How to Distinguish between Internal and External Balances of an Expansion Valve?

2025-02-11

1. Overview and Differences

Based on the different ways the evaporating pressure is transmitted to the valve diaphragm, the thermostatic expansion valve can be divided into the following two types:

Internal balance.
External balance.

Internal balance - The evaporator pressure is transmitted to the diaphragm inside.
External balance - The evaporator pressure is transmitted to the diaphragm from the evaporator outlet through an external connection.

Note: is 10 K (medium temperature – evaporating temperature). If the pressure drop of the evaporator is high, an expansion valve with external balance should be used.
That is to say, a system with a liquid distributor must use an expansion valve with external balance.

2. Differences in Application Scenarios

Now that we know the differences between internal - balance and external - balance expansion valves, what are the differences in their application scenarios?
Due to the pressure drop in the evaporator, the pressure value collected by the internal - balance expansion valve is larger than that collected by the external - balance expansion valve. Since the expansion valve is just a mechanical component, it doesn't know about this deviation and can't correct it, which will cause the internal - balance expansion valve to make wrong judgments.
Therefore, for scenarios with a large pressure drop in the evaporator, it is recommended to use an external - balance thermostatic expansion valve.

How to Distinguish between Internal and External Balances of an Expansion Valve?1739278343183

Specific application scenarios are as follows:

External balance:
1. Scenarios with a large pressure drop, such as those using plate - type evaporators.
2. Units with a large heat transfer capacity, such as those using screw compressors and shell - and - tube evaporator units.
3. Refrigeration systems with a liquid distributor (the pressure drop of the liquid distributor is relatively large).
Internal balance:
1. Scenarios with a small pressure drop.
2. Scenarios with a small cooling capacity (such as household air - conditioners).
3. Mass - produced refrigeration equipment.

3. Advantages and Disadvantages

Advantages of internal balance:
- There is no need to connect a balance pipe, which simplifies the unit installation process, reduces potential failure points during the unit's operation, and the price is relatively cheap.
Advantages of external balance:
- The external - balance thermostatic expansion valve can more accurately control the superheat at the evaporator outlet, make full use of the evaporator's heat - transfer area, and improve the energy efficiency of the unit.