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4. SYSTEM DESCRIPTION
The temperature difference between point 41 and point 42 (∆T h,DES2 ) is
5 ◦ C like for DES1. The temperature of DES2 is 150 ◦ C (point 70 and 79).
The high pressure refrigerant is sent to COND2 inlet (point 71). Like
for COND1 in the single stage cycle configuration, the refrigerant is both
de-superheated and condensed in COND2. Thus the closest approach
temperature difference is found somewhere between point 71 and point
72. Due to limitations in the model it is not possible to set this parameter
directly and instead ∆T min,COND2,r,o is set to 12,5 ◦ C. This corresponds to
∆T min,COND2,mp being 10 ◦ C.
The heat from COND2 is removed by the heat transferring loop (point
31 to point 34) and supplied to low temperature desorber DES1. The loop
is modeled as a water circuit, but in a real double stage unit, COND2 and
DES1 would be integrated as one component.
The saturated liquid refrigerant (qu 72 = 0) is expanded in VA2 point
72 before it is mixed with superheated refrigerant from DES1 point 50 in
MIXR1. The quality of the refrigerant outlet is between zero and one in
point 51.
Unlike COND2, the refrigerant entering COND1 is not superheated.
Thus the closest approach temperature difference exists at the inlet in
point 51 (compared to the temperature in point 36) but the value is still
10 ◦ C. The heat from COND2 is removed by the cooling circuit (point 35
and point 36)
The refrigerant is expanded in VA1 (point 53), then evaporated (point
54), and absorbed in the absorber exactly the same way as in the single
stage cycle. The difference occurs after the weak solution is preheated in
SHEX1 in point 57. In the double stage system only some of the solution
is sent to DES1 but the remaining part is pressurized further by PUMP2
(point 76) and preheated in SHEX2 (point 77) before it enters DES2 (point
78). The strong solution is cooled in SHEX2 (point 80 to 81), expanded in
VB2 and mixed with the strong solution coming from DES1 in MIXL1.
Since the temperature in point 50 is not set as a parameter (as it is the
case for the single stage cycle), another relation is given:
ṁ 70
ṁ 78
= ṁ50
ṁ 58
(4.4)
This relation determines the fraction of solution which is sent to the
DES2. It also implies that concentration of the weak and strong solution
will be the same for the two solutions in the two circuits (w 55 = w 75
90