ACHRNEWS

The Twiddle Factor: Why Techs Can't Leave TXVs Alone

May 22, 2000


You probably had a busy box when you were a child. I know I did, and I still can’t resist playing with the one in the doctor’s office (my kids serve as a ruse).

For those who won’t admit to being mesmerized by these amazingly simple toys, it’s basically a plastic box with numerous buttons, knobs, slides, and other fun things for children to play with. These wonderful devices can keep babies happily occupied for ages (or even a half-hour).

Now there seems to be a high-tech busy box for service technicians — the thermostatic expansion valve (TXV). Being one of the only parts of the system that can be easily adjusted, that knob on the TXV poses a temptation that many technicians just can’t resist.

Maybe it’s because they want to look like they know what they’re doing. Maybe they have a primal need to turn a knob or adjust a screw. Who knows why technicians feel the need to twiddle with it before determining what’s really wrong with the system.

A lot of times, it’s not the right thing to do. The right thing to do is to understand the system and determine whether or not turning that knob is really going to do anything. Because if the TXV isn’t the problem, twiddling with the knob could lead to even bigger headaches down the road.

Many technicians are tempted to simply turn the knob on the TXV, because it's one of the parts that can easily be adjusted. Resist the urge! Instead, take a few minutes and find out what's really wrong with the system.

What it Can (and Can’t) Do

First, it’s probably necessary to state exactly what a TXV can and can’t do. The thermostatic expansion valve is in the system to provide two basic services to the system. The first is to maximize the usable surface area of the evaporator, which it does by controlling a minimum level of evaporator superheat.

The second role that it plays is to protect the compressor. The compressor is a vapor pump, so it only wants to see vapor. The valve makes sure that only vapor returns to the compressor. Another supplemental role it plays is that of a pressure-limiting device, limiting compressor suction pressure.

What a TXV can’t do is control evaporator temperature accurately. In addition, it cannot control evaporator pressure — it can limit it, but it’s not a very good device for controlling a specific pressure. A TXV also cannot fix capacity problems.

Roy Nungesser, research and development manager, Parker Hannifin, Longwood, FL, says that his company often receives complaints on applications in which there is not enough refrigeration capacity.

“We’ll check all the characteristics that the valve is controlling, and it’s controlling properly. So that means there are other problems in the system that the valve can’t fix.”

In fact, Nungesser says that most of the valves that are returned to his company are found to have no trouble whatsoever.

“We’ll check out a valve five ways to Sunday, and the valve’s got the right capacity, it has the ability to control correctly, and the contractor says, well, there’s probably something else wrong.” But some seem to believe it’s easier to change out a valve rather than determine the real problem in the system.

To determine if the valve is the problem, look at the evaporator outlet superheat and the compressor superheat. Superheat is the main system characteristic that the valve is trying to control. If the superheat seems to be in a normal range, which is usually 5° to 15°F, then most likely the valve is working properly.

If the superheat is lower than 5° and the system is very close to or is flooding refrigerant back to the compressor, Nungesser says that the valve is either adjusted wrong, it’s significantly oversized, or there’s something else wrong in the system. That “something else” could be garbage in the system that’s caught in the expansion valve, a poorly mounted bulb, blocked evaporator, or improperly sized system components.

If the superheat is too high, say above 15°, that usually means there is either insufficient charge in the system or there is a clog or a restriction somewhere in the system.

“To come up with a good diagnosis, read the temperatures and pressures in the system. Then look at the superheat level. Spending 10 or 15 minutes taking a good objective look at those things will save you a half hour of pumping down the system, pulling out the expansion valve, putting another one in, and finding that it still doesn’t work,” says Nungesser.

Know Your Refrigerant

Once you’ve determined that the valve is the problem and it needs to be replaced, it’s time to size and select a new one.

The most important factor in selecting a new valve is to know the refrigerant in the existing system. While there is some interchangeability with TXVs and new and older refrigerants, it’s important to check with the manufacturer.

For example, there is usually still an option to use an older R-12 valve on the newer R-134a system, or vice versa. The same holds true for R-22 and R-407C and R-502 and R-404A.

The same valve may also be used with non-azeotropic refrigerants; however, adjustments would have to be made to the TXV, as these refrigerants have a lot of glide. “The expansion valve basically needs to be readjusted to the control superheat based on the dewpoint vapor side of the saturation curve for the new refrigerant,” says Nungesser.

Where there is no interchangeability with TXVs is in the case of R-410A being used as a replacement for R-22. While R-410A has similar operation characteristics to R-22, the pressures in the systems are completely different, so there is no interchangeability between an R-22 valve and an R-410A valve.

Nungesser points out there are also material compatibility issues with the refrigerant and the oil, so it’s necessary to make sure that any materials that the valve is constructed of are compatible with the new refrigerant and the new oils.

Nungesser stresses that most manufacturers try to address those issues for the technician, making their packaging as easy to understand as possible. It should state clearly on the box which type of refrigerant the valve can be used for and the configuration of the valve — so read the packaging.

Don’t Always Replace With Same

When it comes to sizing a new valve, technicians often replace what’s there with exactly the same size valve. That’s not necessarily the correct thing to do.

TXV sizing should be based on the compressor capacity and the evaporating temperature conditions at which the system needs to be operating. It’s also necessary to determine how much pressure drop will be available to the expansion valve.

Nungesser describes pressure drop as being similar to a faucet in your house. If you’ve got a lot of water pressure coming into your house, and you open the faucet all the way, you’re going to get a lot of flow. If you’ve got low water pressure in your area, you’re not going to get a lot of flow, even though it’s the same faucet.

“It’s the same with the expansion valve. The amount of refrigerant that the expansion valve can flow, which is really a measure of the capacity that it can flow, is dependent on how much pressure drop is available to the valve.

“If there’s a lot of pressure drop across the valve, the valve will have a lot of capacity. If there’s a little pressure drop, it has little capacity.” A quick look at a capacity table can prevent over- or undersizing.

Installation

Once those items are determined, and a new valve is properly sized and selected, it’s time to install it. Nungesser stresses that when installing a TXV, cleanliness is next to godliness. That’s because the expansion valve is the smallest refrigerant path in the system (except for the compressor valves).

If there’s garbage floating around in the system, the TXV will be susceptible to clogging up.

If you have to braze or solder onto the valve, there are a few considerations. If you’re going to use hard solder, it is advisable to keep the valve below 250°.

“We build a valve to withstand a lot more than that, but the valves have seals inside of them sometimes, and if you overheat the valves, parts will start melting on the inside. So when you’re brazing, the rule is get in and get out. Heat it up, get it brazed, get a good joint, get out and cool it down,” says Nungesser.

If you’re going to use soft solder, be careful of the flux you use. “The flux is an acid, and a lot of times technicians will just slop it all over the part, then it gets inside, and it starts corroding the inside of the valve over a period of time. The acid floating around in the system isn’t helping the other components either.”

So use the minimum amount of flux you need to, use the minimum amount of brazing, and use the minimum amount of heat. And if all else fails, read the directions — that’s why they’re included in the package.