Tag: TXV


When mounting a TXV bulb or checking bulb placement there are a few important considerations (listed in order of importance)

  1. Mount the bulb on the suction line. Flapping in the breeze is no good.
  2. Mount TIGHTLY it with a proper metallic strap (usually copper or brass). Not zip ties, not tape.
  3. Position it on a flat, clean, smooth, portion of the horizontal suction line. Not on a coupling or an elbow.
  4. Mount it before the equalizer tube (closer to the evaporator than the EQ tube)
  5. When possible mount it at 8 or 4 o’clock on the suction line (or according to manufacturers specs) . This becomes more important the larger the suction line.
  6. When possible, insulate the bulb so that it is not influenced by ambient air temperature. It never hurts to insulate the bulb even inside the cabinet though not all manufacturers require it.
  7. If you do need to mount it vertically, make sure the tube points up not down

Poor bulb contact will result in a bulb that is warmer than desired, resulting in overfeeding and lower than desired superheat.

Finally… be gentle with the bulb and tube. They break easily.

You can read a more detailed description HERE

— Bryan

We get a lot of questions about both evacuation procedure and TXVs so last week we produced videos on both topics including –

  • Before and after testing of piston vs. TXV
  • Using the Bluvac Measurequick app
  • Use of core remover tools for evacuation
  • flowing nitrogen process
  • creating an external equalizer port and much more

If you haven’t hit subscribe on our YouTube channel yet would you mind taking the time to do it today? it would be greatly appreciated. You can do that HERE 

P.S. – I will be at the Rectorseal booth 2545 in Chicago at the AHR conference on January 23rd at 2PM demonstrating the new Pro-Fit flaring tool. If you are at AHR come by and see me and sign up for a chance to win a free Pro-fit!

I walked into a supply house the other day and I was looking at “universal” expansion valve on the shelf. The friendly guy behind the counter saw me and walked over, after saying hello he offered

“That’s a great valve, it’s even balanced port”.

Now I am a bit of a trouble maker, I should have just nodded and said “uh huh” but instead I asked, “what does balanced port mean?”. The counter guy sort of half shrugged and said “I guess it means it works on a lot of different systems?”

I would bet that most people in the industry have heard the term “balanced port” and figure it sounds like a good thing but don’t really know what it does. Not long ago, I would have been one of them.

We have all been taught that there are three forces that act on an expansion valve –

  1. Bulb Pressure is an opening force
  2. Evaporator Pressure (external equalizer) is a closing force
  3. The Spring is a closing force

while the system is within its design operating conditions these forces are the primary forces at work that allow the valve to “set” the evaporator outlet superheat.

There is a fourth force and that is the opening force applied by the refrigerant passing through the needle. When the inlet (liquid line) pressure is within the normal operating range this force is accounted for in a normal TXV. In cases where the liquid pressure is higher than usual the force will be greater allowing more flow through the coil and when it is less it will allow less flow.

The result of this effect is fluctuating superheat based on liquid pressure which may be acceptable in small amounts but can become unacceptable quickly on systems that require accurate evaporator feeding or systems that have a wide swing in condensing temperatures and pressures.

Sporlan largely solved this particular issue in the 40’s when they brought the “balanced port” valve to market. While the technology is nothing new it has been improved on over time.

Balanced port TXVs can vary in design but they solve this problem by allowing the inlet pressure to effect the top and bottom of the needle (orifice) equally. This eliminates (or reduces) the liquid pressure as an opening force and instead turns it into a “balanced” force that neither opens or closes the valve.

If you have an application where the head pressure is allowed to change or “float” over a wide range, the balanced port TXV is a great choice.

— Bryan

 

 

Photo by Ulises Palacios

Refrigerant circuit restrictions can be common things like a plugged filter drier or a restricted metering device. They can also be more difficult to diagnose and exotic issues like a kinked liquid line, blocked evaporator feeder tube or a compressor connected improperly with a discharge line full of solder (I’ve seen it).

To start with let’s talk about the symptoms.

When an undesigned restriction occurs, refrigerant will “back up” against the restriction resulting in more refrigerant being present before the restriction and less afterwards than designed. Think of it like a refrigerant traffic jam with the refrigerant “road” being congested before the restriction and free and clear afterwards. This restriction will result in a pressure drop across the restriction with higher pressure being on the inlet side and lower pressure on the outlet side of the “traffic jam”.

