Capacity is the sum of every consumer's heat duty, plus distribution losses, plus any start-up requirement, read at your actual operating temperature and then given a 10–20% margin. Expressed in the units buyers actually use: 1 kW = 860 kcal/h, so a 2,800 kW heater is a 2.4 million kcal/h heater.
Most sizing errors do not come from the arithmetic. They come from adding nameplate duties without a diversity factor, quoting kcal/h at a different temperature, or forgetting that a generous margin quietly degrades low-load behaviour.
First, the three units that get mixed up
| Unit | Meaning | Conversion |
|---|---|---|
| kW | Kilowatts, the SI heat duty | 1 kW = 860 kcal/h = 3.6 MJ/h |
| kcal/h | Kilocalories per hour, still standard in much of Asia and the Middle East | 1 kcal/h = 1.163 W |
| MW | Megawatts, used above roughly 10,000 kW | 1 MW = 1,000 kW |
| t/h steam (equivalent) | A familiar yardstick for boiler people | 1 t/h saturated steam ≈ 700 kW |
A "1 million kcal/h" system is therefore 1,163 kW — a mid-size unit. A "10 million kcal/h" system is 11.6 MW, which is a large plant heater.
Step 1 — List every consumer and its duty
Write the duty of each consumer in kW, in a table you can defend later. Below is a typical chemical plant loop.
| Consumer | Duty (kW) | Notes |
|---|---|---|
| Reactor jacket A | 400 | Batch, 8 h per cycle |
| Reactor jacket B | 400 | Batch, offset from A |
| Reactor jacket C | 400 | Batch, offset from A |
| Spray dryer air heater | 600 | Continuous |
| Raw material preheater | 300 | Continuous |
| Storage tank coils (4 tanks) | 150 | Intermittent, winter peak |
| Piping and valve tracing | 80 | Continuous |
| Sum of nameplate duties | 2,330 | Not the design load |
Step 2 — Apply a diversity factor
The sum is a worst case that never happens. Three batch reactors at 400 kW each are not usually at peak simultaneously, and tank coils only run in cold weather. A diversity factor of 0.85–0.95 is normal for a mixed batch/continuous plant; use 1.0 only when every consumer genuinely runs at peak together.
2,330 kW × 0.9 = 2,097 kW of diversified process load.
Step 3 — Add distribution losses
Insulated mains lose 5–10% of transmitted heat. Long outdoor runs, damaged lagging or a high proportion of small-bore tracing push it toward 15%.
2,097 kW × 1.08 = 2,265 kW.
Step 4 — Check the start-up (heat-up) load
This is a separate calculation, and for small systems with large fluid inventories it can govern the selection:
Q (kW) = [M × c_p × ΔT] ÷ [t × 3600]
Where M is the mass being heated in kg, c_p the specific heat in kJ/(kg·K) — about 2.2–2.6 for heat transfer oils — ΔT the temperature rise in kelvin, and t the allowed time in hours.
Example: 20,000 kg of oil, c_p 2.4, from 20°C to 300°C in 4 hours:
[20,000 × 2.4 × 280] ÷ [4 × 3600] = 13,440,000 ÷ 14,400 = 933 kW
Compare that with the 2,265 kW process load: the start-up case is satisfied by the process-sized heater, with a comfortable margin. Had the plant demanded a 1-hour heat-up, the number would be 3,733 kW and would have driven the selection instead.
Step 5 — Read the rating at your real outlet temperature
A quoted capacity is meaningless without its design point. Two heaters both sold as "3,000 kW" can differ by 15% in usable output at 330°C because the flue gas to oil mean temperature difference shrinks as the oil gets hotter.
| Outlet temperature | Usable capacity relative to 300°C rating |
|---|---|
| 250°C | ≈112% |
| 300°C | 100% |
| 320°C | ≈94% |
| 350°C | ≈86% |
Ask for the capacity curve, not a single number. If a supplier cannot produce one, that is itself the answer.
