Expansion pipe selection

The expansion pipe (pipe between the expansion valve and evaporator) works in multiple phases depending on the scenario

Scenario 1: only one case (operating point)

If the user has selected only one case, the expansion pipe selection is conducted in two phases

1. First, expansion pipes are filtered out based on the flow regiment in the pipe. If the flow regiment is in the annular, wavy annular or wavy stratified area, the two phase flow will most experience phase separation. Phase separation will lead to poor liquid distribution in the evaporator and therefore the expansion pipes in these flow regiments are filtered out.

2. Second, out of the remaining expansion pipes, which are in the mist, froth or bubble flow regiments, the expansion pipe that is closest to a 25 m/s flow velocity will be selected.

Scenario 2: multiple cases

If the user has selected multiple cases, the expansion pipe selection is conducted in the following phases:

1. First, if there are full load and part load operating conditions, very small part load (heat pump part load less than 50 %) are disregarded from the selection. This is done do prioritize the full load conditions when selecting the expansion pipe. Typically it is difficult to select an optimal pipe that would work in both 100 % load and eg 25 % part load optimally. Therefore the full load case is prioritized.

2. Second, expansion pipes are filtered out based on the flow regiment in the pipe. If the flow regiment is in the annular, wavy annular or wavy stratified area, the two phase flow will most experience phase separation. Phase separation will lead to poor liquid distribution in the evaporator and therefore the expansion pipes in these flow regiments are filtered out.

3. Third, a group of expansion pipe suitable for all cases is formed. For example, if in one case pipes Cu18, Cu22, Cu28, Cu35 are suitable, and in another case Cu22, Cu28, Cu35, Cu42 are suitable, the expansion pipes that will be used for further consideration are Cu22, Cu28, Cu35.

4. Fourth, out of the remaining suitable expansion pipes, the one with a flow velocity closest to 25 m/s in all the user selected cases is chosen.

A multi case selection flow regiment chart is shown below. Note that even though the Winter min load case is in the annular area, the expansion pipe selection is accepted, since the part load in the case was only 30 %. The full load cases were prioritized.

Scenario 3: condenser inlet colder than evaporator outlet

If the condenser inlet temperature is colder than the evaporator outlet temperature, the refrigerant exiting the expansion valve will most likely be completely liquid instead of partially liquid and gaseous. In these cases the flow velocity target is reduced from 25 m/s to 10 m/s in order to keep the pressure drop in the expansion pipe reasonably low. In these cases the flow regiment is in the liquid area as shown in the example below.