Recovery Systems

There are two main approaches to recovering heat from compressors: hot air recovery and water-based recovery.

Air Recovery

Principle

The compressor's cooling air is directed to spaces requiring heating:

    COMPRESSOR ROOM              AREA TO HEAT
    ┌─────────────────┐              ┌─────────────────┐
    │                 │              │                 │
    │  ┌───────────┐  │   Insulated │                 │
    │  │Compressor │══════════════════════►           │
    │  │           │  │   duct      │   Hot           │
    │  └───────────┘  │              │   air          │
    │                 │              │   122-140°F    │
    │  Air 68°F  ←───────────────────────────────────│
    │  (return)      │              │                 │
    │                 │              │                 │
    └─────────────────┘              └─────────────────┘

Components

Duct system:

    ┌────────────────────────────────────────────────────┐
    │                                                     │
    │   Outdoor     Indoor      Centrifugal  Compressor │
    │   damper      damper      fan                     │
    │     │          │            │             │        │
    │     ▼          ▼            ▼             ▼        │
    │   ═══════════════════════════════════════════════ │
    │   ↑          ↑            ↑             ↑         │
    │ Outdoor    Indoor      Forced       Hot air      │
    │(summer)   (winter)     boost        from compressor│
    │                                                     │
    └────────────────────────────────────────────────────┘

Component	Function
Capture hood	Captures hot air from compressor
Ducts	Transport the air
Dampers	Direct flow
Booster fan	Overcomes long duct resistance
Filter	Cleans return air
Thermostat	Automatic control

Capacity Calculation

Available heat (BTU/h):

$$Q = \dot{V} \times \rho \times C_p \times \Delta T \times 60$$

Where:

• $\dot{V}$ = Air flow (CFM)
• $\rho$ = Density (0.075 lb/ft³)
• $C_p$ = Specific heat (0.24 BTU/lb·°F)
• $\Delta T$ = Temperature rise (°F)

Example (100 HP compressor):

• $\dot{V} = 3{,}000$ CFM (compressor fan)
• $\Delta T = 40°F$ (from 70°F to 110°F)

$$Q = 3{,}000 \times 0.075 \times 0.24 \times 40 \times 60 = 129{,}600 \text{ BTU/h} \approx 38 \text{ thermal kW}$$

Duct Sizing

CFM	Duct Diameter	Velocity
1,000	12"	1,300 fpm
2,000	16"	1,400 fpm
3,000	20"	1,400 fpm
5,000	24"	1,600 fpm

Rule: Maintain velocity between 1,000-2,000 fpm for low noise.

Water Recovery

Basic System

┌─────────────────────────────────────────────────────────┐
│              HOT WATER SYSTEM                           │
│                                                          │
│  Oil        ┌──────────────┐        Water               │
│  194°F ═════│Plate heat    │═══════► 131°F             │
│             │  exchanger   │                            │
│  122°F ◄════│              │◄═══════ 59°F              │
│             └──────┬───────┘        (inlet)             │
│                    │                                    │
│                    │ Control                           │
│                    │                                    │
│             ┌──────┴───────┐                           │
│             │   3-way      │                           │
│             │   valve      │                           │
│             └──────────────┘                           │
│                                                          │
└─────────────────────────────────────────────────────────┘

Heat Exchanger Types

Type	Advantages	Disadvantages
Plate	Compact, efficient, cleanable	Medium cost
Shell and tube	Robust, economical	Large, less efficient
Double pipe	Simple, low cost	Low capacity

Plate Heat Exchanger

       Hot oil inlet
              │
    ┌─────────▼─────────┐
    │ ║   ║   ║   ║   ║ │
    │ ║   ║   ║   ║   ║ │◄── Corrugated plates
    │ ║   ║   ║   ║   ║ │
    │ ║   ║   ║   ║   ║ │
    └─────────┬─────────┘
              │
       Cold oil outlet

    Cold water ──────────────► Hot water

Typical specifications:

Parameter	Value
Approach temperature	9-18°F
Oil pressure drop	7-29 psi
Water pressure drop	4-15 psi
Fouling factor	0.0001-0.0002

System Control

                    Thermostat
                       │
                       ▼
    ┌──────────────────────────────────────┐
    │                                       │
    │   Hot oil ══════╗                    │
    │                 ║                    │
    │                 ║                    │
    │            ┌────╨────┐               │
    │            │ 3-way   │               │
    │            │ valve   │               │
    │            └────┬────┘               │
    │                 │                    │
    │     ┌───────────┼───────────┐        │
    │     │           │           │        │
    │     ▼           ▼           ▼        │
    │  Bypass     Exchanger    Mixed       │
    │  (if water   (heat      (proportional│
    │  too cold)   demand)     control)    │
    │                                       │
    └──────────────────────────────────────┘

Specific Applications

Boiler Feedwater Preheat

                     Boiler
                        │
    Feedwater ──────────┤
    59°F                │
                        │
    ┌───────────────────┴───────────────────┐
    │                                        │
    │        With recovery:                  │
    │                                        │
    │   Water 59°F ──► Exchanger ──► 140°F ──► Boiler
    │                        ▲                     │
    │                        │                     │
    │                   Compressor               Savings:
    │                   oil                      40-60%
    │                                           preheat fuel
    │
    └────────────────────────────────────────────┘

Domestic Hot Water

    ┌─────────────────────────────────────────────────────┐
    │                                                      │
    │   Municipal      Heat           Storage            │
    │   cold water ──► exchanger ──► tank                │
    │      │               │               │              │
    │      │               │               │              │
    │      │          Compressor          │              │
    │      │          oil                 │              │
    │      │                              ▼              │
    │      │                        Hot water            │
    │      │                        for use             │
    │      │                        (showers, washing)   │
    │      │                              │              │
    │      │                              │              │
    │      └────── Recirculation ◄────────┘              │
    │                                                      │
    └─────────────────────────────────────────────────────┘

Radiant Floor Heating

    Exchanger ──► Pump ──► Manifold ──► Floor coils
          │                    │              │
          │                    │              │
    Compressor              Zone           Comfort
    oil                    control         104-113°F

Comparison

Aspect	Air Recovery	Water Recovery
Initial cost	Low	Medium-High
Installation	Simple	Complex
Transport distance	Limited (100-160 ft)	Long (330+ ft)
Temperature	104-140°F	131-158°F
Storage	No	Yes (tank)
Summer use	Difficult	Possible (hot water)
Maintenance	Minimal	Medium
Efficiency	70-85%	80-95%

Project Example

System Data

Parameter	Value
Compressor	150 HP (112 kW)
Operation	6,000 h/year
Recoverable heat	80% = 90 kW
Current fuel	Natural gas
Gas cost	$12/MMBTU
Boiler efficiency	85%

Savings Calculation

Annual thermal energy:
  Q = 90 kW × 6,000 h = 540,000 kWh/year
    = 1,843 MMBTU/year

Gas equivalent cost:
  = 1,843 MMBTU × $12 / 0.85
  = $26,014/year

Investment (water system):
  = $25,000

Simple payback:
  = $25,000 / $26,014
  = 0.96 years ≈ 12 months

Pre-Investment Evaluation

Before investing, verify:

1. Match between heat availability and demand
2. Distance between compressor and point of use
3. Process temperature requirements
4. Summer operation (if applicable)