Subject : TXV's ( TEV's ) - Thermostatic Expansion Valves explained

From: (paul milligan)

In article <3qtofi$>, (Joseph Huber) wrote:
~~>In article (Steve Lewellen) writes: A point to remember. Do not install a sight glass in equipment that has fixed orifice metering devices (ie: cap tubes, orfice flow rater, etc.)

~~>What is the reason for this??

Well, no one else is jumping in, so here goes....

A TXV ( Thermal Expansion Valve )is an interactive device, in that it senses system ( evaporator ) pressure and temperature, and adjusts the flow of freon so as to maintain a given superheat. Without this, the coil would be starved ( not getting enough freon to work at capacity ) at some times, and flooded ( too much freon, possibly damaging the compressor ) at others. There are several related devices ( for different purposes ) such as AXV, CPR, etc. that work in a simlar fashion ( to some extent ).

The TXV needs to see 'a solid liquid head' at all times, in other words the input pipe to it needs to be completely full of liquid, in order for it to be able to meter out the freon. There is typically a receiver ( holding tank ) to store excess freon, or else it is allowed to back up into the condensor in some systems. This is extra freon in the system over and above needed quantity to assure that at all times and operating conditions, there is enough freon to 'present a liquid head' to the TXV.

Thus, seeing as the liquid line is designed to be at all times ( running times, anyway )totally full of liquid, a visual reference is relevent. Low level ( and some other problems) will show as bubbles or 'flash' in the glass, giving an indication that there is a problem that needs looking at.

Fixed ( non-moving ) metering devices, such as capillary tubes, orifices, etc., on the other hand, never adjust to system conditions, therfor standard fluid dynamics come into play, where a higher pressure pushing in ( head pressure ), in relation to a lower resisting pressure on the other side ( back or evaporator pressure ) means more freon flow. Lower head pressures mean less flow. Thus, there is no active control of superheat. If you were to charge a system of this type 'to a full glass' under most conditions, you would be incorrectly charged ( generally overcharged, but actually undercharged in some cold weather conditions ). Thus, having a sight glass would be misleading, as it would be telling you a fact that is not relevent to correct charge.

This a rather 'quick and dirty' explanation, and I encourage others to expand or correct as needed. :~)



In <3thl7f$>, (paul milligan) writes:
> I pulled the valve and compared it to another on the shelf to find the difference. The body was the same, sized was denoted by color code and of course I had the wrong color. Installed new valve and haven't heard a peep since 1984.

>Jim Glad to hear it works now, but the color code is to designate the refrigerant cross-charge in the power head ( green for 22, purple for 502, etc., usually ). The size is in the number designation, such as 1 1/4. The letters after the number designate other features, which I forget offhand, but things like GFC, Z ( slow bleed port, I think ), and others.

Andy... You design the things.... got a list of the codes for us ?

From Andy Schoen

TEV manufacturers have used the ASHRAE color codes to designate refrigerants. Here are some......

Refrigerant Color

R-22 green
R-134a sky blue
R-401A coral/pink
R-402A beige
R-404A orange
R-502 purple
R-507 teal (greenish blue)

TEV manufacturers diverge with ASHRAE only on two refrigerants: R-12 and R-717 (ammonia):

Refrigerant ASHRAE TEV Man.
R-12 white yellow
R-717 silver white

Why this discrepancy? These colors were assign way before my time, and I suspect (actually I know) TEV manufacturers were a hard-headed bunch of folks back then, and they probably disagreed with DuPont, and whoever was supplying ammonia to the refrigeration industry. ASHRAE didn't exist at the time. ASRE was the refrigeration organization, and they apparently didn't have enough clout to knock heads together.....

Andy Schoen

In <3thl7f$>, (paul milligan) writes:

> Glad to hear it works now, but the color code is to designate the refrigerant cross-charge in the power head ( green for 22, purple for 502, etc., usually ). The size is in the number designation, such as 1 1/4. The letters after the number designate other features, which I forget offhand, but things like GFC, Z ( slow bleed port, I think ), and others.
Paul, TEV nomenclature used is as follows:

Alco, Parker:

body type  equalizer  nominal capacity  refrigerant code  thermostatic charge
         ( omit if
           internal )


  HF           E           1-1/2             H                   CA


body type  refrigerant code  equalizer   nominal capacity  thermostatic charge
                             ( omit if
                              internal )

G V E 1-1/2 Z

The thermostatic charge is the constituents used in the sensing bulb. A 'cross charge' indicates the 'bulb charge' P-T curve crosses the P-T curve of the refrigerant used in the system. As a result, a 'cross charge' doesn't use the same refrigerant used in the system the valve is being applied.

Nominal capacity is the rating of the valve in tons at 'standard conditions'. For R-22 and R-502, this is 40F evaporator, 100 psi across the valve, and 100F liquid refrigerant entering the TEV.

Bleed ports can be added to any TEV. With Sporlan TEVs. it is indicated as a 'BP/' on the valve.

TEV manufacturers (for the most part) agree on refrigerant letter codes:

Refrigerant Code
R-12 F
R-401A X
R-402A L
R-404A S
R-502 R
R-507 P
R-717 A

We do disagree on a few, though. Here are some.....

  Refrigerant    Sporlan     Alco
R-22 V H
R-134a J M
R-500 D C

Anyway, this is probably more info on TEV nomenclature than you really wanted to know. :-)

Andy Schoen

From Andy Schoen

Paul, FYI, Sporlan has traditionally abbreviated 'thermostatic expansion valve' as 'TEV'. Alco has used both 'TEV' and 'TXV' in their literature, but they typically use 'TXV'. They also use their 'thermal expansion valve' tradename to describe TEVs, instead of calling the valve by their generic name, i.e., 'thermostatic expansion valve'. Parker uses 'thermostatic expansion valve', but abbreviates it 'TXV' like Alco. Guess that makes Sporlan the oddball regarding the abbreviation. I believe Parker is simply followed the lead of the previous owners of Parker's TEV product line, i.e., Jakes Evans and Detroit Lubricator. Ever see a Detroit TEV? Their #673 dominated the market in the 1940s. ASHRAE literature also uses the generic name for obvious reasons, but it doesn't use an abbreviation, probably in deference to Sporlan.

Actually, 'thermostatic expansion valve' is a misnomer, since the name implies the valve controls at a 'static' temperature. Of course, the TEV controls superheat, but 'superheatstatic expansion valve' didn't find favor anywhere (for good reason :-)). Alco's 'thermal expansion valve' is probably a better term, though it still doesn't describe what the valve does. A hand expansion valve could fall in that category. Some who have sought to rectify this 'problem' (university professor types, primarily), and have used 'superheat control expansion valve' ('SCEV' valve? 'SCXV'? :-( ) to describe it. But these same folks also want us to publish all TEV ratings in SI units, i.e., degrees C, bar, kW (which I could, but no one will read them).

Anyway, this is probably more than what you wanted to know about why I call TEVs 'TEVs'.

Andy Schoen

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