Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 162

60 6

50 5

Measured NH 3 /H 2 O liquid
volume lifted per hour / l h -1

40 4

Maximum lifting ratio
b = 4.3

30 3

Lifting ratio

20 2

10 1

Calculated NH 3 -vapour volume
desorbed per hour / l h -1

0 0

145 150 155 160 165 170 175

External generator inlet temperature / °C

Figure 5.49 Liquid and vapour volume flows and lifting ratios as a function of the external generator
inlet temperature

temperature of 165 ◦ C. The design mass flow of the weak solution for a 2.5 kW evap-
orator cooling capacity is 50 kg h − 1 , and the ammonia vapour mass flow is 8 kg h − 1 .
Another way to analyse the performance of the bubble pump without measuring the
volume flow is to calculate the solution mass flows. For that, first the ammonia vapour
mass flow m V has to be calculated. The vapour mass flow is defined as follows:

Q C

m V = (5.8)

c V,NH 3 T VC − T C,s c L,NH 3 /H 2 O T C,s − T LC

+ h V − h L +

Here, Q C is the condenser capacity, c V,NH 3 is the specific heat capacity of ammonia
vapour, T VC and T LC are the measured condenser vapour inlet and liquid condensate
outlet temperatures, T C,s is the condensation temperature, c L,NH 3 /H 2 O is the specific
heat capacity of liquid ammonia and h V and h L are the vapour and liquid enthalpies.
The weak m Sw and rich m Sr solution mass flows are then calculated as follows:

X V − m V

X Sr
X Sr − X Sw

m Sw = (5.9)

X V − X Sw
X Sr −

m V

m Sr = (5.10)

X Sw

X Sr is the rich solution mass concentration, which is given by the initial mass
concentration. X Sw is the weak solution mass concentration, which is calculated using
a modified Clausius-Clapeyron equation (Bourseau and Bugarel, 1986) relating the
generator weak solution concentration, its vapour pressure and the weak solution outlet
temperature. X V is the vapour concentration, which is dependent on the pressure and