Which metering device is often found in portable air conditioning units?

The capillary tube is often found in portable air conditioning units due to its simplicity, low cost, and compact design. This type of metering device consists of a long, thin tube that allows the refrigerant to expand and cool as it flows from the high-pressure side to the low-pressure side of the system. Portable air conditioning units, which are designed to be easily moved and installed without extensive infrastructure, benefit from the capillary tube's straightforward operation and the absence of moving parts, making it a reliable choice for these applications.

Moreover, the capillary tube does not require any external power supply or controls for operation, unlike more complex devices. It effectively maintains the necessary refrigerant flow rate for the size and cooling capacity of portable units, making it an ideal fit for this type of HVAC system.

In contrast, electronic expansion valves and thermostatic expansion valves are typically used in larger, more complex systems where precise control of the refrigerant flow is necessary. Fixed orifices, while used in some applications, do not provide the same level of control as capillary tubes and are less common in portable models. Thus, the capillary tube is the most appropriate choice for portable air conditioning units.