# EEV vs TXV in a DX AHU Coil

How a thermostatic expansion valve and an electronic expansion valve each control superheat in the same split DX air\-handler coil, and why the EEV holds it steadier\.

By Omkar Marathe

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/tRAiPlUbZ4jg)

## Transcript

### 1. One coil, two metering devices

This face\-split DX coil is a teaching rig: the upper circuits are fed by a thermostatic expansion valve \(TXV\), the lower circuits by an electronic expansion valve \(EEV\)\. Both throttle subcooled liquid and regulate the same variable: evaporator outlet superheat, SH = T\_suction \- T\_sat\(P\_evap\)\. Each valve body is drawn in section so its pin and port stay visible\.

Drag anywhere to turn the rig\. Gauge panel: upper dial reads TXV\-circuit superheat, lower dial the EEV circuit\. 0 to 20 K, major ticks every 5 K\.

### 2. Throttling into the distributor

Liquid near 2\.4 MPa \(R\-410A condensing at about 40 °C\) passes the port and flashes isenthalpically to roughly 1\.0 MPa, about 7 °C saturation\. The flash gas and liquid enter a distributor whose equal\-length tubes impose matched pressure drops, so each circuit receives the same quality and mass flow\.

Pressure values are illustrative R\-410A saturation figures; colours are this rig's own convention: blue liquid, pale two\-phase, amber superheated vapour\.

### 3. Where superheat lives

Air at about 26\.7 °C dry\-bulb crosses the fins while refrigerant boils at nearly constant saturation temperature\. Once the last liquid evaporates, the remaining tube length only sensibly heats vapour: that is the superheat region\. Too little superheat risks liquid floodback to the compressor; too much leaves coil surface doing little work\.

Amber outlet stubs and suction headers mean dry, superheated vapour\. Blue there would mean liquid reaching the suction line\.

### 4. TXV: a force balance on a diaphragm

The bulb charge pressure P1 pushes the diaphragm open; evaporator pressure P2 and the superheat spring P3 push it closed: P1 = P2 \+ P3 at equilibrium\. The spring preload sets static superheat, so the pin lifts only after the bulb runs warmer than saturation by that margin\. Raise the spring setting and watch both the lift and the upper dial\.

Slider spans 2 to 8 K static superheat\. Operating SH = static \+ opening superheat, so it rises with load\.

### 5. Why an external equalizer

An internally equalized TXV feels inlet pressure under its diaphragm, but a distributor and long circuits drop pressure before the outlet\. The valve then underestimates superheat and throttles harder, starving the coil\. The external equalizer line taps the suction line beside the bulb so P2 is the true outlet pressure\.

Here a drop worth about 4 K of saturation temperature is modelled; ACR News cites 0\.9 bar in R\-22 as about 5 K\.

### 6. EEV: a closed digital loop

A ratiometric pressure transmitter gives P\_evap, converted to T\_sat, and a Pt1000 sensor clamped at the coil outlet gives T\_suction\. The controller computes SH every second or faster and runs a PI law that drives a bipolar stepper; an ETS 25\-class valve spans 2625 full steps of pin travel\. Watch the rotor index turn as steps accumulate\.

The superheat setpoint is software, not a spring; adaptive controllers search for the minimum stable superheat \(MSS\)\.

### 7. Part load: drift and hunting

Lower the coil load\. The proportional TXV settles at a lower superheat as load falls, and because it is now oversized for the flow, the bulb lag lets it hunt\. The EEV's integral action returns its circuit to the 5 K setpoint at every load with a steady needle\.

Time is compressed and the hunting amplitude is illustrative; real bulb and loop time constants are tens of seconds to minutes\.

### 8. A sudden load step

Step the load from 40 % to 100 %, as when a damper opens or a second compressor stage starts\. The TXV waits for the bulb to warm through its thermal lag, so its superheat overshoots before settling high\. The EEV reads the transmitter directly and steps open within a few control cycles\.

Both responses come from the same simple coil model; only the valve logic differs\.

### 9. Choosing the valve

A TXV is self\-powered, inexpensive and robust, but its proportional, spring\-fixed control trades superheat margin across the load range\. An EEV costs a controller, sensors and power, yet holds a low stable superheat, suits variable\-speed compressors and multi\-coil coordination, and reports diagnostics\. Either way, airflow, charge and commissioning still decide the outcome\.

The coil is face\-split for comparison only; real AHU coils normally use one valve type per refrigerant circuit\.