The first law of thermodynamics states that energy cannot be created or destroyed in an isolated system; energy can only be transferred or changed from one form to another. According to the first law of Thermodynamics: δQ = δW + ΔU When a process is executed by a system, the change in stored energy of the system is numerically equal to the net heat interaction minus the net work interaction during the process: For example, a room is being heated by an electric heater then the amount of electric energy supplied to the electric heater will be equal to the amount of energy transferred to the room air as heat. Now, if the process is reversed it is well known that transferring some heat to the electric heater will not convert it into the electricity, but according to the first law, the process should occur as long as it is not violating conservation of energy, then it is the second law of thermodynamics which explains that process can occur in a certain direction only, i.e. the energy has some quality, and the low-quality energy can not be converted into the high-quality energy completely. Throttling process: A throttling process occurs when a fluid flowing through a passage suddenly encounters a restriction in the passage. The restriction could be due to the presence of an almost completely closed valve or due to sudden and large reduction in flow area etc. The result of this restriction is a sudden drop in the pressure of the fluid as it is forced to flow through the restriction. This is a highly irreversible process and is used to reduce the pressure and temperature of the refrigerant in a refrigeration system.