Heat Recovery

A conventional air compressor converts approximately 85-94% of electrical energy into heat. This heat is normally wasted to the atmosphere. Heat recovery captures this energy for useful purposes.

The Potential

Compressor Energy Balance

For every 100 kW consumed by a compressor, only 4 kW converts to useful work. The remaining 96% dissipates as heat — energy that, without a recovery system, is lost entirely.

Available Energy

Compressor Size	Available Heat	Equivalent
50 HP (37 kW)	120,000 BTU/h	Heat 2,700 ft²
100 HP (75 kW)	240,000 BTU/h	Heat 5,400 ft²
200 HP (150 kW)	480,000 BTU/h	Preheat boiler
500 HP (375 kW)	1.2 MMBTU/h	Industrial process

Heat Sources

Lubricated Compressor

┌─────────────────────────────────────────────────────────┐
│                 LUBRICATED COMPRESSOR                   │
│                                                          │
│   Inlet air ────────────────────────────────► Air      │
│        77°F           Compression             discharge │
│                          ↓                    185°F    │
│                    ┌───────────┐                ↓      │
│                    │           │          Aftercooler  │
│                    │    Oil    │ ◄───────     ↓       │
│                    │  176-212°F│         Condensate   │
│                    │           │                       │
│                    └─────┬─────┘                       │
│                          │                             │
│                    Oil cooler                          │
│                          │                             │
│                    ┌─────▼─────┐                       │
│                    │RECOVERABLE│ ◄── 80-90% of input  │
│                    │   HEAT    │     energy           │
│                    └───────────┘                       │
└─────────────────────────────────────────────────────────┘

Heat Location

Source	% of Total Heat	Temperature
Oil	72-80%	176-212°F
Aftercooler	10-15%	104-122°F
Motor	4-8%	104-140°F
Radiation	2-4%	-

Applications

1. Space Heating

Winter:                            Summer:

    ┌─────────────┐                    ┌─────────────┐
    │ Compressor  │                    │ Compressor  │
    └──────┬──────┘                    └──────┬──────┘
           │                                  │
     ┌─────▼─────┐                     ┌─────▼─────┐
     │ Hot air   │                     │  Air to   │
     │  duct     │                     │ outdoors  │
     └─────┬─────┘                     └───────────┘
           │
     ┌─────▼─────┐
     │ Workspace │
     │           │
     └───────────┘

Typical ROI: 6-18 months

2. Water Preheating

                    ┌─────────────────────┐
                    │   Plate heat        │
    Hot oil ════════│   exchanger         │════════► Oil
    194°F           │                     │           cooled
                    │   ┌─────────────┐   │
                    │   │             │   │
    Cold water ─────│───│             │───│──────► Hot water
    59°F            │   │             │   │        140-158°F
                    │   └─────────────┘   │
                    └─────────────────────┘

Applications:

• Process water
• Boiler preheat
• Domestic hot water
• Industrial washing

3. Industrial Processes

Process	Temperature	Application
Drying	122-176°F	Wood, grain, paint
Washing	104-140°F	Parts, bottles
Tank heating	104-158°F	Chemicals, food
Thawing	104-122°F	Food products

Recovery Systems

Air Recovery

The simplest method:

                Compressor
                    │
              ┌─────┴─────┐
              │   Fan     │
              │   │       │
              │   ▼       │
              │ ┌───────┐ │
              │ │Radiator│ │
              │ └───┬───┘ │
              └─────┼─────┘
                    │
         ┌──────────┼──────────┐
         │          │          │
    Damper      Damper     Damper
    winter      summer      mix
         │          │          │
         ▼          ▼          ▼
    To space    To outdoors  Proportional

Component	Function
Winter damper	Directs hot air indoors
Summer damper	Exhausts hot air outdoors
Mix damper	Temperature control
Thermostat	Automatic control

Water Recovery

                      ┌──────────────────┐
                      │ Oil/water heat   │
    Oil ══════════════│   exchanger      │══════════════► Oil
    185°F             │                  │                122°F
                      │ ┌──────────────┐ │
                      │ │              │ │
    Water ────────────│─│              │─│────────────► Water
    inlet 59°F        │ │              │ │             outlet 158°F
                      │ └──────────────┘ │
                      └────────┬─────────┘
                               │
                         Water pump

Components:

• Plate heat exchanger (oil/water)
• Circulation pump
• Control valve
• Storage tank (optional)

Calculations

Recoverable Energy

$$Q = kW_{\text{compressor}} \times \eta_{\text{recovery}} \times \text{hours} \times 3{,}412 \text{ BTU/kWh}$$

Where:

• $Q$ = Recoverable energy (BTU/year)
• $\eta$ = Recovery efficiency (0.5-0.9)

Example:

• Compressor: 100 HP (75 kW)
• $\eta$: 70%
• Hours: 8,000/year

$$Q = 75 \times 0.70 \times 8{,}000 \times 3{,}412 = 1{,}433 \text{ MMBTU/year} = 420{,}000 \text{ thermal kWh/year}$$

Economic Savings

$$\text{Savings} = \frac{Q \times \text{Fuel cost}}{\text{Boiler efficiency}}$$

Example:

• $Q = 1{,}433$ MMBTU/year
• Natural gas: $10/MMBTU
• Boiler efficiency: 80%

$$\text{Savings} = \frac{1{,}433 \times 10}{0.80} = \$17{,}912\text{/year}$$

Typical ROI

System	Investment	Annual Savings	Payback
Air ducts (simple)	$2,000-5,000	$3,000-8,000	6-12 months
Potable water	$5,000-15,000	$5,000-15,000	12-18 months
Industrial process	$10,000-50,000	$15,000-60,000	12-24 months
Integrated system	$20,000-100,000	$30,000-150,000	18-36 months

Considerations

Limitations

Factor	Impact
Variable heat demand	May not coincide with operation
Distance to use	Transport losses
Required temperature	Limit ~158°F for water
Summer operation	No heating demand

Requirements

System	Requirement
Heat exchanger space	Near compressor
Piping	Insulated
Control	Thermostats, valves
Maintenance	Periodic cleaning

Quick Evaluation

If your plant spends more than $20,000/year on heating and has compressors larger than 50 HP operating more than 4,000 hours/year, heat recovery is probably cost-effective.