THERMO 2 FINALS

Essential Properties of Refrigerants

- Chemical stability under conditions of use is the most important characteristics - Safety codes may require a nonflammable refrigerant of low toxicity for most applications - Cost, availability, efficiency, and compatibility with compressor lubricants and materials with which the equipment is constructed are other concerns

Applications of air conditioning

- Computer rooms - printing - power plants - textiles - photographic - vehicular

4 Main Components in a refrigeration system

- The Compressor - The Condensing Coil - The Metering Device - The Evaporator

cryogenic technologies

- The Joule-Thomson effect - Compression/condensation and expansion of a pure component - Expansion turbines or engines

Refrigerants

- absorb heat at low temperature and low pressure and release heat at a higher temperature and pressure

Primary refrigerants

- are those fluids, which are used directly as working fluids - when used in compression or absorption systems, these fluids provide refrigeration by undergoing a phase change process in the evaporator

secondary refrigerants

- are those liquids, which are used for transporting thermal energy from one location to other - are also known under the name brines or antifreezes

Antifreezes or brines

- are used when refrigeration is required at sub-zero temperatures

Saturation temperature

- can be defined as the temperature of a liquid, vapor, or a solid, where if any heat is added or removed, a change of state takes place

Heat transfer

- can occur when there is a temperature difference between two or more objects

Applications of refrigeration

- cold treatment of metals, medical, construction, ice skating etc. - Chemical and process industries - Comfort air-conditioning - Food processing, preservation and distribution

Linde liquefaction process

- depends solely on throttling expansion - after compression, the gas is precooled to ambient temperature - it may be even further cooled by refrigeration

Claude Liquefaction Process

- gas at an intermediate temperature is extracted from the heat-exchange system and passed through an expander from which it exhausts as a saturated or slightly superheated vapor - the remaining gas is further cooled and throttled through a valve to produce liquefaction as in the Linde process - the unliquefied portion, which is saturated vapor, mixes with the expander exhaust and returns for recycle through the heat-exchanger system

Refrigeration requirements for cryogenic process

- heat ingress from the atmosphere - irreversible thermodynamic losses through heat exchange - the requirement to produce liquid from what is normally a gas

Heat

- is a form of energy that is transferred from one object to another object

Heat

- is a form of energy transferred by a difference in temperature

Cooling Medium

- is a working fluid cooled by the refrigerant to transport the cooling effect between a central plant and remote cooling units and terminals

carnot refrigerator

- is simply a reversed carnot heat engine, with a sequence of four reversible processes

Refrigerant

- is the primary working fluid used for absorbing and transmitting heat in a refrigeration system

Refrigeration

- is the removal of heat from a material or space, so that it's temperature is lower than that of it's surroundings

Liquefaction

- results when a gas is cooled to a temperature in the two-phase region

Heat

- will only flow from a warm object to a colder object

Carnot Refrigeration Processes

1-2: Reversible isothermal expansion 2-3: Reversible adiabatic compression 3-4: Reversible isothermal compression 4-1: Reversible adiabatic expansion

Selection Criteria for Refrigerants

1. Thermodynamic and thermo-physical properties. 2. Environmental and safety properties 3. Economics

Sadi Carnot

A French physicist who proposed an ideal cycle for a heat engine in 1824

ton

A ___ of refrigeration is equivalent to the rate of heat transfer needed to produce 1 ton of ice at 0 C from water at 0 C in 24 hours.

state

A change of _____ transfers a large amount of energy.

1 ton machine

A machine capable of producing a net refrigeration effect of 210 kJ/min or 3.5 kW is called a ____.

Ozone Depletion Potential (ODP)

According to the Montreal protocol, the ____ of refrigerants should be zero, i.e., they should be non-ozone depleting substances.

refrigerant

Air used in an air cycle refrigeration system can also be considered as a ____.

freezing point

An important property of a secondary refrigerant is its ____.

saturated liquid or vapor

As the TXV meters the refrigerant, the high pressure liquid changes to a low pressure, low temperature, ____.

saturation temperature

At ______, materials are sensitive to additions or removal of heat.

discharge pressure

At a given condenser temperature, the ____ should be as small as possible to allow light-weight construction of compressor, condenser, pipe, etc.