First we must be aware that a restriction exists in the first place. In the case of the most common liquid line restrictions on HVAC equipment (with no receiver) we will see low suction pressure, high superheat and normal to high subcooling. In cases like this we know it is not simply “low on charge” because of the subcooling reading, and we also know it isn’t just a an evaporator airflow issue because of the high superheat. This leaves us in the realm of restriction.  Like anything else, some common sense, a look at the system history and a visual inspection can find many restrictions without any fancy diagnosis, but sometimes you have to put on your thinking cap, grab a pipe or a cigar, and go to work.

In a perfect world we could just connect a gauge anywhere in the system and we could find the pressure drop, in the real world we only have two or maybe three points on connection and they are not sufficient for us to pinpoint a restriction. Luckily we have temperature drop as a proxy for pressure drop, whenever the pressure drops there will also be a temperature drop. The trouble is, by the time the temperature drops enough for us to reliably measure it with a thermometer it is usually pretty bad, making minor restrictions hard to find.  It can also be challenging when the metering device itself is a suspect (and it often is), because the metering device is a DESIGNED RESTRICTION. This means that a pressure drop is it’s very purpose, but is it restricting too much?

So to actually FIND a restriction you are left with a few tools in your arsenal.

Common Sense

Get acquainted with the history of the system. How old is it? What has been done on it recently? Has the refrigerant circuit been open to atmosphere?  If you recently had a burnout compressor then it is very likely that suction and liquid line driers could be restricted. If the system has been running just fine for 7 years it is more likely that that TXV element tube rubbed out and now the TXV is slammed down. If the distributor just a leak repaired on it, it is very possible that they accidentally filled one of the feeder tubes with solder when they made the repair. A little common sense can save a lot of random hypothesis. Any experienced technician will agree with the problem solving principle called Occam’s Razor that states

“With all things being equal, simpler explanations are generally better than more complex ones”

This certainly hold true when looking for restrictions.

Temperature Drop 

Grab your most accurate line temperature clamp and start making measurements across possible restrictions like line filter driers and the liquid line itself. If you find any confirmable temperature drop across a line drier than you can knows it’s restricted, just make sure to double check. Across a typical liquid line you will generally only see a few degrees of temperature drop but it does depend on the ambient temperature, condensing temperature and the line length.

Freeze Test

Sometime the exact point of temperature change can be tough to locate. In these cases when the metering device, distributor, feeder tubes, inlet screen or evaporator are all suspects you can do the freeze test. Disconnect the blower and watch the frost patterns. On a properly functioning system the ice will start right at the outlet of the metering device and extend forward though the feeder tubes and work its way fairly evenly through the coil on the coil piping route. Look for inconsistencies in the pattern and you can often find a restriction.

If for example, you see that the frost is starting BEFORE the metering device instead of after, you can bet the restriction is an inlet screen. This test is finicky and requires a trained eye to track the tubing patterns, otherwise you might think a coil is restricted when it’s just the way it’s piped. Also be aware that the designed pressure drop of metering devices that also contain a distributer and feeder tubes is cumulative across all of those restriction points. This means that in some cases you may get more frost after the distributer than you do between the metering device and the distributer, this is to be expected.

Photo by Ulises Palacios

Thermal Imaging 

The holy grail of finding restrictions is the thermal imaging camera. You are able to see restrictions in real time and pinpoint the exact location where the temperature change begins. Thermal imaging can even be used to find illusive restrictions like discharge line restrictions caused by poor brazing practices, condenser feeding issues, evaporator restriction etc…

Photo by Ulises Palacios

So the process for finding restrictions is –

  1. Prove you have one by looking carefully at your readings
  2. Use some common sense and perform a visual inspection
  3. Take lots of temperature measurements until you find it
  4. Whip out the fancy pants thermal imaging camera and spot that sucker in no time flat and be the hero with throngs of adoring fans

Keep in mind it get’s even trickier to diagnose when you are working on a system with a receiver, because the receiver can usually hold a lot of excess refrigerant, often making a liquid line restriction appear more like a low charge in the readings. Also, minor suction line restrictions like a kinked suction line can be very difficult to find because the temperature drop will usually be unmeasurably low.

This is why taking all the system readings in conjunction with some common sense and knowledge of the systems history are your best allies. And when in doubt… get a thermal imager from TruTech tools .

