AC pressure readings and problem with dealer.

John C. said:

I'm guessing then that your system is either on or off? If that is the case, there is no TXV.

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Yes, it is an on/off system.

John C. said:

I'm guessing then that your system is either on or off? If that is the case, there is no TXV.

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I'm confused...Could you clarify that?

A thermal expansion valve is used in a system where you set the temperature you want the cab to be at. The expansion valve opens or closes allowing more or less fluid to the evaporator as the temperature changes. You have a monitor panel that shows a degree reading that you can turn higher or lower which adjusted the expansion valve. Most of the little machines are either on or off. No TXV, just an orifice that passes a set amount of liquid to the evaporator all the time and a pressure switch that turns the compressor on when the pressure goes below a set point. That's at least how they used to be. School me if they are different now.

What I called the anti-icing switch is really a thermostat.

John C. said:

A thermal expansion valve is used in a system where you set the temperature you want the cab to be at. The expansion valve opens or closes allowing more or less fluid to the evaporator as the temperature changes. You have a monitor panel that shows a degree reading that you can turn higher or lower which adjusted the expansion valve. Most of the little machines are either on or off. No TXV, just an orifice that passes a set amount of liquid to the evaporator all the time and a pressure switch that turns the compressor on when the pressure goes below a set point. That's at least how they used to be. School me if they are different now.

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John, experience is something you have, but I think we covered the TXV matter several pages up in this whole mess. I hope I can help a little here. A TXV as you might know them has precisely zero way to be 'controlled' by anything other than temperature. This is quite different from the most modern EEV, which actually is electronically controlled.

A TXV in a mobile system is typically called an "H valve"and has no external bulb on it. It snags the suction temp from damn near where the gas enters.

TXVs are are insanely common today in mobile systems because they add a new layer of adaptation to conditions. This is all done in the valve and nothing any controller can do about it. It is simply a spring, needle valve, and proprietary gas on the power head side to drive the valve. If the gas heats up, it expands, opening the valve more. When things cool off, the valve closes down.

I can tell you with 101% confidence my older CAT excavator has an Hvalve or TXV. I still have the rear trim removed and look right at it when I hop in the cab.

It would be extremely rare for suction pressures to run so low without an TXV. They are able to exploit more capacity in a system. For that reason alone, they are common.

As for having "climate control", whatever that means, that is not likely to have much bearings as a TXV is very much more adaptive making it a better solution for an "on/off" type deal. A/C is never on/off, it is always in a state of establishing balance. This is due to the receiver and variable compressor RPM.

The purpose of the TXV is to make sure the evap stays only 90% full (just throwing a number) and completely vaporizes the liquid before allowing it to return as vapor to the compressor. It does this by comparing the temp of the returning vapor to the pressure inside the evap and opens or closes the liquid feed line by degrees to maintain what it wants to see.

As evap temp gets lower and lower it will begin to get below freezing point. The TXV does not know or care about freezing, so there is another thermostatic switch that turns off the compressor clutch if this happens to cause the evap to stay above the freezing point. This switch only comes into play when the cab is already cooled down and the operator has begun to turn down the air flow so there is not as much heat load on the evap core.

Some cars also had variable displacement compressors or other fancy valves to do this in another way to regulate evap pressure, but I think the simple H-block and thermostatic switch was most commonly used the last 20+ years, until somebody came along and noticed there were no electronics and decided everything needs to have electronics and leave nothing un-electronified.

Please post a diagram of the system in the OP's subject machine. Also could use a diagram of a typical Cat, Deere or Komatsu system. That way we could all see where the components your are speaking of are located in the systems and understand how they work.

The on or off I was referring to is the typical operating modes of the compressors. Push the button or flip the switch and the compressor will or will not operate.

https://shop.bobcat.com/6675534-a-c-valve-block

This should cover a couple models......lol

That doesn't tell me anything about how it works. Everyone can see your expertise but the way you are conveying it is over most of our heads. Do you have a simple line drawing showing the system?

