Chapter 6 Cooling Production Equipment and Systems

Janis, R. R., & Tao, W.K. Y. (2018). Mechanical and Electrical Systems in Buildings (6th ed.). Pearson Education (US). https://bookshelf.vitalsource.com/books/9780134701608

6.1 How do the vapor compression and the absorption cycle differ in their methods of condensing refrigerant?

Vapor compression condenses refrigerant by mechanically compressing it (raising pressure and temperature) then rejecting heat in a condenser; absorption condenses refrigerant using a heat-driven chemical process (uses heat like steam instead of a mechanical compressor).

6.2 A vapor compression refrigeration machine uses 30 kW of electric power to produce 50 tons of cooling. What is its COP?

COP = Cooling / Work input. 50 tons = 175.85 kW → COP = 175.85 / 30 ≈ 5.86.

6.3 An absorption refrigeration machine uses 30 kW and 1200 lb per hour of steam to produce 100 tons of cooling. What is its COP (excluding electric used for operation of auxiliaries)?

100 tons = 351.7 kW. Steam ≈ 351.7 kW → COP ≈ 351.7 / (351.7 + 30) ≈ 0.92.

6.4 What is the difference between direct evaporative air cooling and indirect evaporative air cooling? What are likely applications for these cooling methods?

Direct evaporative cooling adds moisture to air; indirect cools air via heat exchanger without adding moisture. Direct = hot/dry climates; indirect = when humidity control is needed.

6.5 Describe the basic difference between unitary and split DX systems.

Unitary DX = all components in one unit; split DX = evaporator indoors and condenser outdoors.

6.6 What are the limitations of DX equipment that prevent its application to large systems?

Long piping, leakage risk, difficult control, and safety concerns with large refrigerant volumes.

6.7 What types of compressors are typically installed on water chillers?

Centrifugal, screw, and sometimes reciprocating compressors.

6.8 Which compressors are appropriate for smaller machines? Which are appropriate for large machines?

Small: reciprocating and scroll; large: centrifugal and screw.

6.9 Rank compressors in terms of their typical energy efficiency.

Centrifugal > screw > scroll > reciprocating.

6.10 Under what circumstances would an absorption water chiller be an economical choice with respect to the energy cost of its operation?

When cheap heat (waste heat, steam, gas) is available or when reducing electrical demand charges.

6.11 What is the most widely used type of cooling tower for small- and medium-capacity applications, and why?

Induced-draft cooling towers because they are efficient, reliable, and provide good airflow.

6.12 What advantages are offered by air-cooled condensers in comparison with water-cooled systems using cooling towers?

No water use, no cooling tower, less maintenance, simpler system; but less efficient in hot climates.

6.13 Electricity demand charges are a large portion of the cooling bill for large buildings. Since cooling typically contributes to peak demand, the cost per kWh use for cooling is effectively higher than the average cost per kWh. (True/False)

True.

6.14 What design options are available to reduce cooling demand charges?

Thermal storage, load shifting, efficient equipment, staging chillers, and demand control.

6.15 Outline the progress and rationale that has taken the industry from early refrigerants to refrigerants currently under development.

CFCs → HCFCs → HFCs → low-GWP refrigerants (less environmental impact).

6.16 If ammonia is a high-performance, environmentally friendly refrigerant, why is it not used in most applications?

It is toxic and mildly flammable, creating safety concerns.

6.17 What are the pros and cons of low-pressure versus high-pressure refrigerants?

Low-pressure: safer, efficient, larger equipment; high-pressure: compact but higher leak risk and energy use.

6.18 What would be the advantage of using electric chillers and gas-fired chillers in combination? How would you sequence their operation?

Optimize cost: electric during off-peak, gas during peak; sequence based on demand and energy cost.

6.19 What options are available for using natural gas rather than electricity as an energy source for cooling?

Absorption chillers, gas-engine chillers, desiccant systems.

6.20 What is the most widely used type of cooling tower for large-capacity applications, and why?

Induced-draft cooling towers due to efficiency and scalability.

6.21 How do primary and secondary chilled-water loop arrangements save energy in comparison with unit loop arrangements?

Allow variable flow, reducing pumping energy.

6.22 What applications might favor selecting a modular chiller design?

Phased projects, redundancy needs, limited space, reliability.

6.23 What are the major advantages of ice as a thermal storage medium in comparison with water?

Higher energy density → much smaller storage volume.

6.24 Approximately how much chilled-water storage would be required to reduce chiller load by 250 tons for a period of 6 hours? How much ice storage would be required?

1500 ton-hr; water ≈ ~2.4 million gallons; ice ≈ ~150,000–200,000 gallons equivalent.

6.25 Why might chillers of unequal size be installed in a chilled-water plant?

Better part-load efficiency and redundancy.

6.26 What factors need to be considered in locating a cooling tower?

Airflow, noise, drift, access, structure, and distance from air intakes.

6.27 What factors need to be considered in mechanical plant layout for future maintenance and possible future replacement of components and equipment?

Maintenance access, removal paths, expansion space, safety clearances.

6.28 What nonmechanical support spaces should be provided near a major mechanical plant?

Control rooms, electrical rooms, storage, water treatment, maintenance space.