HVAC Site - Professional HVAC Contractors Forum banner
1 - 15 of 15 Posts

·
Tech./Sales Consultant
Joined
·
609 Posts
Discussion Starter · #1 ·
I have a question about txv's and coil temps that I'm not sure I know how to properly explain. The question is that if a txv controls the superheat of a system, does this not keep the coil at a set temperature despite air flow reduction?

I say no. I say that the txv is controlling the superheat but that the same superheat on a coil with less air going across it is going to get colder because it is removing the same amount of heat from less air.

Any suggestions on how to better word this?
 

·
Registered
Joined
·
84 Posts
I have a question about txv's and coil temps that I'm not sure I know how to properly explain. The question is that if a txv controls the superheat of a system, does this not keep the coil at a set temperature despite air flow reduction?

I say no. I say that the txv is controlling the superheat but that the same superheat on a coil with less air going across it is going to get colder because it is removing the same amount of heat from less air.

Any suggestions on how to better word this?
Your discription sounds correct. The TXV's job is not to controll coil temp in any way.
 

·
Tech./Sales Consultant
Joined
·
609 Posts
Discussion Starter · #3 ·
I am being argued with that a vs blower does not reduce coil temperature on a system with a txv because the txv adjusts itself to maintain coil temperature. I am not quite sure where this information originated, but I am getting it from someone who has been in the HVAC industry for quite some time and has been working with air infiltration and IAQ matters for the most part.

There seems to be some sort of coup against the values of vs motor technology that I am trying to understand the reasons for.
 

·
Registered
Joined
·
84 Posts
The coil temp will be affected as a byproduct of the txv controlling the superheat. It (The TXV) senses the suction line temp and pressure weather it is internally or externally equilized. No matter fixed or txv if you lower the blower speed the evap temp will be cooler allowing for better humidity removal.
If the TXV controls coil temp compleatly then restricted air flow from a blocked filter for example should not be able to freez the evap. Since the evap can freez this tells us that a lower blower speed can lower the coil temp.
This is the best way I can describe it. I think this is what you are saying. :blink:
 

·
Tech./Sales Consultant
Joined
·
609 Posts
Discussion Starter · #5 ·
The coil temp will be affected as a byproduct of the txv controlling the superheat. It (The TXV) senses the suction line temp and pressure weather it is internally or externally equilized. No matter fixed or txv if you lower the blower speed the evap temp will be cooler allowing for better humidity removal.
If the TXV controls coil temp compleatly then restricted air flow from a blocked filter for example should not be able to freez the evap. Since the evap can freez this tells us that a lower blower speed can lower the coil temp.
This is the best way I can describe it. I think this is what you are saying. :blink:
Yes, this is what I am going for. You also gave me another way to argue the point from the too low of air volume standpoint. Thanks.
 

·
Administrator
Joined
·
947 Posts
As the CFM through the coil is reduced. So is the sensible capacity. So the coil is colder.

TXV maintains SH, not coil temp.

A coil with a 35°F SST and 12° SH will be colder then a coil with a 40° SST and 12° SH.
 

·
Energy mover
Joined
·
124 Posts
the coil temperature is dependant on the refrigerant pressure. the pressure will dictate the boiling point of the refrigerant which is the coil temp. the superheat is how many degrees above the boiling point the refrig. is at when leaving the evap. the sensing element reads the temp(SH) of the gas as it exits the evap. and throttles in more or less refrig to maintain a certain amount of liquid in the coil so the coil can absord the right amount of btus and return enough cold gas to the compressor.
dampering the airflow over the coil will cause the the btu absorbtion to slow down, dropping the coil pressure hence dropping the coil temp. slower air has less volume of air which means less the amount of btus, however slower air will give more hangtime over the coil for latent heat to be absorded which gives more time for moisture to be removed
 

·
Tech./Sales Consultant
Joined
·
609 Posts
Discussion Starter · #9 ·
  • the coil temperature is dependant on the refrigerant pressure.
    [*]the pressure will dictate the boiling point of the refrigerant which is the coil temp.
    [*]the superheat is how many degrees above the boiling point the refrig. is at when leaving the evap.
    [*]the sensing element reads the temp(SH) of the gas as it exits the evap. and throttles in more or less refrig to maintain a certain amount of liquid in the coil so the coil can absord the right amount of btus and return enough cold gas to the compressor.
    [*]dampering the airflow over the coil will cause the the btu absorbtion to slow down, dropping the coil pressure hence dropping the coil temp.
    [*]slower air has less volume of air which means less the amount of btus, however slower air will give more hangtime over the coil for latent heat to be absorded which gives more time for moisture to be removed
I like this so much, I wanted to put it in a format that my dislexia doesn't have as much problem following.;)

Another way to say it is that no matter what, less air is less load over the same amount of coil, resulting in a colder coil.
 

·
Tech./Sales Consultant
Joined
·
609 Posts
Discussion Starter · #10 ·
Does anyone have a link to any articles written on the affect of lowered air volume on a txv metered coil? The person who is arguing this point with me does not believe anything I am telling him and will only be satisfied if there is a white paper on the subject.
 

·
Administrator
Joined
·
947 Posts
Might want to try Andy Schoen's(spelling?) web site. If any one can tell you what will happen, he can.
 

·
Tech./Sales Consultant
Joined
·
609 Posts
Discussion Starter · #12 ·
Might want to try Andy Schoen's(spelling?) web site. If any one can tell you what will happen, he can.
I was hoping to see Andy at Comfortech at the end of this month, but looks like he won't be making it. Something about chewing through leather....so I'm not asking:001_unsure:
 
1 - 15 of 15 Posts
Top