Page 311 - Sustainable On-Site CHP Systems Design, Construction, and Operations

284 Op erations

where ρ and c = density and specific heat of hot water entering the absorp-
hotwater,i p,hotwater
tion chiller
v = volumetric flow rate of hot water entering the chiller
hotwater, i
T and T = hot water entering and leaving temperatures
hotwater,i hotwater,o
For absorption chillers that use steam to generate the refrigerant:
Q = v ρ [( T P , ) − h( T , )P ] (17-37)
h
,
gen steam, i steam, i i i steam, i o o steam,o
where ρ = density of steam entering the absorption chiller
steam,i
v = volumetric flow rate of steam entering the chiller
steam, i
h(T,P ) = enthalpy of steam entering the chiller at temperature T and pressure P
i i i i
h(T ,P ) = enthalpy of steam leaving the chiller at temperature T and pressure P
o o o o
Absorption Chiller Performance Calculations
For absorption chillers with hot water or steam as the source of heat, the performance cal-
culations are based on Eqs. (17-33), (17-34), and (17-36) or (17-37). For direct-fired absorption
chillers, the performance algorithms are based on Eqs. (17-33) through (17-35). The
density of liquid water or exhaust gases should be evaluated at the same conditions as the
inlet flow rate is measured. The specific heats of water and exhaust gases are assumed con-
stant across the chiller, which is a good assumption for liquid water for the typical range of
temperatures across the chiller, but it should be evaluated at the average of the inlet and
outlet temperatures for exhaust gas. Although, the auxiliary fuel flow rate is not included in
the equations cited for direct-fired absorption chillers, the auxiliary fuel flow rate is input
into the algorithms and converted to an output as the auxiliary rate of fuel use so that fuel
used for supplemental firing can be tracked (it is not included as an output for hot-water-
and steam-fired chillers because it is an output for the HRU in those cases).

Cooling Tower
Cooling towers (CTs) provide an ability to reject heat from the condenser and the
absorber, which is required by the absorption refrigeration cycle. For a water-cooled
condenser, heat is transferred from refrigerant to cool water, which is pumped to the
cooling tower. The cooling tower uses evaporative cooling to reject heat from the con-
denser cooling water to the ambient environment. The fans push (forced draft) or pull
(induced draft) ambient air through the cooling tower.

Efficiency of Cooling Towers
The cooling tower efficiency (η ) is defined as
CT
(T − T )
η = CT, ,wi CT, ,w o (17-38)
CT (T − T )
CT, ,wi wb
where T = inlet temperature of the cooling water return to the tower
CT,w,i
T = outlet temperature of cooled water from the tower (cooling water supply)
CT,w,o
T = wet-bulb temperature of the ambient air to which heat is rejected by the
wb
cooling tower ∗
∗ If heat losses from piping between the absorption chiller and cooling tower are small, then T ≈
CT,w,i
T and T ≈ T
AbChiller,cw,o CT,w,o AbChiller,cw,i