| Scenario | Method | Key inputs | Capacity (approx.) | Notes |
|---|---|---|---|---|
| Temporary chiller yard | Water-side | 220 GPM, ΔT 10°F | 1,100,000 BTU/hr (≈ 91.7 tons) | Use measured supply/return if available. |
| Roof condenser bank | Air-side | 6,500 CFM, ΔT 15°F | 105,300 BTU/hr (≈ 8.78 tons) | ΔT should reflect actual air across the coil. |
| Shop retrofit estimate | Load × factor | 120 tons, factor 1.25 | 150 tons condenser duty | Good for early budgeting and equipment checks. |
| Coil performance check | UA × LMTD | UA 82,000, LMTD 18°F | 1,476,000 BTU/hr (≈ 123.0 tons) | Verify terminal temperatures for valid LMTD. |
Formula used
- Air-side (Imperial): Q = 1.08 × CFM × |ΔT| (BTU/hr)
- Water-side (Imperial): Q = 500 × GPM × |ΔT| (BTU/hr)
- Air-side (Metric): Q ≈ 1.2072 × V̇ × |ΔT| (kW), with V̇ in m³/s
- Water-side (Metric): Q = 4.186 × ṁ × |ΔT| (kW), with ṁ in kg/s (≈ L/s)
- UA × LMTD: Q = UA × LMTD, where LMTD = (ΔT1−ΔT2)/ln(ΔT1/ΔT2)
- Load-based: Qcond = Qload × heat-rejection factor
How to use this calculator
- Select your unit system to match instruments and drawings.
- Choose a calculation mode based on available measurements.
- Enter airflow or water flow plus temperature change.
- For UA mode, enter UA and all four terminal temperatures.
- Add correction and safety values if your project requires.
- Press calculate to view results above the form.
- Download CSV or PDF for submittals, bids, and reviews.
Condenser duty in construction schedules
Condenser capacity is the heat a condenser must reject to ambient air or cooling water. On construction projects, this duty drives equipment selection, temporary power sizing, and commissioning acceptance. When cooling load is known, the condenser is typically larger because it also rejects compressor work. Using a heat-rejection factor helps align early estimates with field realities, especially during phased occupancy.
Air-side measurements that improve confidence
For air-cooled condensers, airflow and temperature rise across the coil are practical site measurements. Stable readings require consistent fan speed, clean filters or screens, and representative sampling locations. A higher observed ΔT usually indicates stronger heat transfer, while low ΔT may signal recirculation, fouling, or undersized airflow paths caused by nearby parapets and temporary barriers.
Water-side heat balance for tower and plate loops
Water-side calculations are often preferred when flow is metered and temperatures are available at headers. In tower loops, verify sensor placement away from bypass mixing. In plate heat exchangers, confirm that approach temperatures remain positive so capacity is physically consistent. In retrofit work, comparing implied flow to pump curves quickly flags balancing valve issues.
UA and LMTD for performance checks
UA × LMTD is useful when you have terminal temperatures and a rated UA value. It supports “what-if” checks for fouling, reduced surface area, and degraded approach. Because LMTD depends on both ends of the exchanger, mismatched temperatures can produce invalid results. Use this mode for verification, not as a substitute for proper selection software.
Example data for a quick site estimate
Use these sample inputs to validate field instruments and workflow:
| Method | Inputs | Expected output |
|---|---|---|
| Water-side | 200 GPM, ΔT 9°F, correction 1.00, safety 5% | ≈ 787,500 BTU/hr (≈ 65.6 tons) |
| Air-side | 7,200 CFM, ΔT 14°F, correction 1.05, safety 0% | ≈ 114,307 BTU/hr (≈ 9.53 tons) |
| Load × factor | 90 tons load, factor 1.25, safety 10% | ≈ 123.75 tons condenser duty |
FAQs
Q1. What does “condenser capacity” represent?
A: It is the heat rejected to air or water. It typically exceeds cooling load because compressor power also becomes heat that must be removed from the refrigerant.
Q2. Which mode should I use on a jobsite?
A: Use water-side when flow and temperatures are reliable. Use air-side when you can measure coil airflow and temperature rise. Use load-based for early budgeting and checks.
Q3. Why is ΔT shown as an absolute value?
A: Capacity depends on the magnitude of temperature change. The calculator uses |ΔT| to avoid negative signs when inlet and outlet points are swapped during field measurements.
Q4. What is a good heat-rejection factor?
A: Many projects start around 1.15–1.35, depending on compressor type and operating conditions. Use manufacturer data when available, then apply a safety margin if required.
Q5. When should I use the correction factor?
A: Use it for known derates or boosts such as coil fouling, reduced fan performance, temporary duct losses, or expected approach deviations during commissioning and seasonal extremes.
Q6. Why does UA × LMTD sometimes fail?
A: LMTD requires positive terminal temperature differences. If hot and cold end temperatures overlap or are entered in the wrong order, the log term becomes invalid and the method cannot apply.
Q7. Can I use the output for final equipment selection?
A: Use it as a practical check and documentation aid. Final selection should still consider manufacturer ratings, ambient design, refrigerant conditions, and project-specific code requirements.