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i was talking to someone the other day about txv bulb location on systems with a heat exchanger. he was saying that to get the most work out of the evap the bulb should be placed after the heat exchanger before setting your superheat to 5K. any thoughts?
page 10
http://www.danfoss.com/NR/rdonlyres/.../PFA00A102.pdf
cheers
richard
G'day Adam
No the Tx valve bulb must not be located after the heat exchanger. As you probably know the bulb senses evaporator superheat. placing it after the heat exchanger would give a false reading and you would not be able to set the valve to ensure correct amount of refrigerant liquid to the Evap.
I would have thought that the correct location for the tx valve bulb is between the outlet of the evap and the external equaliser line if there is one fitted.... Anyone correct me if I am wrong..
Regards
Tonto
Hi Adam,
Who ever told you that, does not understand the roll of the bulb and its relation to the function of the TXV.
The bulb must be, in that case, located at the outlet of the evaporator!
Chemi:)
Hi Adam,
If you mean a suction line heat exchanger/subcooler, the bulb does not go after the heat exchanger. The TXV MUST be placed at the outlet of the evaporator to control the evaporator superheat.
If the bulb is placed after the heat exchanger, the superheat added to the suction gas will cause the TXV to overfeed. You could argue that by doing this the evaporator may get more efficient, since more liquid is allowed into the coil.
The additional liquid in the evaporator would decrease the evaporator superheat and improve the evaporator performance. Theoretically.
However, the suction line heat exchanger also increases the system performance by subcooling the liquid refrigerant slightly. And, the additional superheat, picked by subcooling the liquid refrigerant, can be considered useful. Both of these improve the system performance also.
Placing the bulb after the heat exchanger is questionable I think. It also removes some margin of safety from the system. If the TXV overfeeds there is nothing to provide protection for the compressor (unless a suction accumulator is installed before the compressor.
I would say someone is trying to get too creative in their approach to refrigeration systems.
You can do that but the heat exchanger must not be too big. You also have to set the superheat to more than 5K (atleast 7 - 10K)Quote:
Originally Posted by adam
You can do that but the heat exchanger must not be too big. You also have to set the superheat to more than 5K (atleast 7 - 10K)
Surely this will cause liquid to flood back to the compressor due to the evaporator being flooded... Goes against the principal of a heat exchanger if you ask me.Quote:
Originally Posted by adam
In addition the TXV is being tricked so your actual 5 dgerees superheat will be alot lower.
If this was on a freezer and it was a hot day so it wasnt condensing all that well , how is the tev ever going to close to bring the suction pressure down???Quote:
You can do that but the heat exchanger must not be too big. You also have to set the superheat to more than 5K (atleast 7 - 10K)
this was his argument. he told me that the rating on the coil was factory tested with a flooded evap. i accept that a flooded evap can do more work than an evap that has a 5k superheat at the evap outlet, but i was wondering what would happen on lower or higher ambient days when you have more/less subcooling than usual. wouldn't the change in liquid temp effect the feeding of the coil, due to the bulb being after the heat exchanger?Quote:
Originally Posted by US Iceman
i don't see how you can get liquid flooding back if you have 5k superheat:confused:Quote:
Originally Posted by crit71
I have read your all your comments with interest, because this was also an issue almost 36 years ago when I started in the refrigeration industri in Denmark.
Mounting the TEV sensor after the heat exchanger was frequenly done when we made a custom built air handling unit with a large area evaporator.
It was actually recommended by the supplier of the evaporator and heat exchanger.
In this way the evaporator became smaller and cheaper because a smaller part of the evaporator was used for creating the necessary superheat signal.
It worked and the suction gas superheat corrosponded to the compressor manufactures requirement for R-22.
The next time I was involved in mounting the TEV sensor after the heat exchanger was in connection with the first Danfoss electronic controlled expansion valve type TQ-PHTQ around 1985 on chillers with shell and tube evaporators.
