yes. 100 degrees is needed for proper operation of txv

 

So the txv won't work properly below a 100 degrees saturated? Why?

 

but i will also add something else make sure ur sh/sc is correct

 

Subcooling is defined as the difference between the refrig- erant liquid temperature and its saturation temperature. For example, the amount of subcooling of R-22 liquid at 85°F and 196 psig is calculated as follows:
saturation temperature of R-22 liquid at 196 psig = 100°F subcooling = 100°F - 85°F = 15°F
Adequate subcooling of the refrigerant liquid is necessary to prevent the formation of liquid line vapor due to pressure losses in the liquid line. Vapor in the liquid line, even in small quantities, will measurably reduce valve capacity. Several methods by which liquid line vapor can be pre- vented in spite of relatively high liquid line pressure losses are explained in Bulletin 10-11.


. Determine the liquid temperature of the refrigerant entering the valve — The TEV capacity tables in Bulletin 10-10, are based on a liquid temperature of 100°F for R-12, R-22, R-134a, R-401A, R-402A, R-404A, R-407A, R-407C, R- 408A, R-409A, R-502 and R-507.

 

Nice cut and paste...but that bulletin also states the txv can work and be designed around other temp other than 100 degrees. There is nothing set in stone that the liquid temp has to be 100 degrees......if there was we would have tons of problems with alot of equipment.


To the OP.....can you run under 100 degrees...sure. Your friend said he set it up that way, because the compressor was running hot. What was the discharge temp and superheat?

 

The TXV needs a certain amount of pressure to work with. It is dependent on TXV and evaporator details. Ideally, the cooler the liquid the better provided you have the required pressure.

I have been having some battles at work with another tech who saw fit to remove all water regulating valves. He reason was all the R12 retrofit freons are known to have high head pressure. Try as I might he couldn't get the concept of adjusting TXV for correct superheat and thinking discharge gas temperature, not pressure.

Get the gas back cold enough and you should be able to keep the hot gas temps within reason. IIRC the magic number is 225 degrees or under six inches from discharge valve.

I have some boxes that run 162.5 psi @ 110 SCT and some that run almost a hundred psi more for the same SCT. The ones with the higher head pressure run cooler because they are freezers and the suction gas is a lot colder.

 

Everything so far mentioned is correct. The liquid line must be full of liquid refrigerant at the txv without any vapor present. The combination of liquid line temperature and pressure drop across txv will influence the valve capacity. If the valve that was selected is not the correct match, then it is possible that the saturation temp will be slightly off. The txv superheat setting is of equal importance for it regulates the loading capacity of the evaporator coil.

 

:ditto::yes:

:yes:

 

On the chance that I may be way wrong.....tell me how TXV capacity is reduced by increased sub cooling assuming same actual pressure drop from inlet to outlet of evaporator.

SCT, just like SST, is a number we are all used to and comfortable with. In the end those conditions are only present someplace inside our coils. The majority of the freon is superheated (as I see it). If I could have my liquid line at the same temp as the highest box temp....and could insure a full head of liquid through perfect insulation and such.....I'm thinking I get a lot more out of a given TXV/evaporator, not less.

But maybe I'm way off base and don't know what I think I do???

 

On the chance that I may be way wrong.....tell me how TXV capacity is reduced by increased sub cooling assuming same actual pressure drop from inlet to outlet of evaporator.

SCT, just like SST, is a number we are all used to and comfortable with. In the end those conditions are only present someplace inside our coils. The majority of the freon is superheated In space volume, maybe, but in refrigerant weight...Uhm... NO(as I see it). If I could have my liquid line at the same temp as the highest box temp....and could insure a full head of liquid through perfect insulation and such.....I'm thinking I get a lot more out of a given TXV/evaporator, not less. The compressor VE determines the capacity, not the TX valve, but your close.

But maybe I'm way off base and don't know what I think I do???

Just to keep it somewhat clear, I understand what your trying to say, but be careful. It's easy for the words to get in the way...(I talk with my hands and pictures a lot :yes: )

GT

 

jp,
The question was solely from a hypothetical, theoretical, and informative standpoint. From all the large ac systems I have worked on I was always told that 100 degrees saturated was the target. I naturally thought that there would be a correlation over to the refrigeration world but I guess not. After making the post (other than the leak we have had no other problems with this box), I checked the superheat and found it to be 45 degrees with the box nearly at temp. Of course the sensing bulb was not insulated. If I can make it in tomorrow (thank you winter storm/blizzard) I plan on trying to see if I can do something with it. Thanks for all the suggestions!!!

 

Ryan,

Let's look at this a bit and compare to air cooled units (which I'm much more familiar with)

An air cooled unit will typically have a headmaster valve on it for low ambient control. This valve would have a setting of approximately 180#

Now, depending on the refrigerant, the condensing temp in this case will land between 75 and 85 degrees for most R-22 and R-502 replacement refrigerants. The R-12 replacements use a different headmaster with a lower setting which lands them in the same range of condensing temperatures.


In the final wash, it would depend on how the TX valve was sized what the minimum head pressure is.