saturation pressure

At a given evaporator temperature, the ____ should be above atmospheric for prevention of air or moisture and non-condensable gases ingress into the system and ease of leak detection.

refrigeration cycle

Basically, a _____ is a reversed heat-engine cycle. Heat is transferred from a low temperature level to a higher one, according to the second law, this requires an external source of energy.

cooling media

Chilled water, brine, and glycol are used as ____ in many refrigeration systems.

primary and secondary

Fluids suitable for refrigeration purposes can be classified into ____ refrigerants.

trade-off

For given condenser and evaporator temperatures as the latent heat of vaporization increases, the pressure ratio also increases. Hence a ____ is required between the latent heat of vaporization and pressure ratio.

liquefy

Gases such as nitrogen, oxygen and methane require the use of very low temperatures to ____ and store them at relatively low pressures.

brine

Generally, the freezing point of a ____ will be lower than the freezing point of its constituents.

Sadi Carnot

He proposed that a cycle comprised of completely (internally and externally) reversible processes would give the maximum amount of net work for a given heat input, since the work done by a system in a reversible (ideal) process is always greater than that in an irreversible (real) process.

Reversible isothermal expansion

Heat transfer (+) and boundary work (+) occur in closed system

Reversible isothermal compression

Heat transfer (-) and boundary work (-) occur in closed system

suction pressure

Higher ___ is better as it leads to smaller compressor displacement.

pressure

If the ____ is low, the saturation temperature is low. If ______ is high, saturation temperature is high.

ton

In the refrigeration industry, the unit used is ___.

Compression

In this stage, the refrigerant enters the compressor as a gas under low pressure and having a low temperature. Then, the refrigerant is compressed adiabatically, so the fluid leaves the compressor under high pressure and with a high temperature.

phase changes

Most refrigerants undergo ___ during heat absorption (evaporation) and heat releasing (condensation)

vapor compression

Natural gas plants, petroleum refineries, and petrochemical plants and most of the food and beverage processes are some of the industrial plants that utilize ____ refrigeration systems.

Global Warming Potential (GWP)

Refrigerants should have as low a GWP value as possible to minimize the problem of global warming.

zero

Refrigerants with ____ ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future.

Latent heat of vaporization

Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small

Pressure ratio

Should be as small as possible for high volumetric efficiency and low power consumption

Isentropic index of compression

Should be as small as possible so that the temperature rise during compression will be small

Vapor specific heat

Should be large so that the degree of superheating will be small

Liquid specific heat

Should be small so that degree of sub-cooling will be large leading to smaller amount of flash gas at evaporator inlet

Ozone Depletion Potential (ODP)

Since ___ depends mainly on the presence of chlorine or bromine in the molecules, refrigerants having either chlorine (i.e., CFCs and HCFCs) or bromine cannot be used under the new regulations.

equal

The COP's of all reversible (CARNOT) refrigerators or heat pumps operating between the same high and low temperature reservoirs are always ____, (regardless of type of devices, working fluid, etc.)

throttling valve

The ___ can be either a thin slit or some sort of plug with holes in it. When the refrigerant is forced through the throttle, its pressure is reduced, causing the liquid to expand.

cooling medium

The ___ is often called a secondary refrigerant, because it reduces the extensive circulation of the primary refrigerant

lower

The ___ the temperature at the gas entering the throttle valve, the greater the fraction of gas that is liquefied.

boiling point

The ____ is most important because it is a direct indicator of the temperature level at which a refrigerant can be used.

Thermal Expansion valve (TXV)

The ____ meters the correct amount of liquid refrigerant into the evaporator.

coefficient of performance (COP)

The ____ of a refrigerator (or heat pump) comprised of reversible processes (CARNOT) is always greater than that for a sequence of irreversible (real) processes operating between the same high and low temperature reservoirs (regardless of type of devices, working fluid, etc.).

ideal refrigerator

The ____, like the ideal heat engine, operates on a Carnot cycle, consisting in this case of two isothermal steps.

second carnot principle

The _____ states that the efficiency of a CARNOT heat engine is a function of high and low temperature reservoir temperatures (TH, TL).

brines

The commonly used secondary refrigerants are the solutions of water and ethylene glycol, propylene glycol or calcium chloride. These solutions are known under the general name of ____.

eutectic point

The concentration at which a lowest temperature can be reached without solidification is called as ____.

