BOYLES LAW: PV/T= Constant

All heat pumps use the principle of Boyles law – that the temperature and vapour pressure of a substance are directly and proportionately related at a constant volume.

Since Volume (V), is almost certainly constant in a sealed heat pump Pressure (P) and Temperature (T) are the main variables and are hence related proportionally. If the Pressure is increased whilst the Volume stays the same the temperature must rise (absorbing the energy which creates the pressurisation in the form of heat = temperature rise).

Let’s look at some practical examples of this working

Why does a bicycle pump get hot when you pump up a tyre?

It’s very little to do with friction! – try the same experiment with the pump disconnected from the tyre and although you can actually pump faster but it won’t get hot at all (you will though!).

When pumping up a tyre you’re increasing the pressure of the air inside the tyre which is almost a constant volume (tyre volume is much greater in comparison with pump volume which is changing), therefore the temperature must rise, absorbing the energy you put in to cause pressurisation in the form of heat.

This is why air compressors for pumping tyres or compressed air systems get so hot.

Why does a cup of tea taste better at the Dead Sea than at the top of Mount Everest?

Of course it may be more welcome at the top of Mount Everest than at the Dead Sea but the culinary reason is that the water should be very hot to make good tea and the boiling point of water is related to the air pressure around it. Water boils at approx 66°C at top of Mount Everest (low atmospheric pressure at high altitude) but slightly above 100°C at the Dead Sea (because it is below sea level, high atmospheric pressure) thus making a better cup of tea!

This gives further proof that pressure and temperature are related.

If you change pressure of a substance and keep the volume same/similar then the temperature has to change.

If you change pressure sufficiently you can also change state and then much larger quantities of energy can be exchanged. Just think how much energy is required to boil water into vapour/gas compared with just heating it up from 0 to 100°C. Can you imagine waiting whilst a pan of water is boiled dry (don’t try this at home).

Why does an astronaut (or even Felix Baumgartner) need a pressurised space suit?

Without a pressurised suit the astronauts blood would immediately ‘boil’ because of the extremely low atmospheric pressure. This further demonstrates that fluids can be forced to undergo changes of state and changes in temperature by merely altering its pressure.


Heat Pumps use the laws of physics to evaporate (boil) and condense refrigerant at different temperatures by changing the pressure at which these occur. This enables large quantities of heat to be transferred (pumped) from low temperatures to higher temperatures.

One method of applying this is the vapour compression cycle.