Environmental Engineering Reference
In-Depth Information
Heat recovery
In food-processing plants, there are many opportunities for heat recovery. But before
engaging in any heat recovery project, some technical issues need examination (DOE,
2006g):
1.
Quality of the source . If heat is recovered from flue gases, the presence of particles or
corrosive compounds needs consideration.
2.
Temperature of the primary source . The differential temperature between the primary
source and the process has to be at least 93°C (200°F).
3.
Matching heat demand of secondary source with the heat supply of primary source .
The amount of heat from the primary supply needs to be high enough to supply a large
percentage of demand of the secondary process.
4.
Matching the timing of the heat supply and demand .
5.
Proximity . The closer the heat supply and demand are the better.
Food-processing plants have three main sources that are good candidates for potential heat
recovery:
1. Equipment . The most important are power generation, boilers, kilns, ovens, driers, and
refrigeration equipment.
2. Water used to cool equipment.
3. Process water.
In direct-fired equipment, such as boilers, driers, and furnaces, heat can be recovered
from the flue with an economizer type of heat exchanger. Typically, economizers recover
only sensitive heat from flue gases; however, new “condensing economizers” can go one
step further and cool the flue gases below their dew point thus recovering also the latent
heat portion. Because the quality of flue gases from direct-fired equipment in food-process-
ing facilities is good (low particles and corrosiveness), condensing economizers are an
attractive alternative to make the best of heat recovery with gas-to-gas or gas-to-liquid heat
exchangers.
Refrigeration equipment is designed to capture heat from the space that needs cooling and
reject the heat to the outside. Heat rejection takes place at the condenser, which can operate by
cooling the refrigerant with atmospheric air or with water that is then cooled at cooling towers.
In compression cycle refrigeration, heat can be recovered at several points of the cycle includ-
ing the condenser, desuperheating units between the compressor and condenser, subcooling
units after the condenser, and from compressors and motors.
In ammonia refrigeration systems, the installation of a secondary ammonia compressor can
boost the pressure of compressed ammonia further and produce more heat that then can be
recovered with a water-cooled condenser. This setup allows the generation of hot water with
temperatures of 63°C (145°F) that can be used in the process (Electric Power Research
Institute [EPRI], 2008).
Low-grade heat with a heat pump
Heat naturally flows from higher to lower temperatures. However, a heat pump can take heat
at a low temperature and boost it to higher temperatures with more useful applications. A heat
pump is a mechanical device that basically works as an inverted refrigeration unit. It moves
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