Talk:Thermal expansion valve

Thermal vs thermostatic

Some people call them thermostatic expansion valves, some call them thermal expansion valves. The title says thermal, while the text (used to) say thermostatic. I changed it to thermal in the text for the sake of uniformity. If thermostatic is to be used, the title should be changed as well, correct? - Geekosaurus^(talk) 22:20, 19 June 2010 (UTC)

RE: Thermal vs thermostatic

In refrigeration, many thermal expansion valve types in use are static orifice, some have non-active controls (manual needle valve), and some are active controls. Active controls use the power of the hydraulic system or other means, for example electro-mechanical, to control operation. Some of these active controls have temperature sensing capability. Others sense and control evaporator pressure.

Thermostatic (constant temperature) control is the type referenced in the diagram as the control operates by sensing temperature (usually of the evaporator.) Thermostatic, you can see, is a sub-class of thermal expansion valves used in refrigeration. Thermostatic might be further divided into passive and active, dual and single. The type shown here is active because it uses the high-side pressure to amplify the force of the sensing unit while most thermostatic valves used in automotive applications are passive, meaning they use no power supply. The force needed is generated solely by the thermal expansion of the sensing fluid. I want to put citations here, but I can't find my refrigeration handbook, a sizable text for refrigeration. However the argument is: the title and all references should say thermostatic as there are a number of subclasses of thermal expansion valves which this article doesn't address and the only picture and description is of a thermostatic control.

This article is pretty incomplete even for thermostatic controls since it doesn't address active/passive dual/single. It appears pictured is a dual control that senses pressure differential across (mostly) the compressor and senses temperature at the bulb. Single, temperature only, units are ubiquitous as well: Better GM AC's use passive temperature-only control on the expansion valve and active low-side pressure control on the compressor to vary displacement. Thus, you get only the evaporator pressure needed for the design temperature regardless of engine RPM AND no compressor cycling (yay!!) Dual controls allow the valve to open more at high pressure differentials even though the target temperature has been reached. One application of a dual control is cooling vs dehumidification. High efficiency AC units often run a dehumidification cycle every so often or on-demand for more comfort at higher temperatures. The compressor differential pressure is raised (lower evaporator pressure caused by high-volume/speed compressor), lowering the evaporator temperature set-point for better condensation.

===Function in a refrigeration cycle===

It sounds like the purpose of the valve is to produce cooling via the Joule–Thomson effect. But this section says nothing about the cooling produced and has no link to that effect. It should do so, if my interpretaion is correct. Retired Pchem Prof (talk) 04:22, 30 January 2016 (UTC)

No, JTM valves are for low temperatures like final-stage liquefaction of Helium where liquid-evaporation refrigeration isn't practical.

I think this usage should be mentioned since a JT valve is an expansion valve, but the distinction must be made since JT cooling is fundamentally different from evaporative cooling. MasterTriangle12 (talk) 07:24, 25 August 2020 (UTC)

Article needs improvement

There are many issues with this article , as suggested in the above section Thermal vs Thermostatic, notably "how" and "why" are not discussed, and "what" is not correct. I have a refrigeration textbook somewhere , when I find it, I'll check all my facts below, and propose making major changes to the article, to goal is not to create a "refigeration" article , but describe, compare, contrast the different types of expansion devices.

Should article be renamed "Expansion Valves (Refrigeration)" ? , and then redirect all the different sorts of expansion valves here.

Thermal Expansion Valve, TXV or TX , controls temperature differential across the evaporator, effectively guaranteeing superheat, but is prone to freezing over at evaporator. Typically It works by having a sensing bulb filled with a different refrigerant on one side of the diaphragm, the valve modulates so the pressure is balanced across the diaphragm, and because the boiling points of the two refrigearnts are different at the same pressure, the valve modulates so the sensing bulb is warmer than the liquid flowing into the evaporator, this results in the evaporator coils being 95% filled, and operating at maximum efficiency, and avoids liquid slugging back to the compressor. A TXV effectively controls the cooling capacity of the evaporator (at a constant airflow).

