Objectives Finished Cooling Towers and Adiabatic Humidifiers Cooling

Objectives • Finished Cooling Towers and Adiabatic Humidifiers • Cooling Cycles – Refrigerants

Air Washer • Sprays liquid water into air stream • Typically, air leaves system at lower temperature and higher humidity than it enters

Schematic

Air Washers/Evaporative Coolers • Heat and mass transfer is mutually compensating • Can evaluate based on temperature drop, humidification, or comparison to other energy exchangers

Cooling Tower • Similar to an evaporative cooler, but the purpose is often to cool water – Widely used for heat rejection in HVAC systems – Also used to reject industrial process heat

Cooling Tower

Solution • Can get from Stevens diagram (page 272) • Can also be used to determine – Minimum water temperature – Volume of tower required • Can be evaluated as a heat exchanger by conducting NTU analysis

Real World Concerns • We need to know mass transfer coefficients – They are not typically known for a specific direct-contact device – Vary widely depending on packing material, tower design, mass flow rates of water and air, etc. – In reality, experiments are typically done for a particular application – Some correlations are in Section 10. 5 in your book • Use with caution

Summary • Heat rejection is often accomplished with devices that have direct contact between air and water – Evaporative cooling • Can construct analysis of these devices – Requires parameters which need to be measured for a specific system

Vapor Compression Cycle Expansion Valve

Efficiency • First Law – Coefficient of performance, COP – COP = useful refrigerating effect/net energy supplied – COP = qr/wnet • Second law – Refrigerating efficiency, ηR – ηR = COP/COPrev – Comparison to ideal reversible cycle

Carnot Cycle No cycle can have a higher COP • All reversible cycles operating at the same temperatures (T 0, TR) will have the same COP • For constant temp processes • dq = Tds • COP = TR/(T 0 – TR)

Real Cycles • Assume no heat transfer or potential or kinetic energy transfer in expansion valve • COP = (h 3 -h 2)/(h 4 -h 3) • Compressor displacement = mv 3

Example • • R-22 condensing temp of 30 °C (86 F) and evaporating temp of 0°C (32 F) Determine a) qcarnot wcarnot b) Diminished q. R and excess w for real cycle caused by throttling and superheat horn c) ηR

Comparison Between Single-Stage and Carnot Cycles • Figure 3. 6

Subcooling and Superheating • Refrigerant may be subcooled in condenser or in liquid line – Temperature goes below saturation temperature • Refrigerant may be superheated in evaporator or in vapor (suction) line – Temperature goes above saturation temperature

Two stage systems

Multistage Compression Cycles • Combine multiple cycles to improve efficiency – Prevents excessive compressor discharge temperature – Allows low evaporating temperatures (cryogenics)

What are desirable properties of refrigerants? • • • Pressure and boiling point Critical temperature Latent heat of vaporization Heat transfer properties Viscosity Stability

In Adition…. • • Toxicity Flammability Ozone-depletion Greenhouse potential Cost Leak detection Oil solubility Water solubility

Refrigerants • What does R-12 mean? • ASHRAE classifications • From right to left ← – – # fluorine atoms # hydrogen atoms +1 # C atoms – 1 (omit if zero) # C=C double bonds (omit if zero) • B at end means bromine instead of chlorine • a or b at end means different isomer (b is generally less symmetric)

Refrigerant Conventions • Mixtures show mass fractions • Zeotropic mixtures – Change composition/saturation temperature as they change phase at a constant pressure – 400 series (if commercialized) • Azeotropic mixtures – Behaves as a monolithic substance – Composition stays same as phase changes – 500 series (if commercialized)

More Refrigerant Arcana • Organic refrigerants – 600 series • Inorganic refrigerants 700 + molecular weight

Inorganic Refrigerants • Ammonia (R 717) – Boiling point? – Critical temp = 271 °F – Freezing temp = -108 °F – Latent heat of vaporization? • Small compressors and linesets – Excellent heat transfer capabilities – Not particularly flammable • But…

Carbon Dioxide (R 744) • Recent ASHRAE papers – • • Evaluation of carbon dioxide as R-22 substitute for residential air-conditioning Brown, J. Steven (Department of Mechanical Engineering, Catholic University of America); Kim, Yongchan; Domanski, Piotr A. Source: ASHRAE Transactions, v 108 PART 2, 2002, p 954 -963 Abstract: This paper compares the performance of CO 2 and R-22 in residential air-conditioning applications using semi-theoretical vapor compression and transcritical cycle models. The simulated R-22 system had a conventional component configuration, while the CO 2 system also included a liquid-line/suction-line heat exchanger. The CO 2 evaporator and gas cooler were microchannel heat exchangers originally designed for CO 2. The R-22 heat exchangers employed the same microchannel heat exchangers as CO 2 with the difference that we modified the refrigerant passages to obtain reasonable pressure drops. The study covers several heat exchanger sizes. The R-22 system had a significantly better coefficient of performance (COP) than the CO 2 system when equivalent heat exchangers were used in the CO 2 and R-22 systems, which indicates that the better transport properties and compressor isentropic efficiency of CO 2 did not compensate for thermodynamic disadvantage of the transcritical cycle in comfort cooling applications. An entropy generation analysis showed that the CO 2 evaporator operated with fewer irreversibilities than did the R-22 evaporator. However, the CO 2 gas cooler and expansion device generated more entropy than their R 22 counterparts and were mainly responsible for the low COP of the CO 2 system. (33 refs. ) Cheap, non-toxic, non-flammable Critical temp? Huge operating pressures Often no phase change

Water (R 718) • Two main disadvantages? • ASHRAE Handbook of Fundamentals Ch. 20

Water in refrigerant • Water + Halocarbon Refrigerant = (strong) acids or bases – Corrosion • Solubility – Free water freezes on expansion valves • Use a dryer (desiccant) • Keep the system dry during installation/maintenance

Oil • Miscible refrigerants (11, 12, 21, 113) – High enough velocity to limit deposition – Especially in evaporator • Immiscible refrigerants (717, 744, 13, 14) – Use a separator to keep oil contained in compressor • Intermediate (22, 114)

The Moral of the Story • No ideal refrigerants • Always compromising on one or more criteria • Should be able to look up properties and analyze good candidates for refrigeration cycles