how to increase the portion of liquid after TXV

[Gary]

Alco also advises TXV bulb be placed between evap discharge & TXV equalization line.

Perhaps this could be a good place to discuss the logic, or otherwise, of this choice?

A thought is that, perhaps the original designers consider the equalization line to create a temperature effect on the suction line & that it would be best to set bulb before this disturbance?

The problem with this arrangement in very compact installations, is that the bulb then often lands up fairly close to the evap discharge header, which has large thermal mass. This is not good.

I'd be very interested in reasoning through this matter.

If the seals inside the TEV leak, then a small amount of liquid will flow through the equalizer line to the suction line. If the bulb then senses this liquid it will close the TEV and starve the coil.

This was a common problem back in the good old days, but is now rare with the closer manufacturing tolerances and new seal materials.

[monkey spanners]

I think the idea with putting the txv bulb before the equalizing line is that if the valve lets liquid flow down the equalizer due to a fault it won't shut the valve down, personally having seen a system getting a lot of liquid back due to this fault i wonder if it would have been better for the bulb to have been after the equalizer and with a bit of luck the customer would have noticed poor system performance.

Something else i have been wondering about is equalizing line size, maybe i'll start another thread for it.

Edit: just noticed Gary's post explaining what i was but betterer

[mad fridgie]

Firstly bulb and equalising position, I think gary covered this well, the older valves used to "weep" so if equalizer line was fitted before the bulb, the TXV would close prematurely.
If I have choice, I install, the heat exchanger between evap outlet and TXV bulb. Your choice is normally finacially driven not purely engineering driven. Is it cheaper to have 15% more evap coil/surface area (to allow for superheating) or cheaper to have a vapor/liquid heat exchanger.
If we look at pumped industrial refrig systems, why do they overfeed by so much? Simply it is to ensure that the evap is fully wetted and that the suction has no superheat (thus less suction line pressure drop)
By increasing liquid sub-cooling and removing the need for ssuperheating in an evap, your are reducing the temperature difference between the product and refrigerant for a specific load and surface area, this will increase the system efficiency. (by raising the system SST)

[desA]

If the seals inside the TEV leak, then a small amount of liquid will flow through the equalizer line to the suction line. If the bulb then senses this liquid it will close the TEV and starve the coil.

This was a common problem back in the good old days, but is now rare with the closer manufacturing tolerances and new seal materials.

Thanks Gary. That makes the logic very clear.

The thing is, wouldn't it be more safe for the system if the TXV shut-down mechanism was allowed to happen? It would protect the compressor.

Interesting...

With an adjustable TXV, the weep could be accommodated via a TXV setting offset, could it not?

[Gary]

Thanks Gary. That makes the logic very clear.

The thing is, wouldn't it be more safe for the system if the TXV shut-down mechanism was allowed to happen? It would protect the compressor.

Interesting...

With an adjustable TXV, the weep could be accommodated via a TXV setting offset, could it not?

The theory is that the weep is not enough to flood the compressor, so it is better to place the bulb before the equalizer.

If the weep were predictable and steady, then it could be accomodated... but it is not predictable nor steady.

[HallsEngineer]

We have all dropped off the end of the world, Seriously. Subcooling is good up to a point. If you get too much your system will suffer. The entire system has to be sized and designed to be used with a "true" subcooler i.e. economiser or as an addition to a high stage evap. For best results he needs to work out the best pressure diff for the valves he has or buy an electronic valve with settings for superheat. For best and simplest results use a VFD on your condenser fans to maintain a steady and optimal head pressure and lag your liquid line.

[mad fridgie]

We have all dropped off the end of the world, Seriously. Subcooling is good up to a point. If you get too much your system will suffer. The entire system has to be sized and designed to be used with a "true" subcooler i.e. economiser or as an addition to a high stage evap. For best results he needs to work out the best pressure diff for the valves he has or buy an electronic valve with settings for superheat. For best and simplest results use a VFD on your condenser fans to maintain a steady and optimal head pressure and lag your liquid line.

