We present a lumped parameter thermodynamic model for the internal processes of a scroll pump. This model serves as a tool with which to quickly predict scroll pump thermodynamic performance without worrying too much about details of scroll pump geometry. The model described here selectively incorporates sources of irreversibility in the scroll compression process and iteratively solves for the resultant thermodynamic states of the compression cycle. Inputs to the model are scroll pump displacement volumes, pump inlet state, pump discharge pressure, and motor operating speed. Descriptions of suction and discharge heat transfer, internal leakage, kissing heat transfer and wall conduction are also incorporated into the model. Outputs of the model are mass flow rate of the machine, pump discharge state, closed process polytropic coefficient and pump wall temperatures. Additional sources of irreversibility can also be incorporated into the model using the methodology outlined in the paper.