I told you it wasn’t easy

— Bryan

Here is a great article addressing restrictions in refrigeration systems – Diagnosing A Restricted Liquid Line Can Be Tricky

The piston (fixed orifice) and TXV (Thermostatic Expansion Valve) are the two most common metering devices in use today, with some modern systems utilizing an electronically controlled metering device called an EEV (Electronic Expansion Valve).  It should at least be noted that there are other types of fixed orifice metering devices like capillary tubes, but their use is not common on most modern A/C systems though you will see them in refrigeration.

While the compressor creates the pressure differential to get the refrigerant moving, by decreasing the pressure on the suction and increasing the pressure on the discharge side, the purpose of the metering device is to create a pressure drop between the liquid line and the evaporator coil or expansion line (the line between the metering device and the evaporator when there is one). When the high-pressure liquid refrigerant is fed into the metering device on the inlet the refrigerant flows out the other side and the immediate pressure drop results in an expansion of a percentage of the liquid directly to vapor known as “flashing”. The amount of refrigerant that “flashes” depends on the difference in temperature between the liquid entering the metering device and the boiling temperature of the refrigerant in the evaporator. If the difference is greater, more refrigerant will be “flashed” immediately and if the difference is less than less refrigerant will be flashed.

Piston

A piston is a replaceable metering device with a fixed “bore”. It is essentially a piece of brass with a hole in the center, the smaller the bore the less refrigerant flows through the piston and vice versa. The advantage of a piston is that it is simple and it can still be removed, the bore size changed and cleaned if required.

piston_flow

Some piston systems also allow the reverse flow of refrigerant as shown in the diagram to the above. In a heat pump system when the reversing valve is energized (cool mode), the unit will run in cool mode and the refrigerant will follow the path indicated on the bottom.  This seats the piston so refrigerant must pass through the orifice.  With the reversing valve de-energized the flow reverses.  This unseats the piston and allows the free flow of refrigerant.  In this case, there is a metering device in the condensing unit (outside unit) that meters the flow of refrigerant in heat mode and one inside that meters in cooling mode.

TXV

The TXV can vary the amount of refrigerant flow through the evaporator by opening and closing in response to evaporator heat load.  compared to a fixed orifice a TXV operates more efficiently in varying environmental conditions (theoretically at least).

To operate, the TXV has a needle and seat that restricts the flow of refrigerant and acts as the orifice.  This needle, when opened, allows more refrigerant to flow and, when closed, restricts refrigerant flow.  There are three factors that affect the flow of refrigerant flow through a TXV.  A sensing bulb filled with refrigerant exerts force to open the TXV.  Since gas pressure increases with a rise in temperature, the bulb, which is attached to the suction line after the evaporator coil, “senses” the temperature of the suction line.  If the suction line becomes too warm, the additional pressure created by the heated refrigerant opens the TXV more to allow additional refrigerant flow.  A spring inside the bottom of the TXV exerts pressure to close the valve.  An external equalizer senses pressure in the suction line after the evaporator, and also works to close the valve. In essence, the TXV is a constant superheat device, it sets a (relatively) constant superheat at the evaporator outlet by balancing bulb, spring and equalizer pressures.

The primary method of charging system changes based on the type of metering device. A piston system uses the superheat method of charging and the TXV uses the subcooling method of charging.

No matter what primary method of charging you use it is still important to monitor suction pressure (Evap temperature) head (condensing temperature), Superheat, subcool and delta t (or some other method of air flow verification).

While a TXV and a piston function differently the end result is a pressure drop and boiling refrigerant in the evaporator.

— Bryan

As we talked about in an earlier podcast, a TXV is designed to maintain a specified and constant superheat at the outlet of the evaporator coil. It does this through a balance of forces between the bulb pressure (opening force), Equalizer pressure (Closing force) and spring (Closing force). It is the spring pressure that can be adjusted on some valves, but why and when would this be done?

For the quick, cut to the chase version, turning the adjustment on the bottom of an adjustable valve clockwise = higher superheat and counterclockwise = lower superheat. However, before you start messing with the adjustment, I suggest you read on.

First, the valve must be an adjustable type, many valves on small equipment are not adjustable and have no hex cap at the base.

Here are some other items you need to consider first-

Proper Subcool

Before an expansion valve can function properly and do its job, it must have a full line of properly subcooled liquid refrigerant all the way to the inlet. On a split system checking the subcooling at the condensing unit is a good start but you also need to make sure there isn’t a significant temperature drop all the way up the expansion valve inlet. Keep in mind that some valves have a screen right at the valve inlet, so a restriction even at that point will cause operational issues.