Mounting the TEV sensor after the heat exchanger gave normally a much more stable sensor signal, especially at low load conditions.
The reason for this is the shell and tube evaporator, where we can have superheated gas in the top pipes and non superheated gas or even small amounts of liquid at the bottom pipes.
In such a situation the TEV sensor will sence a signal which is a mixture of what happens from the top to the bottom pipes.
So in this case it was also advantageous to mount the TEV sensor after the heat exchanger.
The conclusion must be:
As usual in refrigeration and air conditioning the truth is normally not either black or white, but can be anything in between.
It depends upon the hardware, the load conditions, the refrigerant, the TEV sensor signal at various loads, and ....
Normally the TEV sensor is not mounted after the heat exchanger, but there can be refrigeration systems where the mounting of the TEV sensor after the heat exchanger can/will improve the system stability and performance.
Replies to the statement from Marc O'Brien
1. It was not normal to have an extra heat exchanger after the TEV sensor.
2. In the water chiller case the idea is to get a much more stable signal for the TEV sensor.
3. I can see your point, but why not insulating the pipe?
4. We made a lot of test, here at Danfoss, in the Lab for the for the TQ/PHTQ on chillers and we found what I described as having different superheat from top to bottom in a shell and tube chiller.
We also found that a so called "demister" was more effective than a traditional heat exchanger.
A demister is a small vessel with steel wool inside, placed in the suction line where the heat exchanger normally is.
The change from two temperature sensors to one temp. sensor and a pressure transmitter came later and has nothing to do with the gravity/distribution problem in the evaporators.
Sorry I forgot to tell you that I have worked for Danfoss for almost 29 years and was involved in introducing the Q-product form our technical marketing department in 1985-1986.
Regards Jorgen
Make No Mistake Bulb Goes Onevap Suction Outlet But Before External Equalising Line
Mounting the TEV bulb and hopefully the equaliser line after a Suction / Discharge HX would result in loss of efficiency. (Tried to paste a picture in with the three components TEV, Evaporator, HX but cannot figure out how to do it)
Assume that the HX adds 5°C to the gas stream and the TEV is controlling to 5°C superheat then the controlled gas stream before the Heat Exhanger / Mixture leaving the evaporator would be at 0°C superheat. We have not completed the removal of heat from the evaporator by acheiving the required superheat. While in this case we would only be adding some sensible heat in the gas phase it is still part of the efficiency of the unit
If the evaporator is operating under conditions of zero superheat, the heat transfer surface is absorbing more heat. This assumes the distribution of the liquid/gas mixture is uniform. Two phase heat transfer is better than single phase, especially with a vapor.Quote:
Originally Posted by adacus1
In effect, with no superheat at the coil exit, the amount of heat transfer should increase using the method described.
If the heat exchanger is used and the bulb is placed after the heat exchanger, the necessary superheat to protect the compressor must be generated by the heat exchanger.Quote:
Originally Posted by adacus1
This brings up another problem; Can the heat exchanger itself generate sufficient superheat with the available contact (or residence) time?
This superheat would be considered useful, since it does contribute to the overall system efficiency by increasing the vapor enthalpy and decreasing the liquid enthalpy.
Jorgen also brings up a interesting point: liquid distribution in shell and tube chillers. I have seen this also, and experienced similar problems with stable superheat control. (I'm assuming he is discussing direct expansion chillers with the refrigerant in the tubes.)
These do not use refrigerant distributors for some reason (probably cost) and the maldistribution of the 2-phase mixture is a real concern.
Again stating the not so obvious, full load operation is easy to design for. All of the in-between operating conditions (part load, ambient effects, etc) can have a direct impact on the operation of the TXV's.
Another point that is sometimes missed on TXV's is: How much superheat is available for control? On a shell & tube DX chiller with a low approach temperature, the evaporator superheat cannot be greater than the approach temperature.
Low approach temperatures help to improve the system efficiency, but at the same time they can also create additional control problems due to the larger surface area when operating at part load conditions.