John C. said:

That doesn't tell me anything about how it works. Everyone can see your expertise but the way you are conveying it is over most of our heads. Do you have a simple line drawing showing the system?

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John, my "lol" was in reference to how many models are listed there, which I would conclude they use about the same system for about any cab.

I don't have a schematic of the system in question in this thread, but I know how they work.

The valve in that picture is a TXV/Hvalve/Hblock/metering device. The little disc on one end is called the "power head" that has a proprietary gas in it that will respond to suction like temps. On a stationary system, that 'hat' is usually connected to a tiny copper tube and "bulb" to sense the suction line temp.

You will also see down one of the barrels there is the needle valve. There is a spring on that needle as well, that works in balance with the power head. It is 100% based on suction line temp. There is no way to control those any other way, and usually not even field adjustable, which limits some level of diagnostic in them.

The TXV was ultimately designed to try to maintain "constant superheat". In a fixed orifice system, superheat will be all over the map, and largely underfeed the evaporator in a hot pulldown, thus high superheat. What needed to happen was the ability for a hot system to 'open up', allowing more refrigerant. However, if you run at full bore once things are cooled off, superheat drops to dangerous levels that risk liquid returning to the compressor. This is where the TXV would throttle down, limiting refrigerant to the evap, ensuring it all boils off and leaves with the proper amount of superheat. A ballpark is about 10* of superheat.

But in any system, a TXV cannot fully shut off refrigerant flow as the compressor is still running. This is where temp and pressure switches come into play and if the suction line gets to a low point, it will cycle the compressor off. Due to the liquid receiver, refrigerant will still continue to flow, but at reduced pressure and thus the suction line temp will increase. This will then tell the AC control to kick the compressor back on.

there are also low pressure cutouts on the suction which were common for orifice systems, but they still play a role in TXV because if that pressure is too low, that could mean low refrigerant charge, which could damage a compressor.

Not sure if any of that helps in the clarity?
https://www.mechanicalbooster.com/2017/12/car-air-conditioning-system.html

Here is a link I found with a decent a simplified picture of how the TXV connects. What is unique about the mobile TXV device is the integrated sensing bulb that is reading the temperature of the suction line going back to the compressor. There are no outside controls for that device unless it is an EEV, which is not common for these.

What you are calling a TXV is working differently than what I have experienced. In a climate control system of yesteryear the bulb that controlled the output of gas to the evaporator was in the space where the cold air or chilled water was going too. In the freezer space the bulb was in the freezer itself. In the chill water systems the sensor was at a designated point in the chill water output. In old cars the sensor was in the car under the dash panel.

In your explanation of the TXV I take it that it is a safety device to prevent liquid from getting back to the compressor. It really has nothing to do with controlling the temperature of the air coming through the evaporator. If that is the case, it is little different than a system with an orifice except that it might be safer for the compressor. I can't understand why it would have a bearing on the OP's problem nor how anyone could tell if it was working properly or not.

Please explain the term super heat. I've heard some people use it but no one provided an understandable explanation of what it is and why anyone should care about it.

John C. said:

What you are calling a TXV is working differently than what I have experienced. In a climate control system of yesteryear the bulb that controlled the output of gas to the evaporator was in the space where the cold air or chilled water was going too. In the freezer space the bulb was in the freezer itself. In the chill water systems the sensor was at a designated point in the chill water output. In old cars the sensor was in the car under the dash panel.

In your explanation of the TXV I take it that it is a safety device to prevent liquid from getting back to the compressor. It really has nothing to do with controlling the temperature of the air coming through the evaporator. If that is the case, it is little different than a system with an orifice except that it might be safer for the compressor. I can't understand why it would have a bearing on the OP's problem nor how anyone could tell if it was working properly or not.

Please explain the term super heat. I've heard some people use it but no one provided an understandable explanation of what it is and why anyone should care about it.