Why will your system suffer, if industrial and on a pump system with high or low side flow float, more sub-cooling the better, no negative effects, I think you are misunderstand the difference between pressure ratio (needed to size valves). On a DX system again sub-cooling (not reduced pressure ratio) all positive unless your liquid gets close to SST, at which gas the liquid/vapour volume is so low that you get very little turbulance within the evap (reduced heat transfer properties)
When designing with sub-cooling (economiser or what ever) you allow for this in equipment selection.
On your pump systems why do you think you overfeed, as far as the evap/valve, the liquid is already highly sub-cooled. The overfeed is to ensure fully wetted surfaces.

[HallsEngineer]

No pumped systems are not sub-cooled as the gas is cooled to saturation and then artificially pumped up to around 2 or 3 bars, not massive amouts of subcooling. Then the liquid is fed through a metering device, not to be confused with expansion device and then returned to the drum having taken in a small amount of heat energy around a 1/4 of that of a DX system. The flooded coils are generally sized to accomodate this. DX is a different kettle of fish completely. For example if i take a 134a system and pull the liquid off the reciever and put it through a true subcooler bigger than the system it'self then thats not effiecient. May as well use a seconadry refrigerant. there has to be an energy trade between subcooling and plant cost.
The other point is startup. you take a system sized for 30'c subcooling through an economiser, and at start up you are boned majorly as the system cannot achieve the required subcooling instantly. HP will cause the system to limit and possibly not be able to achieve temp. The system has to be sized to do most if not all of the duty and then bring in the subcooler to boost capacity as the cooler nears setpoint. This is due to the gas in the flute or cylinder being at a slightly higher pressure than suction. Also as you system pulls down the load on your subcooler is increased as the flow rate through your expansion device is higher. This is why economiser ports on compressors are sized to limit the flow of gas.
True subcooling is good but up to a point. best results - lag your liquid line! why do the A/C boys do it?

[mad fridgie]

again shows limited knowledge, liquid sub-cooling occurs on a pump system prior to entering your pot (drum) commonly called "makeup". the float is the expansion device! On the pump side the liquid is sub cooled as the pressure is being elevated above the pressure in the pot. the valves at the coils being auto or manual are pressure reducers. so in them selves are indeed expansion devices.
How and when an econimiser is best used is more to do with the application and is working pressures.
It would seem that you only work on the very large equipment, but there a lot more refrigeration than just this big stuff. The average commercial refrigeration does not have economiser, VSDs, PLC, Variable VI, Infinate unloading and all the other wonderful benefits of large system.
Now insulating your liquid line is only good if the liquid temp is below the ambient temp in which the liquid is flowing, more approiate if a water cooled systems, but as the most common form of condensing is air cooled insulating the liquid line in many (not all) case has a negative effect.
the reason the AC boys insulated there liquid lines is that the expansion device(s) is in the outside unit, the actual liquid is cold and is at low side pressure plus pressure drops.

[US Iceman]

No pumped systems are not sub-cooled as the gas is cooled to saturation and then artificially pumped up to around 2 or 3 bars, not massive amounts of subcooling.

Not quite right. If you look at a PH diagram, by increasing the pressure of the liquid with a pump you are effectively increasing the subcooling. Most people think of subcooling as only a colder liquid. Strictly speaking this is true, until you plot the pump pressure addition on a PH diagram. The higher the pump pressure above saturation the greater the subcooling, and... the liquid temperature doesn't change at all.

[HallsEngineer]

