Substituting absorption for vapor compression is a way of offsetting the major drawback of the steam-jet refrigeration
system of the preceding Appendix-the high requirement of medium-pressure motive steam. Finding a suitable
absorbent to replace the jet vapor compressor would let us retain those advantages that water exhibits as a refrigerant.
Aqueous LiBr (lithium bromide) solutions have been mentioned (Chapter 2, "Principles of Absorption Refrigeration")
as enjoying extensive use- domestically and industrially-as such an absorbent. A large number of "package units"
operating on this cycle are used for residential and commercial air-conditioning. There are, obviously, opportunities for
process applications in the operating range of this refrigerant.
The fundamental principles and design and operating data for this refrigeration technique are also covered in the
publications of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., New York.
These include the ASHRAE Handbook of Fundamentals and editions of the ASHRAE Guide and Data Book (see
Appendix B). Information is also available in standard reference texts and from brochures issued by designers and
vendors of LiBr absorption refrigeration package units.
Design of Commercial Units
The schematic flow diagram of Figure 2-10 evolves to that of Figure C-l for these units (in their simplest form). The
constraints of the inevitable vacuum operation are mitigated in commercial package designs by combining the
operations into two vessels, thus eliminating energy losses from vapor-line pressure drops. Such an arrangement is
shown in Figure C-2, which includes representative design conditions (in English units). I A more detailed view of the
flow arrangement in one manufacturer's I design is given in Figure C-3; construction details are indicated in the
cut-away photograph of Figure C-4. The major components of the unit can also be contained in one single vessel. This
is done, as shown in Figure C-5, by an internal partition which separate the operations at the two different pressures.
This manufacturer also offers a version wherein the dilute absorbent solution is concentrated in two stages. The
arrangement of its elements and the flow circuitry for the vapor and solutions are shown in Figure C-6. A simplified
schematic diagram which illustrates the principles of such operations is presented in Figure C-7. As seen, the
refrigerant (water) vapor evaporated from the first (high-pressure) stage condenses in and supplies the heat input to the
second stage ("Low-Pressure Concentrator"). A significant energy saving is claimed for this double-effect arrangement.
Energy Consumption
It is beyond our scope to provide data on the specific consumption of heat energy of LiBr absorption refrigeration units
as a function of the evaporating and condensing temperatures and, of the quality of the heat source. Diverse proprietary
designs are available, depending on he refrigeration level and duty and the type of heating medium and coolant for
which they are adapted. Available "off-the-shelf" models can be driven by the combustion of fuel (e.g., natural gas), by
circulating hot water, and by process fluids. Since such standardized designs must be restricted to a manageable
number of models, the characteristics of the heating medium (e.g., steam pressure, hot water inlet/outlet temperatures,
etc.) and the coolant will have a marked effect on the energy efficiency of a particular package unit.
We can, however, estimate the heat energy and coolant requirements of a simple lithium bromide absorption
refrigeration unit without going beyond the basic process design to equipment sizing. Physical properties of and
thermodynamic data on the binary system H2O-LiBr suitable for this purpose are given in various ASHRAE
publications. Those of importance to us are reproduced here, with permission of the American Society of Heating,
Refrigerating, and Air-Conditioning Engineers. They include:
1. Figure C-8: Vapor pressure chart for H2O-LiBr solutions. The solution vapor pressure is given by the vertical
linear ordinate in terms of the boiling temperature of water at that pressure. The auxiliary diagonal scale translates
this water temperature to the actual pressure. The curved lower boundary line represents saturated solutions of
lithium bromide in water.
2. Figure C-9: Enthalpy-concentration isotherms for H2O-LiBr solutions. As in Figure C-8, equations are presented
to permit direct use of these data in computer programs. Note again the curved lines delineating the liquid-solid
phase boundary.
3. Table C-l : Tabulation of the enthalpy-composition-temperature data for aqueous LiBr solutions, as plotted in
Figure C-9. Entrainment-free vapor from these solutions is essentially 100% H2O, superheated due to the boilingpoint rise of salt solutions.
