Factors that may allow one to model a control volume as having negligible (zero) heat transfer include (1) the outer surface of the control volume is well insulated, (2) the outer surface area of the control volume is too small to permit effective heat transfer, (3) the temperature difference between the control volume and its surroundings is so small that the heat transfer can be ignored, and (4) the working fluid passes through the control volume so quickly that there is not enough time for significant heat transfer to occur.