Chapter 11 Thermo, Nader, Helseth

What is the steady-flow energy balance equation?

(qin-qout) + (win-wout) = he - hi

What is the Vapor-Compression refrigeration cycle?

1. 1-2 Isentropic Compression in a compressor 2. 2-3 Constant-pressure heat rejection in a condenser 3. 3-4 Throttling in a expansion device 4. 4-1 Constant-pressure heat absorption in a evaporator.

A refrigeration space is kept at -15*C by a vapor-compression refrigerator in an ambient of 25*C. The space gains heat steadily at 1 kW and the rate of heat rejection to the ambient is 1.4 kW. What is the COP of the cycle and the second-law efficiency of the system?

2.5 and .39 COPr=Ql/W n=COPr/COPr.rev = 2.5 and .39

If a building heating system should supply hot water at 80*C, what is the lowest possible condenser pressure with water as the working substance?

47.39 Psat at 80*C

R-11, R-12, R-22, R-134A, and R-502 account for what percent of the market?

90 Percent.

What is R-502, and what is it used for?

A blend of R-115 and R-22. It is the dominant refrigerant used in commerical refrigeration systems such as those in supermarkets.

When compared with the Vapor-compression systems ARS have one major advantage:

A liquid is compressed instead of a vapor and as a result the work input is very low and is often neglected in the cycle analysis.

What is the cost difference of an ARS and Vapor-Compression system?

ARS are much more expensive, They are complex and occupy more space. they are much less efficient thus requiring much larger cooling towers to reject the waste heat, and they are more difficult to service since they are less common.

What industry is ARS used in?

ARS are primarily used in Large commercial and Industrial installations.

What are Air-Conditioning systems based on absorption refrigeration called?

Absorption chillers.

What refrigerants/chemical do industrial and heavy-commercial sectors use?

Ammonia.(Toxic)

What does a heat pump do?

Another device that transfers heat from a low-temperature medium to a high temperature.

What is the most common energy source for heatpumps?

Atmospheric air (air-to-air systems)

Why is the reversed Carnot Cycle not a suitable model for refrigeration cycles?

Because processes 2-3 and 4-1 are not practical. Process 2-3 involves the compression of a liquid-vapor mixture, which requires a compressor that will handle two phases, and process 4-1 involves the expansion of high-moisture-content refrigerant in a turbine.

Which of the following statements are true regarding absorption refrigeration systems?

Because the work input for absorption refrigeration systems is very small, they are often classified as heat-driven systems The most widely used absorption refrigeration system is the ammonia-water system. Absorption refrigeration systems involve the absorption of a refrigerant by a transport medium. Absorption refrigeration systems are heavier and more complex than vapor-compression systems.

Why are the Ordinary Vapor-compression refrigeration's systems most used?

Because they are simple, inexpensive, reliable, and practically maintenance-free.

The reversed -------- cycle is also known as the gas refrigeration cycle.

Brayton

What is the COP for Heatpumps

COPHP = QH/Wnet,in

What is the COP for Refrigerators?

COPR=Ql/Wnet,in

Which of the following statements is correct for fixed values of Qh and Ql?

COPhp = COPr +1

When the refrigeration cycles are operated in series, the cycles are called ------ refrigeration cycles.

Cascade

What refrigerants maybe be used in refrigeration systems?

Chlorofluorocarbons (CFCs) Amonia Hydrocarbons(propane, ethane, ethylene, etc)

The performance of refrigerators and heat pumps is expressed in terms of their ------.

Coefficients of performance. (COP)

COPabsorption =

Desired ouput/Required input or Ql/Qgen

COPr(refrigerator)

Desired output/required input or cooling effect/work input or Ql/Wnet in

Two common sources of irreversibility's that cause the differences between ideal and actual vapor-compression refrigeration cycles are ------and ------.

Fluid friction Heat transfer

When are innovative refrigeration systems used?

For moderately and very low temperature applications.

Which of the following statements are true regrading gas refrigeration cycles? Gas refrigeration cycles involve simpler, lighter component's than vapor-based refrigeration cycles. A vapor-based refrigeration cycle is more suitable for aircraft cooling than a gas refrigeration cycle. Gas refrigeration cycles have relatively high COPS Gas refrigeration cycle is also known as the reversed Brayton cycle. Gas refrigeration cycles are suitable for cryogenic applications.

Gas refrigeration cycles involve simpler, lighter component's than vapor-based refrigeration cycles. Gas refrigeration cycle is also known as the reversed Brayton cycle. Gas refrigeration cycles are suitable for cryogenic applications.

