Civil Engineering Reference
In-Depth Information
• The head difference between
A
2
and
A
1
represents the saving in power and so in
energy due to the use of a non-dissipative method. This saving depends both on
the head increase and on the difference of efficiency between the two points.
• The energy saved depends on the working hours:
¼
ð
PA
2
PA
1
Þ
ðÞ
ES
working hours Wh
Typical energy saving for a 50 % reduction of the capacity ranges from
25 to 50 % of the corresponding energy needed with non-dissipative
methods.
10.5
Pump and Fan Regulation Methods
Non-dissipative regulation of pumps and fans in industrial applications is an
effective area of energy saving in many processes like paper, chemical, and
refinery, in water-pumping systems, and in HVAC systems.
In the low- and medium-power range, from 1 to 200 kW, electrical variable-
speed drives can conveniently be applied. Synchronous and asynchronous motors,
fed at variable frequency and voltage, are particularly suitable for operating in
hazardous locations with a proper choice of the motor and related equipment.
Higher power ranges, too, can be covered with the same drives, but also gas turbines,
steam motors and turbines, and diesel engines can be employed economically if the
waste heat is recovered as already shown in cogeneration systems (see Chap.
9
).
The amount of energy saved depends on the need for low-rate capacity, working
hours of the system, and electric energy cost, but it is always significant.
Other advantages, in addition to the energy saving which is the main factor, are
greater simplicity due to the elimination of throttling valves, dampers and similar
devices, and simpler design of the mechanical and electric circuits made possible by
softer starting.
10.6
Practical Examples
Example
Compare the energy consumption of a centrifugal pump if flow is
reduced to 50 % of the rated value with (1) a dissipative method and (2) a
non-dissipative method.
An example is given below of technical and economic comparison between a
dissipative system (throttling valve) and a non-dissipative one (electrical variable-
speed drive). This example refers to a low-power pump which works at 50 % of rated
capacity for all the year, but it can be assumed as a basic model for other situations.
Table
10.1(a)
and
(b)
shows the technical characteristics of the pump and the
data elaboration procedures with both SI and non-SI units. The characteristic curves
