TXV Capacity Correction and Valve Sizing

A TXV is an orifice, so its flow follows the square root of the pressure difference across it -- and that is the NET difference, condensing pressure less evaporating pressure less the distributor and line losses, not the system's head pressure. A valve rated at 100 psi of drop passes about 10% more at 120 psi and 30% less at 50 psi. The liquid-temperature factor is the second correction and it runs the other way from intuition: colder, more subcooled liquid entering the valve carries more refrigerating effect per pound, so the valve delivers MORE tons for the same mass flow, gaining capacity below its rating temperature and losing it above. The sizing window is what the corrections are for. A valve under 100% of load starves the coil at design conditions, with high superheat, lost capacity, and a compressor that never satisfies. A valve much over 130% hunts: it overfeeds, floods back, closes, starves, and cycles, and the symptom looks like a bad bulb rather than a sizing error. A 3-ton valve rated at 100 psi and 100 F liquid, installed at 120 psi of drop with 90 F liquid, delivers 3.52 tons and sits at 117% of a 3-ton coil -- comfortable. Put the same valve on a low-ambient day when the drop collapses to 50 psi and it delivers 2.27 tons, 76% of load, and starves the coil. That is precisely why head-pressure control exists on systems that run in winter, and it is a valve problem before it is a compressor problem. A selection check; the valve manufacturer's published capacity tables at the actual conditions govern.

pressure_factor = sqrt(actual_dp_psi / rated_dp_psi); installed_capacity_tons = nominal_tons x liquid_temp_factor x pressure_factor; sizing_ratio_pct = installed_capacity / evaporator_load x 100, against a 100-130% window.

Thermostatic expansion valve capacity correction from the orifice relation -- capacity scales as the square root of the NET pressure drop across the valve -- with the manufacturer's liquid-temperature correction factor applied, by name. The 100-130% of load sizing window is standard refrigeration-service practice. The valve manufacturer's published capacity tables at the actual conditions govern.

The square-root orifice relation is public physics; the nominal rating, its rating conditions, and the liquid-temperature factor come from the valve manufacturer's own data sheet and are entered rather than bundled.

Estimate. AHJ and licensed professional govern.

Field names used by the API: nominal_tons, rated_dp_psi, actual_dp_psi, liquid_temp_factor, evaporator_load_tons, pressure_factor, installed_capacity_tons, sizing_ratio_pct

• Square-root orifice capacity scales as sqrt(actual dP / rated dP), on the NET drop across the valveexpansion valve physics
• Liquid-temperature factor colder liquid means MORE capacity; the factor comes from the manufacturer's tablevalve manufacturer data
• Sizing window 100% to 130% of load; under starves, well over huntsrefrigeration service practice