Which heat pump system has a higher COP but are more complex and more expensive to install?

Geothermal Pumps

What are some examples of Absorption Refrigeration systems?

Geothermal energy, Solar energy, and waste Heat from cogeneration or process steam plants, and even natural gas when it is relatively low price.

Which of the following statements are true regarding heat pumps?

Heat Pumps and air conditioners have the same mechanical components. Both the capacity and efficiency of a heat pump fall significantly at low temperatures. The major problem with air-source heat pump systems is frosting.

What are ARS often classified as?

Heat-Driven systems.

COPhp(heat pump)

Heating effect/work input or Qh/Wnet in

What areas are heat pumps most competitive?

In areas that have a large cooling load during the cooling season and a relatively small heating load during the heating season. in these areas' the heat pump can meet the entire cooling and heating needs of residential or commercial buildings.

What is the objective of a heat pump?

Is to supply heat to a warm medium.

What is the Actual Vapor-Compression Refrigeration Cycle?

It differs from the ideal one owing mostly the irreversibilities that occur in various componenets, mainly due to fluid friction (which causes pressure drops) and heat transfer to or from the surroundings.

What irreversible process does the Ideal Vapor-compression refrigeration cycle have?

It has a irreversible throttling process to make it more realistic model for the actual systems.

What is the objective of a refrigetator?

It is to remove heat (Ql) from the cold medium

What system could be used to liquefy a gas?

Linde-Hampston system

What process occurs at cryogenic temperatures ( below about 100*C) ?

Liquefaction of Gases.

Heat pumps

Maintain the heated space at a high temperature

Refrigerators

Maintain the refrigerated space at a low temperature.

Absorption Refrigeration systems (ARS) involve the absorption of a refrigerant by a transport -------.

Medium

What do CFC's allow?

More ultraviolet radiation into the earth' s atmosphere by destroying the protective ozone layer and thus contributing to the green house effect that causes global warming.

What is the major concern for industrial application?

Most large industrial applications want efficiency, not simplicity.

What are the differences in the Actual to Ideal Vapor Compression refrigeration Cycle.

Non-Isentropic compression superheated vapor at evaporator exit Subcooled liquid at condenser exit Pressure drops in condenser and evaporator.

When Should ARS be considered to be used?

Only when the unit cost of thermal energy is low and is projected to remain low relative to electricity.

In an actual refrigeration system, the refrigerant is slightly subcooled at the inlet of the throttling valve. Which of the following statements is not true for subcooling design?

Power input of the compressor is reduced.

What is R-11 used in?

R-11 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems and offers stron competition to ammonia.

Which of the following cycles are reversed Carnot cycles? Brayton Cycle Refrigeration Cycle Rankine Cycle. Heat pump Cycle.

Refrigeration Heat Pump

The reversed Brayton Cycle can be used for what?

Refrigeration. A Gas refrigeration Cycle.

Replacing the expansion valve on a Ideal vapor-compression refrigeration cycle by a turbine is not practical because?

Replacing the expansion valve by a turbine is not practical since the added benefits cannot justify the added cost and complexity.

Multipurpose Refrigeration Systems with a single compressor.

Some applications require refrigeration at more than one temperature. A practical and economical approach is to route all the exit streams from the evaporators to a single compressor and let it handle the compression process for the entire system. (refrigerator - freezer combo)

What is the Cascade refrigeration Systems?

Some industrial applications require moderately low temperatures, and the temperature range they involve may be too large for a single vapor-compression refrigeration cycle to be practical.

Which of the following statements are true?

The COPr will improve if the evaporating temperature is raised. In an ideal throttling valve, the inlet and outlet enthalpies are approximately the same. All four components associated with the vapor-compression refrigeration cycle are steady-state devices.

A heat pump is installed in a house for heating in winter. When the heat pump is operating, which component of the heat pump does the house gain heat from?

The Condenser

What is the most widely used ARS?

The ammonia-water system, where ammonia (NH3) serves as the refrigerant and water(H20) as the transport medium.

Which of the following statements are true for an ideal Vapor-Compression refrigeration cycle? The pressure losses in the lines and devices are assumed to be zero. The refrigerant enters the compressor as a saturated vapor. The refrigerant leaves the condenser as a saturated liquid. The refrigerant leaves the condenser as a compressed liquid. The refrigerant enters the compressor as a superheated vapor.

The pressure losses in the lines and devices are assumed to be zero. The refrigerant enters the compressor as a saturated vapor. The refrigerant leaves the condenser as a saturated liquid.

