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How to Use the TXV Sizing Calculator
Pick Size a New TXV or Check Existing TXV, then select the refrigerant. Choose the pressure, temperature and capacity units, and enter the site elevation if you read gauge pressures.
Type the evaporator load, the condensing pressure at the valve inlet, the evaporating pressure, any other pressure losses and the liquid temperature entering the valve.
Copy the rated pressure drop and liquid temperature from the valve catalog. Leave the rated evaporating temperature blank to use the design value, and in check mode add the installed valve rating.
Check the correction factors, the required valve rating and the subcooling at the valve. Add the project details, then use PDF, CSV, Copy or Print.
Sizing a TXV from the pressure drop and the liquid
A thermostatic expansion valve is a variable orifice, so the refrigerant it passes follows the orifice equation: flow rises with the square root of the liquid density times the pressure drop. The cooling that flow delivers also depends on the refrigerating effect, which is the heat each kilogram absorbs between the liquid entering the valve and saturated vapour at the evaporating temperature. The calculator compares those quantities at your conditions with the same quantities at the conditions printed in the valve catalog, which gives a correction factor, and divides the load by that factor to get the rating you need.
Why the nominal rating is not the load
A catalog rating applies at one pressure drop and one liquid temperature, and a working system seldom sits there. In the first example, a 36,000 BTU/hr R-410A system with 400 psig condensing, 118 psig evaporating and 60 psi of other losses has a net drop of 222 psi against a 100 psi rating, and 104°F liquid with 12.4°F of subcooling. The factors are 1.490 for the pressure drop, 0.993 for liquid density and 0.975 for the refrigerating effect, which multiply to 1.443. The system needs a valve rated about 24,956 BTU/hr at the catalog conditions. In check mode, a valve rated 36,000 BTU/hr passes 51,931 BTU/hr here, which is 144% of the load. Learn Technique's guide to expansion valve hunting names oversizing as a cause, because excess liquid chills the bulb and the valve closes quickly before it overfeeds again, and it advises selecting the valve for its actual capacity rating and not its nominal one.
Net pressure drop and the lowest head pressure
The drop that counts is the condensing pressure at the valve inlet, minus the evaporating pressure, minus the losses between them, such as the liquid line, the distributor and the evaporator circuits. Leaving the 60 psi of losses out of the first example gives a factor of 1.626 and a required rating of 22,143 BTU/hr, which is 11% too small. Size for the lowest pressure drop the system will see, since a valve that is right on a hot day can be too small on a cool one. If head pressure control holds 300 psig in the same example, the drop falls to 122 psi and the required rating rises to 33,665 BTU/hr, and the 104°F liquid is then above its bubble point, so flash gas reaches the valve. The Trane TEV catalog gives the working formula as the valve rating times a liquid temperature factor times a pressure drop factor, and the calculator follows the same structure.
Liquid temperature and subcooling
Cooler liquid is denser and carries more cooling per kilogram, so the same valve passes more capacity. As a check, the model gives liquid temperature factors of 1.22 at 60°F and 0.77 at 140°F for R-22 at a 40°F evaporator, against 1.23 and 0.76 in the Trane catalog. For R-410A it gives 1.31 and 0.65, against 1.32 and 0.62. The pressure drop factor for a 75 psi drop is 0.87, which agrees with the catalog example of 0.87 when the valve is rated at 100 psi. In the first example 90°F liquid lifts the total factor from 1.443 to 1.608, and 110°F liquid lowers it to 1.368. At zero subcooling the orifice calculation fails, so the page flags it and does not size from it.
Which load to size for
Use the evaporator capacity at the design evaporating temperature, and on a system with several evaporators size each valve for its own coil. For a residential or light commercial air conditioner the load comes from a load calculation, and the AC sizing calculator gives a figure to start from. Enter the load in BTU/hr, tons or kW, and give the rated evaporating temperature when the catalog rating is not at your design value. The calculator uses the design evaporating temperature when you leave that field blank.
Checking an installed valve
Switch to check mode, enter the rating from the valve label and the oversize you will accept, and the page reports what the valve passes at the real conditions as a share of the load. Below 100% the valve is too small, and above your limit it is large enough to hunt. Hunting also comes from a poorly placed or uninsulated bulb, so confirm the superheat and the symptoms with the refrigerant fault diagnosis tool before replacing a valve that checks out. The model treats the valve as an orifice with subcooled liquid at the inlet. It does not model the opening superheat, the distributor or the bulb charge, so the manufacturer catalog has the final word.
Frequently Asked Questions
Find the rating that passes the load at your operating conditions, not the nominal size. The calculator scales the valve flow with the square root of liquid density times pressure drop, and scales the capacity with the refrigerating effect. In the example, a 36,000 BTU/hr R-410A system with 400 psig condensing, 118 psig evaporating, 60 psi of other losses and 104°F liquid has a net drop of 222 psi against a 100 psi rating. The total correction factor is 1.443, so the valve needs a rating of 24,956 BTU/hr at the catalog conditions.
Catalog ratings are at a standard pressure drop and liquid temperature, and the real drop is usually larger. In the example, a valve rated 36,000 BTU/hr at 100 psi passes 51,931 BTU/hr at 222 psi, which is 144% of the load. That excess is the reason oversized valves hunt. Select the valve for its actual capacity at your conditions, and let it equal or slightly exceed the load.
An oversized valve is the most common cause: it overfeeds, the bulb is chilled by the extra liquid, the valve closes quickly, and then it overfeeds again. Check the bulb position, which should be on a horizontal run of the suction line near the evaporator outlet and positioned as the manufacturer directs, and check that the bulb is well insulated. Compare the installed rating with the load at the real pressure drop in check mode before replacing the valve, and measure superheat to see the swing.
A cooler liquid is denser and gives more refrigerating effect per kilogram, so the valve passes more capacity. In the example, 90°F liquid instead of 104°F raises the total factor from 1.443 to 1.608, and 110°F liquid lowers it to 1.368. Once subcooling reaches zero, flash gas enters the valve and it passes less than the calculation shows, so the calculator flags that case and does not size from it.