21.Why are throttling devices commonly used in refrigeration and air-conditioning
applications?

1. To bring down the pressure of the refrigerant

2. To meet up with the load to be refrigerated (the amount of heat to be evacuated)

Explanation:

1. To bring down the pressure of the refrigerant

The high pressure of the refrigerant coming from the condenser require reduction to enable vaporization in the evaporator at the proper temperature

The throttling valve as a small aperture through which the refrigerant flows that lowers the pressure of the refrigerant to a point  at which the refrigerant vaporize of which the refrigerant then passes into the evaporator in a partly as liquid and vapor at a low temperature and pressure

2. To meet up with the load to be refrigerated (the amount of heat to be evacuated)

The throttling valve allows more refrigerant to flow through it when there is an increased load at a higher temperature to be refrigerated

Similarly, in a condition of reduced refrigeration load, hence, a lesser amount of heat to be evacuated, the throttling valve restricts the amount of flow of the refrigerant through it.