Something else to consider. If you lower the head pressure setting more than a little bit, you will probably have to adjust the unit charge as lower head pressures will typically require more refrigerant to flood the condenser.

 

I think this is a topic worthy of more discussion. Energy savings are becoming more important all the time. Many systems are operating the same way we would have set them up 30-40-50 years ago. I'll have to learn more about pulse width modulating expansion valves. Depending on how they operate I think it would be possible to greatly lower head pressure and still obtain good oil return. I also think superheat could be lowered.

It will make for a more complicated system....digital scroll compressors, oil separators, bypass solenoid valves, heat exchangers, etc....but I remain convinced we could do a lot better than what has been the norm.

I keep thinking like this and I'll need a laptop to service a walk in....LOL!

 

In the final wash, it would depend on how the TX valve was sized what the minimum head pressure is.

Agreed JP , I work on racks where outdoor ambiant is about 10f right now, lots of subcooling and low head pressures. No problems when using ballanced port valves less dependent on highside pressure to maintain superheat. We insulate drop leg and liquid lines to maintain subcooling. Standard txv will hunt under these conditions.

 

To the original poster,

Your fellow tech was a bit confused I think. I believe he confused 100º saturation temp with 100º liquid temperature. TX valves are sized for 100º liquid temperature, not saturation temperature. The saturation temp would need to be higher than the liquid temperature. There are correction factors for system designers to use when liquid temp will be cooler.

As for the setting of the water regulating valve, your fellow tech may have also been confused by a rule of thumb. Typical condenser water design temperature is 85º. Figuring a 10º temperature difference across the condenser will provide a 95º leaving water temperature and approach temperature 10º higher than leaving water. This should leave you with a saturated refrigerant temperature of 105º. This is typically what I will set a water regulating valve for.

But with a setting of 92-94º saturated, there should not be a big issue with the way the system operates. It may reduce txv capacity some, but if it working okay, and the customer is happy, then why worry about it?

 

Question for the pros on this, I am not fluent on walk-in freezers or refrigeration systems, I’m an hvac guy.

I am working on a customers freezer system that trips on high discharge temps. Emerson electronic controller.

Found controller tripped on DLL for DLT. 404a. Reset it and measured 30 degrees superheat and 212 discharge temp
6 degrees subcool.

I recovered all refrigerant and replaced txv, evacuated tight and weighed in new 404a to just under receiver capacity.

Adjusted water cooled condenser to achieve 96 degrees liquid line temp and adjusted txv to 9 degrees superheat per spec .

8 degrees subcool .
Does this look good?
Discharge temp runs around 190 degrees, controller is default set to trip at 220 degrees and has before when they leave the door open..
Should it be set higher for a freezer or is there an issue with system?

Clear site glass after drier and cycling , maintaining a 10 degree box.

 

Adjust the SH for 20 - 40* 6” from the compressor.
Bet the discharge comes down to the 175 range! Compressor SH overrules evaporator.

Get
Modern Refrigeration for the A/C Tech by Dick Wirtz!

 

What is the norm for superheat and discharge temps on a freezer with 404a , 10 degrees box.?

What is an ideal liquid line temp when adjusting the water valve?

 

Edit

 

Is this system on a closed loop water cooling or pump and dump?

Is there a water regulating valve on the coaxial condenser?

You should set the high side to 105F condensing so about 254 psig for 404A. With 8 degrees SC with a 96F LL you are pretty close.

Sent from my SM-G965W using Tapatalk

 

If you needed 100 degrees condensing I wouldn't be able to buy the headmasters that I'm installing regularly.
Setting to 100 degrees condensing is old school, just like Setting an old r22 condenser to 68 psi and ignoring everything else.
With a water cooled condenser it's a balancing act if it's a dump system.
You have to weigh the price of water vs the price of electricity with higher head pressure.
My experience has been that I've never had a txv not work properly even at 65 degrees condensing.
If you can set the superheat, then it's functioning properly.

 

I don't know where this gibberish about a TXV needing a set condensing temp. Came from.....

TXV's have 3 PRESSURES that make them function.....ZERO to do with temperature......other than the fact that they are intended to control supeheat.

The "100" thing is likely pressure also, since a good rule of thumb is that most TXV's need at least 100psi of PRESSURE differential to operate properly. And even that is not true in all cases, since many can operate with 60psi. Differential.

 

TXV's have 3 PRESSURES that make them function.....ZERO to do with temperature......other than the fact that they are intended to control supeheat.

Yes and no.

Balanced port TEVS are built in such a way as to negate the influence of liquid pressure.

A non-balanced port valve is influenced by liquid pressure.

3 forces are what we teach younger techs because it's easier to understand, but there's more in the world than JUST what we teach young techs

 

There is a connection between TXV and 100psi. When looking up the actual TXV tonnage on the TXV Extended Capacity Chart it shows that a 1ton r-22 TXV is rated for 1 t at 100psi(just like an oil burner nozzle) . That 100psi rating is for r22 & r502 and replacements. For r12 and replacements the psi test point is just 60psi. So a 1 t "R" TXV is a 1 ton TXV rated at a 60psi PD.

 

I’ll start I new thread I guess I hyjacked this one.