EXAMPLE C-1
LiBr Unit Mass and Heat Balances
The heat energy requirements of a lithium bromide absorption refrigeration unit are readily calculated from the above data.
The procedure is illustrated below using the process design conditions and nomenclature of Figure C-2.
Assume a refrigeration duty of 10.000 x 106 Btu/h (10.155 x 106 kJ/h) at the 42° F (5.6° C) boiling temperature shown. Calculate:
1. The evaporation rate for the refrigerant water
2. Flow rates for the weak and strong LiBr solutions
3. Heat transfer duty of the solution exchanger
4. Heat transfer duty of the heating element in the boiler ("Generator")
5. Heat rejected to cooling water
Solution
The procedure is based on striking balances around the equipment items identified in Figure C-l. The enthalpy values used below
are from the above-noted sources at the pertinent pressure-temperature-composition conditions.
1. The refrigerant vaporized from the Evaporator, absorbed in the Absorber, and finally liquefied in the Condenser is
R = Qe / (He -hr) (C-l) = (10)(106)/(1079.4- 82.9) = 10,035 Lb/h (4,552 kg/h)
2. Absorber mass balances yield: overall:
R + Lb = La (C-2)
LiBr:
Lbxb = Laxa (C-3)
...Lb = Rxa / (Xb -Xa) (C-4) = (10,035)(59.5)/(64- 59.5)
= 132,685 Lb/h (60,185 kh/h)
and:
La = 10,035 + 132,685 = 142,720 Lb/h (64,737 kg/h)
3. The duty of the solution exchanger may be calculated from the cold side, the hot side, or both:
Qx = La (hf- ha) (C-5a) Qx = Lb(hb- ho) (C-5b )
La:
Qx = 142,720 (77.9- 51.9)
= 3.709 x 106 Btu/h (3.914 X 106 kJ/h)
Lb:
Qx = 132,685 (108.2- 73.4)
= 4.611 x 106 Btu/h (4.865 x 106 kJ/h)
This inequality indicates an error in the solution temperatures reported on Figure C-2. We will correct the imbalance by revising
the temperature of the weak solution (La) leaving the Exchanger:
hf = 51.9 + (4.611)(106) / 142,720 = 84.2 Btu/Lb (196 J / g)
From the temperature-enthalpy relationship for a 59.5% LiBr solution: tf = 180.5° F (82.5° C)
4. The heat input to this refrigeration unit is
Qb = RHb + Lbhb -Lahf (C-6) = 10,035(1155.9) + 132,685(108.2) -142,720(84.2) = 13.93 X 106 Btu/h (14.70 x 106 kJ/h)
5. The dut). of the refrigerant condenser is
Qc = R(Hb -hr) (C-7) = 10,035(1155.9- 82.9)
= 10.77 x 106 Btu/h(II.36 X 106kJ/h)
The duty of the absorber intercooler is
Qac = RH. + Lbho -Loho
= 10,035(1079.4) + 132,685(73.4)-142,720(51.9) = 13.16 x 106 Btu/h (13.89 x 106 kJ/h)
The heat rejected to cooling water is then
Qc + Qac = 23.93 x 106 Btu/h (25.25 x 106 kJ/h) which agrees with the total heat input: (Qe + Qb) 6. 30\/J steam flow:
S = (13.93)(106)/(1163.8 -218.5)
= 14,746 Lb/h (6,688 kg/h)
Commentary
The process design for this unit can be completed when the characteristics of the heating medium, the coolant, and the stream to
be refrigerated are specified. If package unit(s) are to be used, the problem may simply boil down to choosing the vendor and
model best suited to the particular design conditions and site requirements.