Automatic Expansion Valves (AXV) These are simple pressure regulators, using a diaphragm and spring (with adjusting screw) Controlling the pressure automatically controls the boiling temperature in the evaporator, these are used where evaporator temperature control is critical, e.g. in air driers where the evaporator temperature is just a few degrees above freezing , and slushy drink machines , where the temperature is a few degrees below freezing, these are true thermostatic expansion valves, disadvantages are the evaporator can completely fill with liquid so measures are needed to control this (e.g. a heat exchanger between compressor suction and pressure lines or hot gas bypass or electrica switching). Another disadvantage is the reference pressure is atmospheric, so the evaporator controlled temperature will get slightly colder at higher altitudes, could be an issue in aircraft installations.

Electronic Expansion Valve (EEV) , this is the heated bimetallic or servo operated or proportional solenoid , these can be operated in different modes depending on what sensors & electronics drives them, a simple primitive system has a thermistor strapped to a refrigerant line directly controlling current to a bimetallic lever wrapped with a heating element to drive the valve pin.

Capillary Tube , a primitive expansion device.... (included for completeness, mention advantages cheap; disadvantages - critical refrigerant weight - slow pulldown prone to icing - prone to blockage from debris/sludge - prone to liquid slugging into compressor ....)

Thermostatic control valves , maybe some tradesmen call them this through ignorance, but it is confusing at a minimum , many refrigeration systems have a thermostat that switches the compressor on and off with room temperature, any use of "thermostatic" needs to carefully differentiate usage. Larger systems have condensor pressure switches which are effectively thermostatic control of the condensor. An AXV is by definition a Thermostatic Expansion valve, it is called an AXV to avoid confusion with a TXV.

Other Comments:

Diagram needed , e.g. a simple diagram with the different refrigerants colored differently. Preferably for all the common expansion valves.

Should capacity control valves and hot gas bypass valves be briefly mentioned? Reverse cycle operation. Evaporator pressure control. Should we mention challenges of a refrigeration system e.g extreme swings in ambient, and how different EV's help with this. Maybe a comparison table of different EV's, and desirable features , cost , superheat, icing , evap temp, liquid slugging, hot restart, variable load, efficiency, ambient extremes ?

Why is there a bypass?, most expansion valves incorporate a bypass, that gaurantees a minimum flow, this is essential with small systems to unload the pressure in the system , so the compressor can start on the next cycle, particularly in a hot ambient condition. How is it done? simplest is to cut a small nick in the valve seat, otherwise use a small capillary.

Large (multi-evaporator) vs small (single evaporator systems) briefly describe how the TXV allows multiple evaporators to run on the same system. Somewhat clarified now but not fully explained MasterTriangle12 (talk) 07:29, 22 March 2021 (UTC)

Salbayeng (talk) 23:55, 18 July 2020 (UTC)

Thermostatic expansion valve, appears that '~static' takes on an industry meaning of 'stable'; as used in this Thermostatic Expansion Valve definition, "A thermostatic expansion valve (TXV) is [sic] precision device used to meter the flow of liquid refrigerant entering the evaporator at a rate that matches the amount of refrigerant being boiled off in the evaporator." WikiIndie (talk) 19:27, 23 August 2020 (UTC)

WikiIndie (talk) 19:27, 23 August 2020 (UTC)

Thanks for finding that article , WikiIndie, I think there is a lot of mis-information out there in the refrigeration industry, the glossary entry is probably a good example. An evaporator is a closed vessel, so the mass flow going into the evaporator must always equal the mass flow going out of the evaporator (except during startup and shutdown when the liquid level changes slightly), the TXV has nothing to do with basic conservation of mass (which is what the HVAC glossary claims). What the TXV does do is to maintain a "constant" "temperature" difference across the evaporator, this guarantees there is always vapour leaving the evaporator, and forces the evaporator to be always about 7/8 liquid, and directly causes the exiting vapour to be (say) 5degC hotter than the incoming liquid, this is known as "superheat", so the TXV is really a "precision controller" of superheat. By controlling superheat you can achieve maximum cooling action, and also ensure that the compressor is not sucking any liquid.Salbayeng (talk) 22:55, 23 August 2020 (UTC)

