Complete the sentences with the correct verbs in brackets

1. The pressure usually (start) ________to decrease.

2. The pressure (start) _________to decrease now.

3. The pressure (start) ________to decrease 10 min. ago.

4. The pressure (start) _____ just _______to decrease.

5. The pressure (start)__________to decrease in some minutes.

The basic refrigeration cycle.

Key words

Final pressure, fraction, expand, refrigerant, partly, required cooling effect, saturated vapour,

superheated, condensation pressure, thermal energy, energy exchange, increase, to be ahead, reach, exchange, remain, isenthalp.

Read the text, find the sentences with the key words and guess the meaning. Use a dictionary to check the answer.

The basic refrigeration cycle.

Part 2

The liquid has reached the final pressure and the frac­tion evaporated can be found from the lines for constant vapour fraction, in the example the refrigerant has ex­panded to 1.63 bar/-30 °C with 33.9 % vaporized.

The partly vaporized refrigerant enters the evapo­rator. Here the remaining liquid refrigerant evaporates, thereby producing the required cooling effect. The refrig­erant at first reaches point C, as 100 % saturated vapour, but usually leaves slightly superheated at D.

The vapour leaves the evaporator at 1.63 bar/-30 °C and superheated to -25 °C.

The vapour is compressed to the condensation pres­sure in the compressor. The compression should if possi­ble be ideal, i.e. mechanical but no thermal energy is ap­plied to the vapour, until the pressure has reached the required level, in the example 15.3 bar/40 °C.

If so, the vapour should move along the isentropical. This has no energy exchange with the surroundings, thus it moves along the isenthalp. Here there is an addition of mechani­cal, but not thermal, energy; thus, the vapour moves along the isentrop. With compression, the temperature in­creases. The increase of the temperature is ahead of the pressure increase, i.e. the refrig­erant not only remains as a vapour but also superheats.