Liquid Line

The condenser changes the high pressure refrigerant from a high temperature vapor to a low temperature, high pressure liquid and leaves through the ____.

Latent heat

The heat required to change a liquid to a gas (or the heat that must be removed from a gas to condense it to a liquid), without any change in temperature.

greatest

The heat transfer is ____, when there is a large temperature difference between two objects.

Discharge Line

The high pressure refrigerant flows to the condenser by way of the ____.

Thermal Expansion valve (TXV)

The high pressure refrigerant then flows through a filter dryer to the ____.

Condensation

The high pressure, high temperature gas releases heat energy and condenses inside the "condenser" portion of the system. The condenser is in contact with the hot reservoir of the refrigeration system. (The gas releases heat into the hot reservoir because of the external work added to the gas.) The refrigerant leaves as a high pressure liquid.

steam engine

The idea of an ideal cycle came about because engineers were trying to develop a _____ (a type of heat engine) where they could reject (waste) a minimal amount of heat.

Throttling

The liquid refrigerant is pushed through a throttling valve, which causes it to expand. As a result, the refrigerant now has low pressure and lower temperature, while still in the liquid phase.

Suction line

The low pressure, dry vapor is then returned to the compressor in the ___. The cycle then starts over.

Evaporation

The low pressure, low temperature refrigerant enters the evaporator, which is in contact with the cold reservoir. Because a low pressure is maintained, the refrigerant is able to boil at a low temperature. So, the liquid absorbs heat from the cold reservoir and evaporates. The refrigerant leaves the evaporator as a low temperature, low pressure gas and is taken into the compressor again, back at the beginning of the cycle.

coefficient of performance

The measure of the effectiveness of a refrigerator is its ____.

refrigeration effect

The rate at which heat is absorbed from space to be cooled is termed as the ____.

superheated

The refrigerant at this point is a high temperature, high pressure, ____ vapor.

increasing

The refrigerants are arranged in ____ order of atmospheric boiling point, from air at -194.3 C to water at 100 C.

ice

The refrigeration effect can be compared with the production of ___.

Vapor Compression Cycle

The simplest explanation of this system is a heat engine working in reverse, technically referred to as reverse Carnot engine. In other words, it is the transfer of heat from a cold reservoir to a hot one.

Vapor compression cycle

The simplest explanation of this system is a heat engine working in reverse, technically referred to as reverse Carnot engine. In other words, it is the transfer of heat from a cold reservoir to a hot one.

concentration

The temperature at which freezing of a brine takes place depends on its ____.

operating temperatures

The thermodynamic efficiency of a refrigeration system depends mainly on its ____.

saturated liquid or vapor

This ____ enters the evaporator and is changed to a low pressure, dry vapor.

Refrigeration

This cycle is based on the physical principle, that a liquid extracts heat from the surrounding area as it expands (boils) into a gas.

evaporator, condenser

Two different pressures exist in the refrigeration cycle. The _____ or low pressure, in the "low side" and the _____, or high pressure, in the "high side". These pressure areas are divided by the other two components. On one end, is the metering device which controls the refrigerant flow, and on the other end, is the compressor.

Latent Heat

When refrigerant absorbs the unwanted heat, this raises the refrigerant's temperature ("Saturation Temperature") so that it changes from a liquid to a gas — it evaporates. The system then uses condensation to release the heat and change the refrigerant back into a liquid. This is called ______.

Reversible adiabatic compression

Work input (-), but no heat transfer and Win >>> Wout

Reversible adiabatic expansion

Work output (+), but no heat transfer

Liquefaction

____ may be achieved in several ways: 1. By heat exchange at constant pressure. 2. By an expansion process from which work is obtained. 3. By a throttling process.

Thermal conductivity

_____ in both liquid as well as vapor phase should be high for higher heat transfer coefficients

Water

______ is an example of how saturation property of a material, can transfer a large amount of heat.

Refrigerants

______ use the same principles as ice. For any given pressure, _____ have a saturation temperature.

Low pressure

_______ vapor refrigerant is compressed and discharged out of the compressor.

Viscosity

should be small in both liquid and vapor phases for smaller frictional pressure drops