Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 161

Table 5.5 Pressure levels, concentration ranges and solution circulation in the large generator
of prototype 1 and the smaller prototype 2 generator

Prototype 1 Prototype 2

Total pressure/10 5 Pa 20 18 20 12

NH 3 initial concentration/% 38 30 40 37

Rich solution concentration X sr /% 45-42 30 40 37

Weak solution concentration X sw /% 31-34 24-26 31-33 24-29

Degassing width E /% 14-8 6-4 9-7 13-8

Vapour concentration X V 1 /% 88-91 82-85 89-91 84-89

Specific solution circulation f /- 4-7 11-15 6.5-8.5 4.5-7.5

weak solution, called the temperature flank method, is described in detail by Biesinger
(2002). For the investigation of the generator, the lifting ratio b , which is the ratio of
lifted liquid volume V L to simultaneously expelled vapour volume flow V V , is an
important value used to quantify the performance of the generator (Cattaneo, 1935)

V L
V V

b = (5.6)

Another important characteristic value of the generator is the specific solution
circulation index f . This index is defined as the ratio of the mass flow rates of the rich
solution m Sr to the ammonia vapour m V , or alternatively as the ratio of the vapour
concentration X V 1 and weak solution concentration X Sw difference to the degassing
width E (which is the difference between the weak and the rich solution concentration
X Sw and X Sr )

X V 1 − X Sw
X Sr −

m Sr
m V

f = = (5.7)

X Sw

An evaluation of the measurement results of the volume flows was done with the
corresponding measured pressures and vapour concentrations. For the maximummea-
sured volume flow of the lifted weak solution of 53 l h − 1 , the evaluated vapour volume
flow is 12.2 l h − 1 and thus the maximum lifting ratio is 4.3 when the external generator
inlet temperature is 165 ◦ C. Figure 5.49 shows the three curves of the measured liquid
volume flow, the calculated vapour volume flow and the lifting ratio of prototype 1.
The volume flow of the vapour was continually increased because of the changing
flow regime. The slug flow collapsed and converted to annular flow, and thus nucle-
ate boiling changed to film boiling. Therefore, at heating temperatures above 165 ◦ C,
more vapour is generated but the solution lifting decreases.
Converted to mass flow, the lifted weak solution volume flow shows a maximum
of 42.0 kg h − 1 and a specific solution circulation of 8.0 at an external generator inlet