses up about 25% of the liquid refrigerant going through the valve. This is a direct loss in refrigerating effect and is a negative thing from an efficiency standpoint, so we try to make sure we have as much subcooling as possible to the liquid before it enters the valve to minimize this loss in capacity.

 Master of Cold 10-07-2011 10:26 AM

If we are going to get picky...
The txv is not responsible for the temperature drop of the refrigerant. The refrigerant must pass through the orifice in the distributor, where the pressure drop is created. The suction from the compressor creates a low pressure area at the back of the orifice. The hydraulic pressure from the liquid refrigerant, pushes the refrigerant through the small hole, causing the refrigerant to atomize, and allow it pick up heat, by increasing the surface area of the refrigerant.

 Master of Cold 10-07-2011 10:28 AM

Quote:
 Originally Posted by dlloyd "Heat went to the condenser" No
I beg to differ....

 HVACDave 10-07-2011 10:38 AM

[QUOTE]
But look at that TXV—It has no electrical power but it has moving parts.
It is thermostaticly controlled with a bulb that picks up heat and transmits it to a diaphram that actually moves to change the orifice size. That takes energy. That energy comes from the heat energy in the refrigerant. I rest my case.

Sorry, but you are mistaken, The energy to move the diaphram comes from the refrigerant pressure that is tied up in the sensing bulb. The refrigerant in the bulb is a sealed system working to create pressure on the top of the diaphram which then moves the valve stem to open the valve. The refrigerant in the evaporator provides pressure on the other side of the diapram to create closing force as well as the spring tension (which is ussually adjustable) is transmitted to the bottom of the diaphram throuhg push rods inside the valve to add additional pressure to close the valve. This is how you adjust the superheat setting of the valve.

The heat energy has nothing to do with this operation other than the heat which is transmitted through the sensing bulb mounted to the exit of the evaporator warming up the liquid refrigerant trapped inside that bulb which then increases tha pressure against the top of the diaphram which opens the valve. As the suction line cools off then, the refrigerant in the bulb condenses and there is less pressure pushing against the diaphram which allows the evaporator pressure and spring pressure to close the valve. When the system is operating, the valve isn't likely to be fully open, or fully closed, but modulating consistently in a range around midpoint.

 JJboy 10-07-2011 11:00 AM

Quote:
 Originally Posted by dlloyd (Post 743722) Thanks for your comments but we must go deeper. I was not sure of the answer of where the heat goes as the refrigerant enters the TXV but now I am. "Heat went to the condenser" No, the refrigerant has already gone past the condenser and still has 118 degrees F of heat energy. "Heat is spreading out" Where? "Heat goes from warmer to colder body" Yes, that is the 2d Law of Thermodynamics but where is it going? The temperature of the refrigerant is colder after the TXV. You guys are partially right. Some of the heat energy is flowing to the colder metal of the TXV and then into the cooler air. But look at that TXV—It has no electrical power but it has moving parts. It is thermostaticly controlled with a bulb that picks up heat and transmits it to a diaphram that actually moves to change the orifice size. That takes energy. That energy comes from the heat energy in the refrigerant. I rest my case. Now, I will give you a harder geothermal heat pump question. The compressor raises the pressure of the gas and the temperature increases almost proportionatly. But the temperature of a substance cannot increase unless heat energy is introduced. So, where does that heat energy come from?
This law has the following important consequences:
1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
2. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.

 dlloyd 10-08-2011 10:52 AM

TXV Heat

Ah, HVAC Dave and Master of Cold are saying the same thing and it is starting to make sense:

They say that as the refrigerant goes thru the orifice, the pressure of course drops and then some of the liquid refrigerant "boils off" or "atomizes" from a liquid to a gas, lowering the temperature. So why is that lowering the temperature? I think it is because when you go thru a phase change from a liquid to a gas, it takes up and absorbs energy in order to loosen the molecular bonds that make it a liquid. That energy must come from the refrigerant,lowering its temperature.

So, there are three factors involved, a small temperature drop due to heating the metal, a small amount lost to operate the TXV and a large loss due to the change of state or phase. I love it!

 dlloyd 10-08-2011 11:05 AM

To TechNow

The question is the opposite of the TXV question. When the compressor raises the pressure the temperature increases. In a pressure cooker this happens but it is energized by the stove heating coil.What does that in a heat pump? In order for any substance to increase its temperature, it requires the insertion of heat energy.

"Heat of compression" Can you explain that better?

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