Development of a stepped capillary tube as a low cost throttling device for a residential heat pump system

• Stepped capillary tube is employed as a cheap throttling device in heat pumps.
• Operating principle of the stepped capillary tube is explained.
• Mathematical model of the stepped capillary tube is proposed.
• Sizing charts for fast designing stepped capillary are developed.

An assembly of two capillary tubes and a by-pass check valve is widely used to achieve different mass flow rates in residential heat pump systems. In order to reduce the manufacturing cost, a stepped capillary tube consisting of two serial-connected capillary tubes with different diameters is employed as an alternative. As a result, nearly half of the cost can be saved as no check valve is needed. The operating principles of the stepped capillary tube under different operating modes are explained. To size and rate the stepped capillary tube, a mathematical model is proposed and validated. Sizing charts for easy design are also presented in the paper. A successful case study is included at the end of the paper to demonstrate the design and application process of a stepped capillary tube in a residential heat pump system.