Which of the following statements are true regarding the second-law analysis of vapor-compression refrigeration cycles?

The second-law efficiency is equal to the ratio of actual and maximum COPS for the refrigeration cycle. Exergy destruction in a component can be determined directly from the exergy balance equation. For the condenser, if the temperatures of the high-temperature medium and the environment are the same, then recoverable exergy is zero. COPR is inversely proportional to the temperature difference Th-Tl for both ideal and actual refrigeration cycles.

Liquefaction of gases involve

The separation of oxygen and nitrogen from air, preparation of liquid propellants for rockets, the study of material properties at low temperatures and the study of superconductivity.

Without regeneration, the lowest turbine inlet temperature is T0. what is T0

The temperature of the surroundings or any other cooling medium.

What do refrigerators do?

The transfer of heat from a low-temperature region to a high-temperature.

Despite their relatively low COPS what benefits do the gas refrigeration cycles involve?

They involve simple, lighter components, which make them suitable for aircraft cooling, and they can incorporate regeneration.

What do ground-source (geothermal) heat pumps use as a source?

They use the earth as the energy source.

What is absorption Refrigeration systems?

This is economic when there is a source of inexpensive thermal energy at a temperature of 100*C to 200*c.

Please select all the devices that are components of a vapor-compression refrigeration cycle.

Throttling valve Compressor Evaporator Condenser

In an actual refrigeration system, the refrigerant is slightly superheated at the compressor inlet. Which one of the following statements is the purpose for this light overdesign?

To ensure that the refrigerant is completely vaporized when it enters the compressor.

Cascading improves the COP of a refrigeration system. some systems use three or four stages of cascading. true or false

True

The reversed Carnot cycle is the most efficient refrig. cycle operating between Tl and Th. True or False?

True

The simple vapor-compression refrigeration cycle is the most widely used refrigeration cycle, and its adequate for most refrigeration applications.

True

True or False : A cascade refrigeration system is suitable for applications with large temperature differences between the two heat reservoirs.

True

True or False : The storage and transportation of some gases are done after they are liquefied at very low temperatures. (several innovative cycles are used for the liquefaction of gases. )

True

True or False. The COP of an ARS is usually less than one.

True

True or False: Absorption refrigeration systems are used when there is a source of inexpensive thermal energy at a temperature of 100*C to 200*C

True

True or False: For some applications the simple vapor-compression refrigeration cycle is inadequate and needs to be modified.

True

True or False: Ground-source heat pumps have higher COP values than air-source heat pumps.

True

True or False: Most air-source heat pumps require a supplementary heating system such as a electric resistance heaters or gas furnace.

True

True or False: The cycle that operates in the counterclockwise direction on a T-s diagram of a Carnot cycle is called a reversed Carnot cycle.

True

True or False: When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a flash chamber. Such systems are called multistage compression refrigeration systems.

True

True or false. A heat pump can be used to heat a house in the winter and to cool it in the summer.

True

Both COPs increase as the difference between the two temperatures decreases, that is, as Tl rises or Th falls. True or False

True Because COPr = 1/(th/tl - 1) COPhp = 1/1- Tl/Th

True or False: Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.

True.

Why is the Vapor Compression refrigeration the ideal model for refrigeration systems?

Unlike the reversed Carnot Cycle, the refrigerant is vaporized completely before is is compressed and turbine is replaced with a throtlling device.

Which of the following cant be used as a refrigerant? R502 water R134a Ammonia Vegetable Oil

Vegetable oil

Some other ARS systems include -

Water - Lithium bromide Water - lithium Chloride systems Both of these systems use water as the refrigerant. These systems have limited applications such as A-C where the minimum temperature is above the freezing point of water.

What do water-source heat pump systems use?

Well water

What are Multistage Compression Refrigeration Systems?

When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber ( called a flash chamber) since it has better heat transfer characteristics.

When do Absorption chillers perform best?

When the heat source can supply heat at a high temperature with little temperature drop.

Do the gas refrigeration cycles have a low COP compared to the Vapor-Compression Cycle and the reversed Carnot Cycle?

Yes

The reversed Carnot cycle consumes a fraction of the network. but produces a greater amount of refrigeration.

Yes

With regeneration , the high pressure gas is further cooled to T4 before expanding in the turbine.

Yes

What are the Two important parameters that needed to be considered in the selection of a refrigerant?

are the temperatures of the two media (the refrigerated space and the enviroment) with which the refrigerant exchanges heat.

What happens to the capacity and the efficiency of a heat pump when the temperature is low?

it decreases.

Cophp = Copr + 1

the fixed values of Ql and QH