Refrigerant distribution optimization for microchannel evaporators: Part II

Continuing its research on reducing quality-induced maldistribution in microchannel evaporators, Kaltra introduces the vapor/liquid separation method combining flash gas bypassing (FGB, discussed in the previous article) and vented inlet manifolds.

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In microchannel tubes, vapor bubbles can rapidly grow up to a size comparable to the hydraulic diameter of the tube port, then expand in a longitudinal direction along the port, forming elongated bubbles. This introduces pressure spikes that may lead to overcoming flow inertia and pressure in the inlet manifold, causing reverse flow. Reverse vapor flow, in turn, causes the flow maldistribution among microchannel tubes, as bubbles in each port may extend into different directions.

The FGB approach essentially eliminates quality-induced maldistribution caused by flashing vapor generated during the isenthalpic expansion process. Vapor is bypassed to the compressor suction by a gas separator installed in front of the evaporator inlet manifold, allowing only liquid refrigerant to be fed into the inlet manifold.

However, the inlet manifold is not filled up only with liquid. Liquid refrigerant fills the bottom of the manifold, while gas phase concentrates at the top (vapor pocket). This vapor pocket is formed by vapor bubbles that are gradually discharged out of the microchannel tubes (periodic reverse flow).

To get rid of the reverse vapor flow and minimize its negative impact on the evaporator performance, the method of venting reverse vapor trapped in the inlet manifold back to the upstream flash gas separator can be used. For this purpose, venting valves were installed on both manifold ends; a low-pressure drop massflow meter was installed on the venting line to control the flow resistance and prevent liquid entrainment from the inlet manifold.

Test Setup

Measurements on air conditioning systems with evaporators with flash gas bypass with and without manifold venting were conducted and compared. The refrigerant used for field tests is R134a. The flash gas bypass in the test setup represented a T-junction that was not equipped with separation enhancers. The tube diameters of the T-junction were selected to be close to the inlet manifold’s inner diameter of the evaporator. This separation device was installed downstream of the expansion valve at a distance of 200mm from it, as R134a two-phase flow is always separated at approximately that distance. Under extreme conditions (massflow, flow velocity; for inlet vapor quality of 25%), observed liquid separation efficiency was 98.5%, while vapor separation efficiency was not lower than 70.0%. It is recommended to use a Y-junction to prevent liquid phase splashing, which occurs due to increased inertial forces at high massflow rates.

In both tested setups, evaporators were not equipped with refrigerant distributors.

Experiments were performed with oil-free R134a and R134a with 100% synthetic POE blend. With POE oil, at lower mass flow rates and higher vapor quality conditions, there was a noticeable increase in droplet entrainment in the flow above the liquid layer. As the mass flow rate increases, the flow becomes less separated and contains even more entrained droplets and bubbles. At high flow rates, the droplet entrainment becomes intensified.

Test Results

The surface temperature profile of the evaporator provides indirect information regarding refrigerant distribution inside microchannel tubes, as tubes receiving less liquid refrigerant form larger superheated regions. Fig. 1 depicts evaporator surface temperatures for FGB and FGB with vented manifolds (FGBV). The distribution rating parameter (DRP) is about 6% higher for the system with vented manifolds, confirming the improvement in refrigerant distribution among evaporator tubes.

Figure 1: Surface temperature profiles

The oscillation amplitude of surface temperatures in the FGB cycle is higher than 3°C, while the corresponding oscillation in the FGBV cycle is less than 1°C, indicating stable two-phase flow and heat transfer within the microchannel tubes in the FGBV case  (Fig.2). Fluctuation and amplitudes of pressure within microchannel tubes for the FGBV cycle are much lower because removing vapor reduces the quantity of the backflow vapor in the vapor pocket in the inlet manifold).

Figure 2: Surface temperature oscillation

Performance

System performance (COP) was compared under different inlet air temperatures at the same compressor speed for FGB and FGBV operating cycles (Fig. 3). COP improvement is consistently obtained in all operating conditions for FGBV cycle (up to 15% higher COP over FGB cycle). It should be noted that the evaporating temperature for the FGBV cycle is 2.5°C higher for matching cooling capacities. This increases compressor suction pressure and reduces the compressor work.

Figure 3: System performance

Pressure Drop

Reverse vapor increases the pressure drop in the evaporator due to re-entrainment, but manifold venting allows this problem to be overcome. The reverse vapor massflow rate vented out can be estimated at around 7.5% of the total refrigerant liquid flow rate entering the evaporator. This translates to approximately 15% lower pressure drop across the FGBV evaporator.

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Kaltra, a leader in microchannel heat exchanger technologies, continues its research in collaboration with US and German university teams to improve the technology further and adapt it for modern applications. We anticipate resolving many of today’s challenges associated with environmental impacts, making the refrigeration industry climate-friendly and efficient.