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A TXV's purpose is actually to optimize system efficiency by being able to modulate refrigerant flow. It goes into a long explanation but as a byproduct of its function, it does give some protection for the compressor. In a fixed orifice system, you can actually watch the superheat drop, and it will drop all to way to 0, meaning liquid may enter the compressor. One of the fixes for this problem of yesterday was using an 'accumulator', which is basically a small tank on the vapor side, to give more time for refrigerant to boil and help prevent liquid death.

As for superheat, I use water an an example because it is easier to understand it. Basically everyone 'thinks' water boils at 212F.....NOT! It boils at 212F at sea level atmospheric pressure. If we take water up in an airplane, the boiling point drops. As well, we pressurize engine cooling systems to increase the boiling point! As you increase the pressure of water, you will increase the boiling point where it turns to steam. This is precisely what refrigerant does, but at much lower pressures!

So what is superheat? Superheat is the superheating of a gas beyond its boiling point. If you make steam at 212F, but you keep heating that steam to lets say 222F, congrats, you have 10* of superheated steam.

In the refrigeration cycle, having superheat, means that your refrigerant being boiled in the evaporator has not only completely turned to vapor, but it has collected additional heat to 'ensure' it is full vapor. Technically you could run a system with only 1* of SH, but that is riding a dangerous line.

But as with the water example, we see that water's boiling point is a function of its pressure. Refrigerant is the same. This is why you can never know the superheat without knowing the pressure it is running at.

If you reference the pic I put in this thread I think, you will notice there are two pressures and two temperatures. Those values are used to calculate the superheat and subcooling of the refrigerant system. Those values are critical to properly diagnosing a system. Working with just pressures, you are only guessing.

John, you said freezer and chiller so I assume you are speaking of things like meat lockers. In yesteryear, those had fixed orifices because they were usually running under the same conditions all the time.

The TXV is taking over those systems and home AC as well because it is more efficient (and complicated) like everything these days.

The TXV feeds in the max allowable refrigerant to be safe for the compressor. It comes in handy like Fastline said because if it is real hot out and the refrigerant still has bubbles of vapor, it will open farther because those bubbles do you no good, they just pass on through. It always lets in the correct amount of liquid.

They seldom have problems, but when they do, the whole system is screwed up. Almost everything uses some kind of TXV these days.

I was on a ship and water temperature and air temperature changed all the time. What you guys are telling me is that the technology and terminology has changed. So cab temps are exclusively controlled by means of an air mixing door in the evaporator unit. Is that correct.

My next question then is how would you use superheat temperature to check and troubleshoot a problem? How would you even measure it? What would make you want to check it?

Everything has superheat. Aboard ship I am sure refrigeration techs checked superheat. Orifice systems are pretty hardy but you know they always want to make things more complicated.

Automotive refrig systems have used TXV of one sort or another since the 50s. They were never exclusively fixed orifice, just 2 ways to skin the cat.

Superheat is the number of degrees the return line is over the saturated liquid temperature at that pressure.

https://learnmetrics.com/r134a-pt-chart-pressure-temperature-chart/

The evap is usually runs 90% full of liquid. Let's say it is running 30F. That is 26 psi. The liquid comes in the bottom and after it has a chance to flash off, all that liquid from bottom to top will be 30F. Once all the liquid is gone at the top, it is all vapor and that vapor gets superheated another 10F so now it is leaving at 40F. That way no liquid can be carried back.

https://www.acservicetech.com/post/the-hvac-total-superheat-charging-method-explained

Birken Vogt said:

Everything has superheat. Aboard ship I am sure refrigeration techs checked superheat. Orifice systems are pretty hardy but you know they always want to make things more complicated.

Automotive refrig systems have used TXV of one sort or another since the 50s. They were never exclusively fixed orifice, just 2 ways to skin the cat.