Step 6 — Confirm the flow rate and temperature drop
Flow follows directly from duty and the chosen drop:
m (kg/s) = Q (kW) ÷ [c_p × ΔT]
For 2,800 kW, c_p 2.4 and ΔT 25 K: 2,800 ÷ 60 = 46.7 kg/s, about 168 t/h, or roughly 210 m³/h at 800 kg/m³.
Two sanity checks that catch errors fast:
- Rule of thumb: about 100 m³/h of flow per 1 million kcal/h at a 20 K drop. Deviating far from that suggests a units mistake.
- Velocity check: the pump and pipe sizes must hold 1.5–2.5 m/s in the coil at that flow. If the required flow cannot be achieved without excessive pressure drop, widen ΔT rather than choosing a bigger pump.
Step 7 — Apply the margin, but not too much
| Margin | When it is right | What it costs |
|---|---|---|
| 10% | Well-understood process, steady load, room to add later | None |
| 15–20% | Normal industrial practice with future expansion expected | Slightly higher capex |
| Above 25% | Almost never justified | Short cycling, poor low-load efficiency, coking risk at low flow |
Oversizing is a real cost, not a safety feature. A 4 MW heater running at 25% load cycles its burner constantly, wastes purge losses and runs the coil below its intended velocity.
Worked example, end to end
| Step | Calculation | Result |
|---|---|---|
| Sum of nameplate duties | 400 + 400 + 400 + 600 + 300 + 150 + 80 | 2,330 kW |
| Diversity factor 0.9 | 2,330 × 0.9 | 2,097 kW |
| Distribution losses 8% | 2,097 × 1.08 | 2,265 kW |
| Start-up case | 20 t oil, 280 K, 4 h | 933 kW — satisfied |
| Margin 15% | 2,265 × 1.15 | 2,605 kW |
| Selection | Next standard size | 2,800 kW (2.41 million kcal/h) |
A 2,800 kW gas-fired unit also gives the plant headroom for one additional reactor of the same size, which is usually the cheapest expansion option available.
Six mistakes that produce the wrong number
- Adding nameplate duties with no diversity factor — overstates by 10–30% on batch plants.
- Forgetting the heat-up case — undersizes small systems with large oil inventories.
- Quoting kcal/h at a different ΔT — a 2 million kcal/h unit at 20 K drop is not the same machine at 40 K.
- Ignoring fuel calorific value and altitude — burner output falls with altitude and with lower-grade fuels; the coil may be fine while the burner is short.
- Sizing on average load — the heater must meet the peak, not the monthly mean.
- No allowance for fouling — a coil with two years of carbon deposit loses measurable capacity; the 10% margin is largely for this.
Where to go next
Once you have the number, the fuel choice sets the rest of the design:
- Gas and oil fired thermal oil heater — 100–28,000 kW, ≥92% efficiency
- Coal and biomass fired thermal oil heater — ≥86% efficiency, lowest fuel cost where agro-waste is cheap
- Skid-mounted thermal oil heater — pre-piped and pre-wired, installs in days
- Chemical industry solutions and food processing solutions — worked configurations for these duty profiles
If you would rather have the calculation done against your own numbers, send the consumer list, required temperatures, fluid volume and allowed heat-up time, and the sizing comes back with the quotation.
Key takeaways
- 1 kW = 860 kcal/h; a 2,800 kW unit is 2.4 million kcal/h, and 1 t/h of steam is roughly 700 kW.
- Capacity = diversified process load + 5–10% distribution losses, checked against the heat-up case, read at your real outlet temperature.
- A 10–20% margin is right; more than 25% is a defect, not insurance.
- Confirm the flow rate (about 100 m³/h per million kcal/h at a 20 K drop) so the coil holds 1.5–2.5 m/s.
- Demand a capacity curve, not a single rating — usable output falls roughly 14% between 300°C and 350°C.