Dear All
Let's clearly define the purpose of having a HE in a system:
1. To acheive high refrigeration capacity in evaporator.
2. Maximize utilisation of evaporator on setting the thermostatic expansion valve for minimum superheat.
3. To prevent sweating and iced-up of suction lines.
If the bulb is placed after the HE:
* Suction line will be iced-up
* More non-useful superheat will be pick up along the suction line outside the refrigerated box(higher TD between ambient & suction line)
* The net refrigerating effect will be decrease
* Higher risk of liquid flooding back
Please correct me if I am wrong.
Cheers
Freddy
i can't see how you could have liquid flooding back if you have measured a superheat of 5k:confused:Quote:
Originally Posted by freddyng
Hi Adam
You are absolutely right. There shouldn't be any floodback if you are measuring 5 degree K superheat. But, when during a warm start-up(with sensing bulb located after HE), the evaporator pressure will drop rapidly when the compressor is started and the bulb pressure remains high until the temperature of the bulb is cooled to the normal operating temperature by the suction vapor.
Naturally, because of the high bulb pressure, the valve will be unbalanced in the open direction during this period and overfeeding of the evaporator will occur until the bulb pressure is reduced. This will eventually cause liquid slugging in the compressor.
Hope my reply does make sense.
Cheers
Freddy
How is the superheat at the TEV bulb location determined ?
Hi,
My bulb for the TXV is located on the suction line, also my TXV is externally equalized.
I get a bit of sweat and small layer of ice on the suction line but I have been told it's normal, from reading this thread though it seems like it's something bad.
Thanks.
Hi
This issue seems to be a never ending story.
The advice could be --> mount the TEV sensor where you have the most stable signal.
My expirience is: It can be in many different places on the suction line in a system.
Advice is: Try and get some expirience.
Regards jorgen
what about the location of the heat exchanger in all of this and what type,, ie plate or wrap arround, if the situation arises that the HX is positioned close to the evap and the TEV bulb is fitted after the HX you get as suggested liquid flooding through the HX and partiely back to the compressor, that is not healthy for the HX,even recently i had a head scratching situation where the system had low back pressure on a med temp cold room,i eventully found the cause to be a imploded HX more than likely due to frost damage,just thought i would throw that one into the equasion..:D
Download and read this...Quote:
Originally Posted by gsjsio67
http://sporlan.jandrewschoen.com/10-9.pdf
:Dpeople are obviously reading that brian
We live in hope ;)
so what you or your friend are saying is that every system should operate with a flooded evaporator and the compressor should be protected with a heat-x.Quote:
Originally Posted by adam
That flies in the face of what most of us i think were tought.
The tev is actually a superheat control valve and by introducing a heat-x you are actually relying on the heat-x to provide the 5k superheat that the design demands for efficient operation. you could argue that the heat-x needs to be close to the evaporator outlet, or even in the refrigerated space to meet the design, but surely this is complicating things outside of that which we were tought. You are making the heat-x an integral part of the control strategy, instead of using it to increase the efficiency by reducing the liquid temperature.
i can only rely on my experience with food storeage systems utillising dx cooling coils, but, by potentially allowing liquid to exit the evaporator and enter the heat-x this type of system with a mechanical valve may not respond fast enough to prevent liquid from entering the compressor.
i can see certain designs that would operate safely using this method but surely we are entering the realms of bad practises are we not.
Now i am going to re-read the sporlan info to see if my theory is backed up.
I must have had a no brainer day when I wrote that:p :o.Quote:
Originally Posted by adam
BUT......I would almost certainly say that...even with 5K SH, the chances of liquid getting back to the compressor are higher. This is a risky game to play in my mind. The TEV is after all, a mechanical valve....and not a particurlally fast one at that. If it suddenly started snowing in the middle of summer for example, this would almost certainly cause liquid to enter the compressor :o:D:p