, a reasonable pressure drop between the evaporator and absorber is 1.5 psi Consider a large NH3/H2O plant operating per the schematic be
For the NH3/H2O cycle the following relationship give the solution flow rate per unit refrigeration rate:
WFS_A * X - WFS_G(X-1) = 1
where
WFS_A = mass fraction of NH3 in solution form the absorber
WFS_G = mass fraction of NH3 in solution from the generator
X-1 =mass of solution from the generator per unit mass of refrigerant flow
For large systems, a reasonable pressure drop between the evaporator and absorber is 1.5 psi
Consider a large NH3/H2O plant operating per the schematic below:
Known/given:
i. Refrigeration load = 500 tons
ii. Evaporator temperature (state point 10) = 41.1 F
iii. Evaporator pressure (state point 10) = 75 psia
iv. Absorber pressure (state point 11) = 73.5 psia (per rule of thumb above regarding pressure drop between
the evaporator and absorber)
v. Strong aqua solution temperature (state point 3) = 105 F
vi. Condenser temperature (state point 8) = 100 F
vii. Condenser and tower pressure (state point 7) = 211.9 psia
viii. Concentration split, WFS_G - WFS_A =6% by weight (which allows for sufficient flow rate and adequate
wetting of plain horizontal tubes up to 1 inch diameter in optimum arranged absorbers when using gravity
feed for large systems, and it also ensures reasonable maximum liquid flows for cost effective heat exchangers
and towers and a practical minimum temperature of heat source for the generator)
ix. Cooing tower water temperature = 85 F
Assume a 3% increase in the theoretical refrigerant flow rate to account for heat losses from the high temperature shells
and heat gain through insulation for the evaporator, thus the LIFT = 1.03*500 tons = 6,180,000 BTU/hr.
a. Fill in the table below (assume equilibrium as needed) here SA = strong aqua, WA = weak aqua
| State Point | Hardware | T (F) | P (psia) | SA (%) | WA (%) | hl (BTU/lb) | hv (BTU/lb) |
|
1 |
71.4 |
|
2 |
118 | -14.21 | |||||
| 3 | -28.3 | ||||||
| 4 | 48.3 | ||||||
| 5 | |||||||
| 6 | |||||||
| 7 | 638.7 | ||||||
| 8 | 155.2 | ||||||
| 9 | |||||||
| 10 | 623.02 | ||||||
| 11 | absorber | 105 | 73.5 | 49 | 43 |
| 12 |
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