Calculator Inputs
Formula Used
- Q = UA × LMTD (overall heat transfer relation)
- LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂), where ΔT₁ = T_in − T_evap and ΔT₂ = T_out − T_evap
- UA = Q / LMTD
- Area_base = (UA / U) × fouling_multiplier
- Area_final = Area_base × (1 + safety_factor/100)
- Airflow ≈ (Q × SHR) / (ρ × c_p × ΔT_air) (quick sensible airflow check)
Use realistic U-values for your coil type and conditions. This tool is for preliminary sizing and verification, not detailed manufacturer selection.
How to Use
- Select a unit system that matches your project notes.
- Enter cooling capacity (total load) and target inlet/outlet air temperatures.
- Set an evaporating temperature based on refrigerant and approach expectations.
- Choose an overall U-value appropriate for fin density and cleanliness.
- Add fouling and safety factors to reflect real site uncertainty.
- Press Calculate to view coil area, UA, LMTD, and airflow.
- Use the CSV/PDF buttons to save the latest results.
Example Data Table
| Scenario | Capacity | Air In / Out | Evap Temp | U | Fouling | Safety | Typical Result |
|---|---|---|---|---|---|---|---|
| Office AHU (Metric) | 12.0 kW | 27°C / 14°C | 5°C | 65 W/m²·K | 1.10 | 10% | ~4.0 m² final area |
| Small Retail (Imperial) | 36,000 BTU/hr | 80°F / 58°F | 40°F | 12 BTU/hr·ft²·°F | 1.15 | 10% | ~55 ft² final area |
| Dusty Site Allowance | 20.0 kW | 28°C / 15°C | 4°C | 60 W/m²·K | 1.25 | 15% | Area rises noticeably |
Professional Guide to Evaporator Sizing on Construction Projects
1) Why evaporator sizing matters
Undersized coils struggle to reach setpoints, while oversized coils short-cycle and reduce dehumidification. This calculator estimates required surface area using the same heat-transfer structure many coil selection tools start with, helping teams align load assumptions before procurement.
2) Start with a defensible cooling load
Use a total load that reflects occupancy, lighting, envelope gains, and equipment diversity. For many temporary or phased areas, loads shift as trades mobilize. A practical safety factor of 5–15% can cover schedule-driven uncertainty without forcing extreme oversizing.
3) Temperature targets define the driving force
Leaving-air targets commonly sit near 12–15°C (54–59°F) for comfort systems, but vary with ventilation and humidity strategy. The tool uses inlet, outlet, and evaporating temperatures to compute LMTD, the effective mean temperature difference that drives heat transfer.
4) Choosing an evaporating temperature
Evaporating temperature is tied to refrigerant and approach. A few degrees of approach between evaporating and coil surface is typical, and overly high evaporating temperatures can make it hard to reach low leaving-air targets. If your LMTD becomes small, required area rises quickly.
5) Interpreting U-values with real site conditions
Overall U depends on fin density, airflow, moisture on fins, and cleanliness. Finned coils often fall around 40–120 W/m²·K (about 7–20 BTU/hr·ft²·°F) in preliminary checks. Dusty sites, filter bypass, and access limitations justify a fouling multiplier above 1.0.
6) Fouling and safety: two different buffers
Fouling multipliers cover performance loss over time from dirt or film; safety factors cover unknowns in load and operating points. Keeping them separate helps review meetings: one factor belongs to maintenance realities, the other belongs to design risk management.
7) Airflow sanity-check using SHR
The airflow estimate uses the sensible portion of the load, set by the sensible heat ratio (SHR). Comfort applications commonly land near 0.70–0.85, but high latent conditions lower SHR. If airflow looks unusually low or high, revisit temperatures, SHR, and duct constraints.
8) Turning results into procurement-ready decisions
Use the calculated coil area and UA as a screening metric when comparing equipment options, then confirm with manufacturer selection data, fan curves, and coil face velocity. Document the assumptions and export CSV/PDF so stakeholders can review the same inputs and outputs.
FAQs
1) What does “evaporator size” mean here?
It refers to estimated coil heat-transfer surface area needed to meet the cooling load at the stated temperatures and U-value, including fouling and safety allowances.
2) Is this the same as tonnage selection?
No. Tonnage is capacity. This tool starts from capacity and estimates coil area and UA. Final equipment selection should still use manufacturer performance data at your airflow and conditions.
3) What if my LMTD is very small?
A small LMTD means the temperature driving force is weak, so required UA and area increase sharply. Adjust evaporating temperature, leaving-air target, or confirm the load and airflow assumptions.
4) How do I choose a reasonable U-value?
For finned air coils, preliminary checks often use 40–120 W/m²·K (7–20 BTU/hr·ft²·°F). Use the coil type, face velocity, and cleanliness expectations to narrow the range.
5) Why include a fouling multiplier?
Construction environments can load filters quickly and introduce dust. Fouling reduces heat transfer, so a multiplier (like 1.10–1.25) helps keep performance closer to target over time.
6) Does the airflow result replace duct design?
No. It is a sanity-check based on sensible load, SHR, and air temperature change. Duct sizing, static pressure, and fan selection must still be engineered separately.
7) Can I use this for chilled-water coils?
The math structure is similar, but evaporating temperature is replaced by chilled-water temperature profiles and approach. Use this for quick screening, then verify with detailed coil selection for water-side conditions.
Accurate coil sizing reduces risk, costs, and callbacks later.