HEAT RECOVERY
The drying plants in the paper industry require lot of heat energy. It is therefore necessary first to see that drying process is as economical as possible. An optimized heat recovery system directly reduces the prime heat consumption.
The most common dryer types in paper industry are the multi-cylinder machine, the Yankee machine and Air Dryers.
HEAT RECOVERY- MULTI-CYLINDER MACHINES
The superheat of exhaust air from a PM Hood is often in the range of 15 to 20°C, i.e. the dry bulb temperature of the exhaust air is 15 to 20°C higher than the dew point. This, however, depends on the temperature of make-up air, the quantity of infiltration air and the steam pressure in the cylinders.
The supply air to the hood is heated in a heat recovery plant to an economically optimum temperature which is around 50 to 60°C and this can be attained by an air / air heat exchanger.
The table below shows approx heat consumptions obtained in a multi-cylinder machine provided with heat recovery. Here an open hood with dew point 40°C is compared to a closed hood with dew point of 53°C and 58°C
| | |
Open
Hood |
Closed
Hood |
Closed
Hood |
| Exhaust air dew point |
ºC |
40 |
53 |
58 |
| Exhaust air humidity |
Kg/Kg |
0.05 |
0.10 |
0.14 |
| Exhaust air
temperature |
ºC |
55 |
75 |
82 |
| Supply air humidity |
Kg/Kg |
0.01 |
0.01 |
0.01 |
| Supply air Temperature after Heat Recovery
|
ºC |
35 |
55 |
60 |
|
Ambient air temperature in M/c room |
ºC |
25 |
25 |
25 |
| Proportion of heated supply air/ exhaust air
|
|
0.3 |
0.70 |
0.85 |
| Approx.
heat consumption |
Kg/Kg H2O |
3400 |
3125 |
3000 |
HEAT RECOVERY - HIGH EFFICIENCY HOOD
Yankee hoods may be steam -heated by means of steam coils or may be gas or oil heated in the air circulation unit. In the
former case, the air impingement temperature lies in the range of 140°C to 160°C and in the latter case, the drying temperature
can be as high as 350°C to 450°C.
For this system a well designed heat recovery unit is essential to recover a part of the exhaust heat in pre-heating the fresh air intake to the hood.
TYPES OF AIR TO AIR HEAT RECOVERY UNITS
Two types of air to air heat exchangers are commonly used -Channel design and tube design. This cross-flow heat exchanger ;
- has no moving parts
- is easy to adapt to different flows and temperature efficiencies
- is available for high temperatures and corrosive atmospheres
- is easy to clean.
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