DX Evaporator Calculator
Detailed guide for direct expansion evaporator coil design: refrigerant selection, superheat control, two-phase heat transfer, and capacity balancing.
1. Overview
The DX Evaporator Calculator models a finned-tube heat exchanger where refrigerant evaporates inside the tubes while air passes over the finned surface. The engine uses a segmented solver that divides the coil into discrete elements, tracking the refrigerant quality from inlet (two-phase mixture) through complete evaporation to superheated vapor at the outlet.
The solver supports both Rating mode (evaluate a known geometry) and Sizing mode (find the refrigerant mass flow rate that delivers a target capacity). An energy balance bisection algorithm ensures that Q_air matches Q_ref within 0.01%.
Supported Refrigerants
- R32 (GWP: 675, A2L)
- R410A (GWP: 2088)
- R134a (GWP: 1430)
- R22 (GWP: 1810, ODP: 0.055)
- R290 — Propane (GWP: 3, A3)
- R1234yf (GWP: 4, A2L)
2. Air-Side Inputs
| Parameter | Unit | Description |
|---|---|---|
| Dry Bulb Temperature | °C | Entering air dry bulb (typically 24–35°C) |
| Wet Bulb Temperature | °C | Entering air wet bulb (determines latent load) |
| Air Flow | m³/h | Face velocity or volumetric flow rate |
| Altitude | m | Installation altitude (affects air density and pressure) |
3. Refrigerant-Side Inputs
| Parameter | Unit | Description |
|---|---|---|
| Refrigerant | — | Select from supported refrigerants list |
| Evaporating Temperature | °C | Saturation temperature (typically 2–10°C for comfort AC) |
| Superheat | K | Target superheat at coil outlet (typically 5–10 K) |
| Inlet Quality | — | Vapor fraction at inlet after expansion valve (typically 0.15–0.30) |
| Mass Flow Rate | kg/h | Refrigerant mass flow (Rating mode) or auto-calculated (Sizing mode) |
Important: In Sizing mode, the solver automatically finds the refrigerant mass flow rate that closes the energy balance (Q_air = Q_ref). In Rating mode, you provide the mass flow and the solver reports the resulting balance error.
4. Coil Geometry
DX evaporator coils typically use 3–6 rows with staggered tube arrangements. The number of circuits determines the refrigerant velocity inside the tubes — too few circuits leads to excessive pressure drop, too many leads to poor heat transfer.
- 3–4 rows: Comfort cooling (7–12°C evaporating)
- 4–6 rows: Low temperature or high latent load
- 6–8 rows: Dehumidification applications
5. Understanding Results
| Result | Description |
|---|---|
| Total Capacity | Sum of sensible + latent capacity (kW) |
| Sensible Capacity | Heat removed by cooling the air (temperature reduction) |
| Latent Capacity | Heat removed by condensing moisture from the air |
| SHR | Sensible Heat Ratio = Q_sensible / Q_total |
| Energy Balance Error | Difference between Q_air and Q_ref (target < 1%) |
| Leaving Air Conditions | Temperature and humidity of air leaving the coil |
6. Segmented Solver Method
The engine divides the coil into small segments along the refrigerant flow path. In each segment:
- Local refrigerant properties are evaluated (quality, density, viscosity)
- Two-phase or superheated heat transfer coefficient is calculated
- Air-side heat transfer with wet/dry surface detection
- Local heat flux and enthalpy change are computed
- Refrigerant state is updated for the next segment
This approach captures the variation in heat transfer coefficient as the refrigerant transitions from two-phase to superheated vapor, providing much higher accuracy than a single-zone LMTD method.
7. Design Guidelines
- Face velocity: 2.0–3.0 m/s (higher velocity increases capacity but also pressure drop and moisture carryover risk)
- Superheat: 5–8 K for TXV systems, 10–15 K for capillary tube systems
- Inlet quality: typically 0.20–0.25 after the expansion device
- Circuit count: aim for refrigerant velocity of 3–8 m/s in the tubes
- Evaporating temperature: at least 3–5°C below the target leaving air temperature