Heat Units

Heat units are essential for sizing cooling systems like cabinet coolers, heat exchangers, and heat recovery systems.

Main Units

Unit	Name	System
BTU/h	British Thermal Units per hour	Imperial
kcal/h	Kilocalories per hour	Metric
W	Watts (Joules/second)	SI
kW	Kilowatts	SI
TR	Tons of Refrigeration	HVAC

Definitions

• 1 BTU = energy to raise 1 pound of water by 1°F
• 1 kcal = energy to raise 1 kg of water by 1°C
• 1 TR = 12,000 BTU/h (capacity to melt 1 ton of ice in 24h)

Conversion Table

Unit	BTU/h	kcal/h	W	kW
1 BTU/h	1	0.252	0.293	0.000293
1 kcal/h	3.968	1	1.163	0.001163
1 W	3.412	0.860	1	0.001
1 kW	3,412	860	1,000	1
1 TR	12,000	3,024	3,517	3.517

Quick Conversions

$$Q_{BTU/h} = Q_{kcal/h} \times 3.968$$ $$Q_{kcal/h} = Q_{BTU/h} \times 0.252$$ $$Q_{W} = Q_{BTU/h} \times 0.293$$ $$Q_{kW} = Q_{BTU/h} \times 0.000293$$

Rule of thumb

• 1 kW ≈ 860 kcal/h ≈ 3,400 BTU/h
• 1 TR ≈ 3.5 kW ≈ 12,000 BTU/h

Cabinet Coolers

Cabinet coolers use compressed air to cool electrical enclosures. Their capacity is typically specified in BTU/h or kcal/h.

Typical Capacities

Size	BTU/h	kcal/h	Application
Small	275-550	70-140	Small enclosures, individual VFDs
Medium	900-1,700	225-430	Medium enclosures, PLCs
Large	2,500-5,600	630-1,400	Large enclosures, multiple drives

Heat Load Calculation

$$Q = P_{dissipated} \times \eta_{loss}$$

Where:

• $Q$ = heat load to remove
• $P_{dissipated}$ = component power
• $\eta_{loss}$ = loss factor (typically 0.03-0.05 for VFDs)

Air Consumption

Cabinet coolers consume compressed air continuously. A 1,000 BTU/h model can consume 5-15 scfm depending on design.

Heat of Compression

Approximately 80-90% of compression energy converts to heat:

$$Q_{heat} = P_{compressor} \times 0.85$$

Example: A 100 kW compressor generates ~85 kW of recoverable heat.

Compressor Power	Heat Generated
10 kW	~29,000 BTU/h
50 kW	~145,000 BTU/h
100 kW	~290,000 BTU/h