Work to heat

Heat engine

Receives heat from and outputs some useful work, rejects waste heat to sink

Must reject heat

Reservoir

Very large area whose temperature is constant

Kelvin-Planck

It is impossible for any device that operates in a cycle to extract heat from a single reservoir, and do an equivalent amount of work on the surroundings

Clausius

It is impossible to construct a system which will operate in a cycle and transfer heat from a cooler body without working being done by the surroundings

Heat pump

Receives heat and work and then outputs to a hot reservoir

Refrigerant

A chemical compound that is used as a heat carrier which changes from gas to liquid and then back to gas in the refrigeration cycle

Pure substance

A substance that has a fixed chemical composition throughout. Can have a mixture of elements or compounds but the mixture must be homogeneous

Do liquid and gaseous water have the same composition?

Yes

Do liquid and gaseous air have the same composition?

No

Latent heat

The amount of energy absorbed or released during a phase-change process

Latent heat of vaporisation

The amount of energy absorbed/released during a vaporisation or condensation process

Latent heat of fusion

The amount of energy absorbed/released during a melting or freezing process

Saturation temperature

The temperature at which a pure substance changes phase

Saturation pressure

The pressure at which a pure substance changes phase

Critical point

The point at which saturated liquid and saturated vapour states are identical

What table shows saturated water- Temp

A4

What table shows saturated water- Pressure

A5

What table shows ssuperheated water

A6

What table shows compressed liquid water

A7

What table shows saturated refrigerant- Temp

A11

What table shows saturated refrigerant- Pressure

A12

What table shows superheated refrigerant?

A13

Quality

The ratio of the mass of vapour to the total mass of the mixture

Linear interpolation

For properties in between data points on a property table we use linear interpolation

What is so good about the reversed carnot cycle?

Ideal cycle

Most efficient

Good to find upper limits of performance

What happens during the reversed carnot cycle?

1→ 2 - isothermal heat addition

2→3 - Isentropic compression

3→4 - Isothermal heat rejection

4→1 - Isentropic expansion

What must the phase be during the reversed carnot cycle?

Saturated- Isothermal processes require it

What does the area under processes 4→1 in the Vapour Compression Refrigeration cycle on a Ts graph show?

Heat absorbed by refrigerant in evapouration

What does the area under processes 2→3 in the Vapour Compression Refrigeration cycle on a Ts graph show?

Heat rejected by the condenser

What happens in the Vapour- compression Refrigeration cycle?

1→2 - Isentropic compression in a compressor

2→3 - Isobaric heat rejection in a condenser

3→4 - Throttling in an expansion device

4→1 - Isobaric heat absorption in an evaporator

What happens to the phases in the VCR cycle?

Starts as saturated vapour

Superheated vapour

Saturated liquid

Saturated mixture

Saturated vapour

What happens to the SFEE in a heat exchanger?

• Two moving fluid streams exchange heat without mixing

• Mass flow rate of each fluid must remain constant

• No work done

  • Negligible changes in potential energy and kinetic energy

What happens to the SFEE in a compressor?

• Uses work to generate a hot, high pressure fluid

• Negligible heat transfer

• Negligible changes in potential energy

  • Changes in kinetic energy often neglected

What happens to the SFEE in throttling/expansion valves?

• Flow restricting devices that cause a significant pressure drop at constant enthalpy

• Flow assumed to be adiabatic

• No work done

• Can assume negligible change in potential energy and kinetic energy

Why can we apply the SFEE?

FLuid flows through a control volume steadily