I know this. I work on industrial systems all the time the term expansion means the refrigerant boils itself in order to cool it's self. This does not happen in a liquid overfeed system the liquid entering the evap doesn't mystically lower its temperature lower than it already is! The subcooling as you call it has no effect on the refrigerant at all. It purely ensures better transfer of the liquid. It is not useful subcooling. Usefull subcooling means the liquid does not have to expend as much latent energy to cool it's self to the correct saturation temp. That is useful. A liquid overfeed system purely has throtling valves to meter the flow and balence the system and not over tax your pump. A ratio of 4:1 or close as should be maintained. HP float systems are the bane of the world, but people like them because they are simple and easy to work. Like two stage systems because they are more complicated doesn't mean they are worse. The AC boys lag their liquid line to make heat transfer to the liquid minimal thats the point im trying to make. I know the expansion device is in the outdoor but it is not always (VRV)! I have worked on many small commercial systems also. As soon as you put any refrigerant into any vessel where there is a mixture of gas and liquid it is at that saturation temp. Whether this be a HP reciever or LP reciever. So as the liquid exits your LP vessel there is NO subcooling. If you have subcooling on your reciever, purge it you have air in there. and the float is not the item in question its the metering near the coil. Don't say i have limited knowledge of pumped overfeed systems HP or LP control. And actually most commercial systems now have VSD on condenser fans as they are peanuts for systems that size. Yes you won't always have an economiser but you wont get a lot of subcooling out of an air cooled condenser without detremental effects on the compressor side. You would have to run the head a lot higher than ambient taking more power. And are we discussing SUBCOOLING or not as in liquid leaving my aircooled condenser cooler than the head pressure, which should be only a little higher than dry bulb anyway.
As time goes on and people finally realise how much fridge costs to run they will change their attitude.

[mad fridgie]

Expansion means filling a large void from a small void. in refrigeration terms this means turning a liquid to vapour by means of changing pressure "its boiling point".
In a liquid over feed system, where does the LP liquid come from, you have guessed from expanding HP liquid. This HP liquid is sub-cooled by what ever method. It then goes through an expansion device of some for. I totally agree about sub-cooled liquid coming out of a high side liquid reciever "it is not" it gives the impression of being sub-cooled because the liquid temp is compared against compressor discharge pressure. (I had a thread about this)
A thottling valve is a expansion device, if not then the pressurerised subcooled liquid (out of the pump) would just act like a heat transfer fluid. You have to have apressure drop to bring the liquid pressure back to an evaporating pressure thus latent exchange can happen. You pump pressure is there to ensure that the liquid does not boil down the pipe during transportation.
You overfeed your coils to ensure that the internals of the evap are totally wetted ensuring optium heat transfer co-efficients and to ensure that the suction has no superheated vapour, so reducing piping pressure drop.
Go to your local butchers shop, see if he has a VSD on his 3HP unit, or his little display counter. I think not.
If we look at VRV this liquid line is more likely to be insulated to aid the heating cycle (when keeping energy "in the system" is more important.
Again i will agree with you on when people realise the cost of running refrigeration. This comes down to one simply issue (as far as a compressor, the power user in most cases) "compression ratio" we need to keep SCT low, SST high, then manage all the other problems

[US Iceman]

There are two forms of subcooling. One where the temperature is reduced (the only everyone thinks of) and increasing the pressure above saturation. Both help to prevent flash gas.

Colder liquid helps to increase the net refrigeration effect. Pumping the liquid does not increase this, it just allows you overcome pressure losses (friction or static) so that 100% liquid is delivered to the expansion device, just like subcooling where the temperature is reduced.

You can have similar problems with either form. If the liquid is colder than the evaporating temperature the liquid will not boil until it warms up to the evaporating temperature.

If the liquid pressure is too high on a pumped system the hand expansion valve has to reduce the pressure down to the evaporating pressure before the liquid will boil. This is why you see liquid overfeed evaporators experience something called brining. The liquid does not boil. The same thing occurs if the pumped liquid is too cold also.

[oldesky]

The idea stems from liquid being able to bleed down the equalizing line from the Tx valve, affecting the temperature of the suction line at the bulb and therefore destabilising superheat. Honeywell (Flica) valves say that their passage is sealed so the bulb can be mounted after the equalising line.

[Aik]

Firstly, I want to know: which should be more? liquid or vapor? after TXV. In my opinion, liquid should be much more than vapor.
Thanks a lot!

As example check this:
http://www.sporlanonline.com/20-10.htm
Figure 1 shows the answer...see weight, because mass flow in system is constant.

[AlwaysLearning]

After I install a new TXV or re build one I always adjust it after my temp is reached, turn the adjuster CCW to lower superheat and CW to increase, more liquid lower superheat