Environmental Engineering Reference
In-Depth Information
Distribution
Pressure
reducing valve
Combustion
gases
End use
Combustion air
preheater
Forced
draft fan
Process
heater
Heat
exchanger
Economizer
Process
heater
Generation
Boiler
Steam
trap
Steam
trap
Fuel
Condensate
receiver
tank
Feed
pump
Condensate
pump
Deaerator
Recovery
Figure 11.4 General schematic of the four principal areas of a steam system.
From Department of Energy (DOE), 2004.
Pressure in the boiler drives steam into the distribution system that take it to points of use.
Steam distribution is done by a network of pipes that carry the steam at one pressure or at
multiple pressures created by intermediate pressure-regulating valves. Typically, steam with
pressure between 0.1 MPa (15 psi) and 0.689 MPa (100 psi) is considered “low pressure,” and
when its pressure is higher than 0.689 MPa is considered “high pressure.”
At the end-use points, steam delivers its energy to processes mainly by condensing, which
releases its latent energy. In food processing, steam has use in multiple applications, such as
direct injection into food products, sterilization in containers, sterilization of liquid or semi-
solid foods in heat exchangers, cooking, and heating in general. When using indirect heating
through heat exchangers, steam is contained in the exchanger by a steam trap that allows only
condensate to pass but not steam. Condensate coming from steam traps is often, but not
always, recovered and returned back to the boiler.
Direct-fire heating in food processing
In direct-fire heating systems, gas, or other fuel, is burned directly inside the equipment.
Typical examples are fryers, ovens, flame sterilizers, roasters, flame peelers, and dryers. In
direct-fire heating, heat can be transferred directly from the flame (by radiation), from com-
bustion gases to the food product (e.g., in drying), or indirectly by using a heat exchanger.
Direct heat transfer is done when the process allows it, and the fuel produces a clean flame
(e.g., natural gas).
Opportunities for energy-efficiency improvements
One of the cornerstones of sustainability is the reduction of energy consumption based on
improvements of energy efficiency. Because most of energy consumption in food processing
happens at the processing level, this would be a good place to start making efficiency improve-
ments by targeting direct and indirect heating, then equipment powered by electric motors,
and later to improvements at the building's level.
 
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