Superheat is the number of degrees the return line is over the saturated liquid temperature at that pressure.

https://learnmetrics.com/r134a-pt-chart-pressure-temperature-chart/

The evap is usually runs 90% full of liquid. Let's say it is running 30F. That is 26 psi. The liquid comes in the bottom and after it has a chance to flash off, all that liquid from bottom to top will be 30F. Once all the liquid is gone at the top, it is all vapor and that vapor gets superheated another 10F so now it is leaving at 40F. That way no liquid can be carried back.

https://www.acservicetech.com/post/the-hvac-total-superheat-charging-method-explained

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Have to disagree on this small matter. Running liquid in 90% of the evap would be a serious over feed issue, and be asking 10% of the coil to perform the magic. And that magic is the latent heat of vaporization. In short, no thermal magic happens until that liquid turns to a gas.

It would be more typical to see a heavily loaded evap at 50% liquid, and lightly loaded down to around 20%. This state change can usually be caught on a FLIR camera.

John C. said:

Please post a diagram of the system in the OP's subject machine. Also could use a diagram of a typical Cat, Deere or Komatsu system. That way we could all see where the components your are speaking of are located in the systems and understand how they work.

The on or off I was referring to is the typical operating modes of the compressors. Push the button or flip the switch and the compressor will or will not operate.

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This is from the service manual. I took the pics with my phone, if they aren't good enough I can have my wife make pdf files.

Oops, sorry. I'm about as good at adding files as I am understanding what you guys are talking about. Just meant to do thumbnails. Here's the last one.

The 90% was just a number I was throwing out there for illustration purposes. The technician has no way to know how much of the evap is liquid and how much is vapor. So long as the superheat number is correct, it should work as intended.

Birken: Thanks for the links and the references. They are a great aid in understanding what is supposed to be happening in a properly operating system. I can see now that just putting a set of gauges in the system might not be the whole answer to troubleshooting a problem. I very much doubt that any wrenches in the industry carry a chart that would let them know if the system is properly charged.
fastline: It sounds like you work on these systems all the time and have developed your instincts and procedures for troubleshooting. I appreciate now that there is more to it than just plugging in gauges.
Doug580l: Thanks for the diagram. It helps a lot by providing the graphic showing the metering valve and the bulb placed by the output of the evaporator. The diagram does not show it being connected to the vapor stream and instead indicates it is in the air stream monitoring temperature of the air through the evaporator. That is like the systems I am familiar with.

Since the machine is new and under warranty you shouldn't be messing around with the system until you have no other choice. You may think you are there now. After making sure that you operate the system the way it was designed, cab closed and the cab air flow recirc is on. I would suggest that you run the machine in a test of actual system operation in the conditions that make the problems occur. Write down exactly what is happening and a time for each check you do. You may have to do this in front of the dealer salesman or service manager. Make the paper legible by typing it out if need be. I would next write out questions at the bottom of the test paper. You will need to convince a manager or factory rep that there is an issue and create doubt in people's minds that the troubleshooting was completely thorough.

What was done when the technician checked the system operation?
Gauges plugged in? What were the readings?
Was the covers over the evaporator unit removed?
Was the operation of any heated air door checked?
Was the temperatures of the inlet and outlet to the evaporator checked?
Were those readings checked for proper system charge?
Was the unit tested when the ambient temperature was around 90 degrees?
Were the drains removed and blown out to make sure there was no plugs?

Last thing to bring up is that manufacturers very commonly assemble new machines with minor glitches. Perhaps there is some insulation covering a ducting connection. Maybe the evaporator is partially blocked. Maybe the bulb for the metering valve is covered or in the wrong place. All these things create some doubt in the manager's mind that possibly the technician didn't do a complete job of troubleshooting possibly because he wasn't fully trained. You might even consider finding a qualified outsider to check the system in the dealer's shop. If that person says its right then you pay, if he finds something wrong, then the dealer pays.

I know it is a pain and you didn't buy a new machine to have to go through all this but it is unfortunately part of the process when things like this occur.

Good Luck!