Required Pressure Drop 

For an expansion valve to function there needs to be a significant pressure differential between the evaporator design pressure and the liquid pressure entering the expansion valve (In many cases 100 PSIG +). During cooler times of the year the outdoor condensing pressure / temperature may drop to the point that the required difference in pressure may not exist and in these cases, the valve may no longer be able to maintain the target superheat. While low ambient controls may be employed to rectify the issue is some cases, in many cases, you must simply be aware that the valve will not function as expected.

Improper Bulb Placement 

Ensure that the bulb is mounted on the suction line flat and tight with a proper strap. It is never a bad idea to insulate the bulb, and anytime it is exposed to ambient air it is a necessity.

When to Consider Adjustment 

Now you are at the point where you can consider whether that valve could use some adjustment. First, read your superheat right at the evaporator outlet in the same general location as the TXV bulb and equalizer in most cases the superheat at that point should be 5-8 degrees but refer to manufacturers specs when in doubt. In some cases, you will not have a pressure port at the evaporator so you must rely on a pressure reading outside. Use common sense when assessing the situation and realize that there may be some pressure drop on a 100′ line set and there should be very little in a 10′ line set. Make some allowance according to the situation.

If the system is running VERY low or VERY high suction pressure and /or superheat readings that are way out of range, it is very unlikely that adjusting the valve will remedy it. Usually, valve adjustments are only for small superheat changes up or down.

The Forces at Play

The bulb pressure is the opening force of the valve, so when the bulb is warmer it exerts more opening force resulting in a more “open” orifice, and when it’s cooler it exerts less opening force resulting in a more “closed” orifice.

The equalizer is a closing force so the higher the suction line pressure, the more the valve is forced closed and the lower the suction line pressure the more the valve is forced open.

The spring is also a closing force and on an adjustable valve increasing the spring tension/force results in lower flow and higher superheat, decreasing the spring tension/force results in more flow and lower superheat. In short, counterclockwise = lower superheat, clockwise = higher superheat.

Making an Adjustment

Before you adjust anything, the system must have been running for a good long while, and you have observed that the superheat has stabilized. You then must check the entire system and surmise that everything else is functional, the valve is being provided with a fully liquid, properly subcooled, high enough pressure feed of refrigerant. If at that point you find it is out of range then you can make adjustments.

  • CAREFULLY remove the hex cap from the base of the valve with a properly sized wrench and a backing wrench exposing the adjustment screw.
  • Turn 1/2 turn at a time clockwise to increase superheat or counter-clockwise to decrease superheat.
  • After a 1/2 turn adjustment, replace the panels and allow the system to run and stabilize.
  • Recheck the superheat and not the change.
  • Repeat as needed until the maximum setting is reached. NEVER force the adjustment screw too far, it should require minimal force to turn other than possibly initially to “unstick” the screw.

Adjusting a TXV / TEV is an advanced skill for a technician who has a good grasp on their readings and the forces at play. Tread carefully.

— Bryan

P.S. – Here is a great resource from Parker / Sporlan and I am also in the process of uploading a video onto the YouTube channel that will be up later tonight.

The TXV powerhead or power element is the part of the valve that sits on top of the valve to which the sensing bulb is attached. The power head provides the opening force for the valve by translating force from the bulb to a diaphragm in the element that forces the valve open.

The sensing bulb and power element contain a mixed liquid / vapor charge, when the bulb temperature increases the pressure in the bulb increases which opens the valve (increases the orifice size, feeding more refrigerant into the evaporator). When the sensing bulb temperature decreases the pressure in the bulb decreases which closes the valve (decreases the orifice size, feeding less refrigerant into the evaporator).

In some cases the power head and bulb can lose the charge, usually due to a cracked bulb tube. This causes the bulb and element to lose all of the pressure and the expansion valve will fail closed / evaporator will starve. The result will be abnormally high suction superheat at the evaporator outlet accompanied by low suction pressure, high subcooling and often freezing at the center to the outlet of the valve (on A/C applications).

Good practice is to first confirm that you have proper subcool all the way to calve inlet. Next, remove the sensing bulb and warm it in you hand with panels on and the system running. With a properly functioning element the suction pressure will increase and the superheat will decrease. If you do not get this response then either the power element has lost it’s charge or the valve is severely blocked.

Also keep in mind that some expansion valves are field adjustable. In this case ensure that the valve is adjusted full counterclockwise (open) before condemning the valve as failed closed.

Keep in mind that some valves have replaceable power elements, check to see if you can replace the element instead of the entire valve to say time and expense for the customer.

 

— Bryan

 

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