I believe that the use of the term "thermostatic" should be mentioned only as a misnomer since it is quite a misleading term that leads to a lot of people mis-understanding the function of the device. I believe it is the original term however, and the "thermal" variant might (just guessin') have only been created to keep the acronym while being more correct, but I still think "thermal expansion valve" should be the only definitional term used. I think renaming the page to "expansion valves (refrigeration)" would be an excellent idea since it covers all the related valves, capillary tubes can easily be shoehorned in, and even JT valves could be added despite the different thermodynamic principle. I think some other valves like hot gas bypass could be mentioned but it must be clear that these are only related by their usage in refrigeration unless the page is renamed to something like "valves used in refrigeration" to expand it to all thermal/pressure controlled valves used in refrigeration, but I do not think that is ideal since it would be better if a new page was made for the over-category which links to this one. An entry for AXVs is sorely needed too, I am only familiar with the operating principles and mechanics of refrigeration valves rather than the usages and available devices, but I might take a look at that eventually. I created the SVG diagrams currently on the page so if anybody would like more detailed images or images of other valves I can put those together (the current ones were made very crudely so I'm not too keen to re-work them). MasterTriangle12 (talk) 07:29, 22 March 2021 (UTC)

Under theory of operation, why condenser is linked to HVAC air coils under Heat Exchanger and not Condenser(Refrigeration)? Unanimous350 (talk) 16:20, 26 March 2021 (UTC)

I think you mean Condenser (heat transfer), which I think would be more appropriate if that page was improved. MasterTriangle12 (talk) 03:45, 27 March 2021 (UTC)

Yes, sorry for my mistake. I meant Condenser(heat transfer).

So, you're saying the page Condenser(heat transfer) needs to be improved? Unanimous350 (talk) 11:54, 27 March 2021 (UTC)

Kinda? The choice of reference is debatable. The overlap with heat exchanger needs to be reconciled since there is a lot of duplication, but the 'heat exchanger' page currently has more information that relates to the vapor-compression cycle, so I think it is currently the better reference. I can see two option for improving the 'condenser' page though: MasterTriangle12 (talk) 05:20, 1 April 2021 (UTC)

1. Make the Condenser (heat transfer) page only about things that are particular to condensers and their operation, move such content from heat exchanger and just leave links. Most things related to construction can just be referenced to 'heat exchanger' rather than repeated on the 'condenser' page. Content related to vapor-compression operation should mostly be on the 'condenser' page and so the link in this article should go there too. The 'heat exchanger' page is very long so this could help to shorten it.
2. Merge the Condenser (heat transfer) page into heat exchanger. It makes sense except that it adds to the problem of sprawl.

Yes, this is a good suggestion. Unanimous350 (talk) 17:30, 4 April 2021 (UTC)

References

excessively long sentence

This is a whopper of a sentence:

Cross charges, that is, sensing bulb charges composed of a mixture of different refrigerants or also non-refrigerant gases such as nitrogen (as opposed to a charge composed exclusively of the same refrigerant inside the system, known as a parallel charge), set so that the vapour pressure vs temperature curve of the bulb charge "crosses" the vapour pressure vs temperature curve of the system's refrigerant at a certain temperature value (that is, a bulb charge set so that, below a certain refrigerant temperature, the vapour pressure of the bulb charge suddenly becomes higher than that of the system's refrigerant, forcing the metering pin to stay into an open position), or even different kinds of bleed passages that generate a minimum refrigerant flow at all times, help to reduce the superheat hunt phenomenon by preventing the valve orifice from completely closing during system operation, at the cost, however, of determining a certain flow of refrigerant that won't reach the suction line in a fully evaporated state while the heat load is particularly low, and that the compressor must be designed to handle.

Ccrrccrr (talk) 12:52, 24 January 2021 (UTC)

A page for "superheat" is needed

This is a common enough term in steam and vapor compression engineering that I think it needs it's own page. There is a page for "superheating" but this refers only to superheated liquids, I added an about tag linking to the section in vapor compression refrigeration just to clear it up a little. It should be combined with supercooling too, since they are intimately linked in usage. I am not familiar enough with the thermodynamics to put much into such a page, if it isn't created after a while then I might take a crack at it, but mine would probably just be a basic definition page. MasterTriangle12 (talk) 06:30, 22 March 2021 (UTC)

Dead Link

The Sporlan link doesn't work. Here's a working link. I don't know how to fix it or else I would. https://web.archive.org/web/20160303232605/https://sporlanonline.com/literature/education/10-9.pdf 96.27.35.128 (talk) 04:18, 19 September 2023 (UTC)