Environmental Engineering Reference
In-Depth Information
Maintain heat transfer surfaces
Similar principles to those described for improving boiler efficiencies apply to direct heating.
Equipment that uses radiant tubes or heat exchangers (with the flame or heated gases in one
side and the heated medium on the other side of the tube) can develop deposits of soot, scale,
and sludge on the heat transfer surfaces. These deposits slow down heat transfer, and as a
result, reduce the efficiency of fuel utilization, which can be detected as an increase in the
temperature of flue gases.
Insulate and maintain equipment
As was described in the section on improving insulation for steam systems, insulation is an
important way to reduce energy consumption of equipment subjected to continuous service.
In the case of direct-fired equipment, such as ovens and dryers, which are operated at high
temperatures, heat loss by radiation is an important factor that needs consideration. Hot sur-
faces radiate energy to surfaces at lower temperature than are in the line of sight at rates that
vary with the fourth power of the absolute temperature. For instance, a surface at 204°C (400°F)
losses 3.52 MJ/h m
2
(300 Btu/h ft
2
), and at 409°C (800°F), the rate of heat loss is 7.62 MJ/h m
2
(650 Btu/h ft
2
) (DOE, 2006f).
Efficiency of mechanical systems
Electric motors
It is estimated that over its typical operating life of 10 years, a continuously operated electric
motor consumes in electricity 50 times the initial purchase price (Washington State University
[WSU], 2003a). Even when initial prices are higher, selection of energy-efficient motors
instead of ones with standard efficiency has a significant impact on electricity use in the long
run. To be considered energy efficient in the United States, a motor has to meet or exceed the
nominal full-load efficiencies established by the National Electrical Manufacturers Association
(NEMA). Threshold efficiencies are established in the MG 1 standard by NEMA for different
horsepower, enclosure types, and rotational speeds. For all motors, standard- or energy-
efficient, efficiencies increase with horsepower. For instance, a 1 hp energy-efficient open
motor running at 1800 rpm has an efficiency of 82.5 percent, a 10 hp 89.5 percent, a 100 hp
94.1 percent, and a 500 hp 95.8 percent. Enclosed motors follow a similar trend to that of open
ones (“Buying an energy-efficient electric motor,” n.d.).
Selection of a motor does not depend only on its efficiency but also on the load. Underpowered
motors consume more electricity, overheat, and eventually fail. However, overpowered motors
are also inefficient because they operate outside the range of maximum performance.
The efficiency of both standard- and energy-efficient motors is designed to operate above
60 percent load with peaks around 75 percent of the full-rated load (McCoy et al., 1993).
Especially for smaller sizes, motors that operate at less than 50 percent of the full-rated load
are good candidates for downsizing to a lower power (Table 11.5) (McCoy et al., 1993).
Purchasing energy-efficient motors has to be considered:
For new installations.
●
When purchasing new equipment that contain electrical motors (e.g., compressors, HVAC
systems, and pumps).
●
When modifying facilities and processes.
●
When purchasing spares or replacing failed motors.
●
Instead of rewinding old standard-efficiency motors.
●
