Heating and cooling

BADI Year 3

John Errington MSc

Definitions

• Refrigeration– Removing Heat from Cold Region to a Hot Region

• Specific Heat– Amount of heat required to raise the temperature of 1 pound of

substance 1F (water is 1 Btu/lb F)• Sensible Heat

– Heat that is given off or absorbed by a substance without changing its state

• Latent Heat of Fusion– The heat that must be removed from a liquid to change it into a

solid or added to go from solid to liquid without any change in temperature

• Latent Heat of Vaporization– Amount of heat required to change the state of a substance from

liquid to a vapor without a change in temperature

Heating• Combustion

– Coal, Oil, Petrol, Natural Gas (non-renewable)– Wood, Alcohols, Biomass, Hydrogen (renewable)

• Electricity– Resistive heating– Heat pumps

• Solar– Solar collectors for water heating

• Fuel Cells– Produce electricity and heat at efficiencies up to 85% as

compared to about 30% for combustion in power stations and vehicle engines.

• Thermal gain – from windows, lighting, internal equipment and occupants

Cooling & Refrigeration

• Heat transfer e.g. ice pack

• Induced vaporisation of a liquid– Evaporative cooling– Forced vaporisation by reducing pressure

• Peltier effect

• Endothermic chemical reactions

Insulation

• Volumes to be heated or cooled efficiently need to be insulated

• The effectiveness of insulation is measured as a U value, R value, or K value.– A low U or K value indicates good insulation– A high R-value indicates good insulation

K-valueUsed for single materials• K = Thermal conductivity • How well the material will conduct heat• k = W / m deg C

A low k-value indicates the material is a useful insulator Stainless Steel k = 16Glass Wool k = 0.038

U-valueUsed for composite constructions e.g. walls

and roofs

• Thermal transmittance or overall heat transfer coefficient

• U = W / m2 deg C

• A low U-value ( >>1) indicates the structure is well insulated

R-value

• Thermal resistance of a material

• R = m2 deg C / W

• Used to calculate total thermal transmittance of a structure

Total thermal transmittance of multiple insulating layers

• Add together the R-values of all the materials in a construction, plus an allowance for inside and outside air layers.

• The reciprocal of this sum is the U-value for the construction

Insulation: values terms and calculations

Comparison of K values for materials

Examples of cooling by vaporisation

• When you heat a liquid it evaporates more quickly.

• When a liquid evaporates it uses heat.

Keep on heating a kettle of water, and the water will never get hotter than 100C. The heat you apply is used up converting the water from liquid to steam.

Evaporative cooling

When we are hot we sweat. The evaporation of the moisture cools us down.

Q: How can you keep a bottle of wine cool on a picnic?

A: Wrap a thin wet cloth around it, put in a draught in the shade. Evaporation cools the cloth and keeps the wine cool.

• Energy is needed for a change from liquid to vapour

• This energy comes as heat from the liquid, which thus becomes cooler.

• Practical refrigerators use substances like ammonia. This is a gas at room temperature and pressure. Compress it and it turns into a liquid. Let it expand and it turns again to gas

Latent Heat of Vaporization:Amount of heat required to change the state of a substance from liquid to a vapor without a change in temperature

Compressor

Expansion valve

Block diagram of refrigerator / heat pump

Coldgas

Less coldgas

Warm gas

Cool liquid

A thermostat turns the compressor off when the fridge is cold enough, and back on when it warms up

Insulated box

Heat pump cycle

Work function and contact potentialWhen two dissimilar metals are connected together, a voltage called the “contact potential” is produced. It’s not easy to observe, because however you connect the wires, the voltages cancel out.It happens because some metals lose electrons more easily than others. Reactive metals (Lithium, Magnesium, Aluminium, Zinc, Iron) lose electrons easily, while less reactive metals (platinum, rhodium, chromium, copper) do not easily lose electrons. The energy needed to remove an electron from a material is called it’s work function.

V

Copper

Copper

Iron

Seebeck effect - thermocouples

If one junction is heated and the other cooled the contact potentials change. Now a small but useful voltage can be generated.

V

Hot junction

Cold junction

Copper

Copper

Iron

Peltier effect

If the same thermocouple has a current passed through it the reverse effect occurs. One junction heats up and the other cools down.

It’s important to use good conductors because Joule heating (W = V * I) also occurs and this can easily exceed the Peltier effect cooling.

The other junction heats up

One junction cools down

Copper

Copper

Iron

Applications

Thermocouples are frequently used to measure temperatures in the range of -100C to +1200 C.Semiconductor junctions have higher junction potentials and give bigger voltages. They can be used to generate electricity, as thermo-generators.Peltier coolers are used for spot cooling for microscope slides and computer chips.