Environmental Engineering Reference
In-Depth Information
Turbine/generator
Heat
reject
Waste
heat
Figure 11.10
Schematic of rankine cycle use to transform waste heat into mechanical work.
Adapted from Barber-Nichols, n.d.
Vapor recompression has limitations though. One restriction is that the compression ratio
has to be no more than two per stage (DOE, 2001). But when the pressure of the waste steam
is too low, which would require multistage compression, a better alternative is likely to be the
use of a heat pump.
Applications of recovered heat
Recovered heat has multiple applications, for instance, preheating boiler makeup water, plant
cleaning, and heating process water. When waste heat is abundant, it can be used to produce
mechanical energy that then can be turned into electricity. Devices capable of taking low-
temperature heat to transform it into mechanical energy are governed by the “Rankine Cycle”
(Fig. 11.10) with organic compounds as working fluids with boiling temperatures lower than
water. Potential working fluids are ammonia, isobutene, toluene, isopentane, and gases used
in refrigeration equipment (Barber-Nichols, n.d.).
Absorption refrigeration
Absorption refrigeration works according to the absorption cycle. Similarly to a vapor com-
pression system a refrigerant absorbs heat in the evaporator during expansion and releases
heat in the condenser. The difference with a vapor compression cycle is that the absorption
cycle uses a liquid to absorb the refrigerant at the low-pressure section of the cycle at the
absorber and then the liquid (with the dissolved refrigerant gas) is pumped to the high-pressure
section instead of compressed. At the high pressure section, the refrigerant gas is taken out of
solution by application of heat at the generator. The refrigerant gas then goes into the con-
denser and gets converted into liquid that goes through the expansion valve and then to the
evaporator. Liquid goes back to the absorber via a valve and is reused in a continuous fashion
(Fig. 11.11).
Basically an absorption cycle needs two fluids: a gas (the refrigerant) or absorbate and a
liquid (the absorbent). The absorbate has to be soluble in the absorbent. Typical working fluids
for absorption refrigeration systems are water-lithium bromide, in which water is the
refrigerant and lithium bromide the absorbent, and ammonia-water, in which ammonia is the
