Temperature Glide
One important limitation of simplified analysis of absorption cycle performance is that the heat quantities are assumed to be at
fixed temperatures. In most actual applications, there is some tem- perature change (temperature glide) in the various fluids supplying or acquiring heat. It is most easily described by first considering sit-uations wherein temperature glide is not present (i.e., truly isother- mal heat exchanges). Examples are condensation or boiling of pure
components (e.g., supplying heat by condensing steam). Any sensi-
ble heat exchange relies on temperature glide: for example, a circu-lating high-temperature liquid as a heat source; cooling water or air
as a heat rejection medium; or circulating chilled glycol. Even latent heat exchanges can have temperature glide, as when a multicom-
ponent mixture undergoes phase change.
When the temperature glide of one fluid stream is small compared to the cycle lift or drop, that stream can be represented by an average
temperature, and the preceding analysis remains representative.
However, one advantage of absorption cycles is they can maximize benefit from low-temperature, high-glide heat sources. That ability derives from the fact that the desorption process inherently embodies temperature glide, and hence can be tailored to match the heat source glide. Similarly, absorption also embodies glide, which can be made
to match the glide of the heat rejection medium.
Implications of temperature glide have been analyzed for power cycles (Ibrahim and Klein 1998), but not yet for absorption cycles.
.WORKING FLUIDS
Working fluids for absorption cycles fall into four categories, each requiring a different approach to cycle modeling and thermo-
dynamic analysis. Liquid absorbents can be nonvolatile (i.e., vapor phase is always pure refrigerant, neglecting condensables) or vola-
tile (i.e., vapor concentration varies, so cycle and component mod- eling must track both vapor and liquid concentration). Solid
sorbents can be grouped by whether they are physisorbents (also known as adsorbents), for which, as for liquid absorbents, sorbent
temperature depends on both pressure and refrigerant loading (bivariance); or chemisorbents (also known as complex com-
pounds), for which sorbent temperature does not vary with loading, at least over small ranges.
Beyond these distinctions, various other characteristics are either necessary or desirable for suitable liquid absorbent/refrigerant pairs, as follows:
Absence of Solid Phase (Solubility Field). The refrigerant/ absorbent pair should not solidify over the expected range of com-
position and temperature. If a solid forms, it will stop flow and shut down equipment. Controls must prevent operation beyond the
acceptable solubility range.
Relative Volatility. The refrigerant should be much more vola-tile than the absorbent so the two can be separated easily. Otherwise,
cost and heat requirements may be excessive. Many absorbents are effectively nonvolatile.
Affinity. The absorbent should have a strong affinity for the refrigerant under conditions in which absorption takes place. Affin-ity means a negative deviation from Raoult’s law and results in an activity coefficient of less than unity for the refrigerant. Strong
affinity allows less absorbent to be circulated for the same refriger- ation effect, reducing sensible heat losses, and allows a smaller liq-
uid heat exchanger to transfer heat from the absorbent to the pressurized refrigerant/absorption solution. On the other hand, as affinity increases, extra heat is required in the generators to separate
refrigerant from the absorbent, and the COP suffers.Pressure. Operating pressures, established by the refrigerant’s
thermodynamic properties, should be moderate. High pressure requires heavy-walled equipment, and significant electrical power
may be needed to pump fluids from the low-pressure side to the high- pressure side. Vacuum requires large-volume equipment and special means of reducing pressure drop in the refrigerant vapor paths.
Stability. High chemical stability is required because fluids are subjected to severe conditions over many years of service. Instabil-
ity can cause undesirable formation of gases, solids, or corrosive substances. Purity of all components charged into the system is crit-
ical for high performance and corrosion prevention.
Corrosion. Most absorption fluids corrode materials used in construction. Therefore, corrosion inhibitors are used.
Safety. Precautions as dictated by code are followed when fluids are toxic, inflammable, or at high pressure. Codes vary according to
country and region.
Transport Properties. Viscosity, surface tension, thermal dif-fusivity, and mass diffusivity are important characteristics of the
refrigerant/absorbent pair. For example, low viscosity promotes heat and mass transfer and reduces pumping power.
Latent Heat. The refrigerant latent heat should be high, so the circulation rate of the refrigerant and absorbent can be minimized.
Environmental Soundness. The two parameters of greatest concern are the global warming potential (GWP) and the ozone depletion potential (ODP). For more information on GWP and ODP, see Chapter 29.
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