If the immediate purpose is limited to comparing the energy efficiency of the LiBr absorption route with alternate refrigeration
cycles, we now have a reasonable approximation of the heat input and coolant requirements of the simple LiBr case. Of interest is
the Coefficient-of-Performance (Chapter 18, "Energy Efficiency") obtained in this example:
COP = Q,/Qb = 10/13.93 = 0.7174
Optimization of this design will entail reexamining the concentrations chosen for the lean and rich LiBr solutions and those key
temperatures affected by the terminal approaches allowed in the various heat-transfer devices.
We again have most of the advantages of the steam-jet refrigeration cycle. The moving parts are limited to the control system plus
the solution pumps. Spare pumps may be provided when necessary, to ensure continuity of operation.
As in steam-jet refrigeration, ice formation may occur in abnormal circumstances. There is the further hazard of overconcentration of the LiBr solution during misoperation or upsets. This will precipitate solid LiBr and may result in blockage of the
piping and equipment.
Note the reversal of terminology here. The "weak" and "strong" designations in this case customarily refer to the solute (LiBr)
content of the absorbent solutions, and not to the refrigerant (H2O) content. In ammonia absorption refrigeration, usage dictates
the reverse-i.e., "strong-aqua" refers to the solution rich in refrigerant (NH3) and low in absorbent (H2O) concentration.
EXAMPLE C-2
Comparable AAR Unit
For comparative purposes we will evaluate servicing the refrigeration load
of Example C-1 with an aqua-ammonia absorption refrigeration unit. A first pass at this with a simple AAR flow scheme indicated
a stand-off in energy requirements with the LiBr route (as designed in Example C-l ). It also appeared that the aqua-ammonia
version was penalized to a greater degree by the restricted operating temperature range of Example C-l.
To depict a more representative AAR design, this example will be based on 30 psia (2.04 atm abs) steam heat, a 240° F (115.6° C)
fractionator bottom temperature, and minimum 90° F (32.2° C) condensate temperatures. To "squeeze" the most out of the AAR
technique, we will reclaim some of the heat normally rejected to the coolant by partial condensers on the fractionation and
absorption units. The process flow diagram for this arrangement is shown in Figure C-10.
Commentary
The LoP 0 steam consumption for thi-s aqua-ammonia variant is 15% less than that of the LiBr version (Example C-l). More
factually, this would drop to a 10% gain if the LiBr example were done at the same design temperatures as in Example C-2 (240°
F Generator and 95° F absorbate / condensate). The use of the two-stage LiBr solution evaporation scheme (Figures C-6 and C- 7)
might regain the lead for the LiBr route. A review of this first-trial AAR design indicates that its efficiency can also be increased.
It thus appears that a careful study, with parallel designs and pricing, will be required for choosing between the aqua-ammonia and
LiBr routes in cases such as those of Examples C-l and C-2. The characteristics and costs of the available heating and cooling
mediums will have considerable influence in this choice. The scale of operation will also have a great bearing on the selection-it is
feasible to design a single AAR unit for the entire ~~ load; the nature of the LiBr absorption refrigerator is such that a plurality'
of modules (or package units) may be required for large industrial needs. The AAR route then benefits from the economy of scale
and from avoiding the problem of dividing the load equitably between parallel units.
Nomenclature
A = Ponchon swing-point ordinate, BtulLb (J/g)
F = feed to fractionation, Lb/h (kg/h)
h = liquid-phase enthalpy, BtulLb (J I g)
H = vapor-phase enthalpy, BtulLb (J Ig)
L = solution flow, Lb/h (kg/h)
Q = heat transfer duty, 106 Btu/h (106 kJ /h)
R = refrigerant flow, Lb/h (kg/h)
S = heating steam flow, Lb/h (kg/h)
t = temperature, ° Fahrenheit (0 Celsius)
x = solution concentration, wt% LiBr (wt% NH3)
Subscripts
a -absorber
ac -absorber-condenser
b- boiler ("Generator")
c -condenser
e- evaporator
f- feed to boiler ("Generator")
ic -intermediate condenser
o -outlet of exchanger
r- refrigerant (or reflux)
w- water
x- solution exchanger