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 (T0, TR) will have the same COP

• For constant temp processes

• dq = Tds

• COP = TR/(T0 – TR)

Real Cycles

• Assume no heat transfer or potential or kinetic energy transfer in expansion valve

• COP = (h3-h2)/(h4-h3)

• Compressor displacement = mv3

Example

• R-22 condensing temp of 30 °C (86F) and evaporating temp of 0°C (32 F)

• Determinea) qcarnot wcarnot

b) Diminished qR and excess w for real cycle caused by throttling and superheat horn

c) ηR

Comparison Between Single-Stage and Carnot Cycles

• Figure 3.6

carnot

carnotR

wAA

qA

21

2

1

1

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 (R717)– 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 (R744)

• 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 954-963Abstract: This paper compares the performance of CO2 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 CO2 system also included a liquid-line/suction-line heat exchanger. The CO2 evaporator and gas cooler were microchannel heat exchangers originally designed for CO2. The R-22 heat exchangers employed the same microchannel heat exchangers as CO2 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 CO2 system when equivalent heat exchangers were used in the CO2 and R-22 systems, which indicates that the better transport properties and compressor isentropic efficiency of CO2 did not compensate for the thermodynamic disadvantage of the transcritical cycle in comfort cooling applications. An entropy generation analysis showed that the CO2 evaporator operated with fewer irreversibilities than did the R-22 evaporator. However, the CO2 gas cooler and expansion device generated more entropy than their R-22 counterparts and were mainly responsible for the low COP of the CO2 system. (33 refs.)

• Cheap, non-toxic, non-flammable• Critical temp?• Huge operating pressures• Often no phase change

Water (R718)

• 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