The Pumping Up Phenomenon of Double-Stage Bubble Pump with Water and Aqueous LiBr Solution

The double-stage bubble pump, using thermal energy as driving force to transport the solution, can replace the mechanical solution pump in the double-effect lithium bromide absorption chiller. By building a bench, a lot of experimental research and analysis were conducted with water and different concentrations of lithium bromide solution as the working fluid of the bubble pump. The first-stage bubble pump in the experiment pumps up by the external heat source. The heat for driving the second-stage bubble pump is provided by refrigerant steam produced from the first-stage bubble pump. The experiment data shows that the heating of refrigerant vapor is only one of the elements of pump-up phenomenon. Another is that the intermediate solution flashes to vapor to become bubbles. The pump-up phenomenon of double-stage bubble pump has much to do with the pressure difference of intermediate solution and first-stage refrigerant vapor. With water as the working fluid, when the pressure difference between refrigerant vapor and the intermediate liquefied refrigerant is 3.5-3.9 kPa, the bubble pump can pump up and run for some time and the start-up time decreases with the driving head. When the working fluid is lithium bromide solution, the pressure difference of the double-stage bubble pump increases with the solution concentration and is bigger than that of water. The start-up time increases with the concentrations of lithium bromide solution within the range of 45.5 to 54% and decreases within the range of 54-59.5%. The start-up time is largest at 54% under this experimental condition. The experimental result is also compared with the single-stage bubble pump. The start-up time of double-stage bubble pump decreases with the driving height, which is contrary to the single-stage bubble pump.


INTRODUCTION
The driving force of the pump-free lithium bromide absorption refrigeration device is supplied by the bubble pump, which absorbs the external heat (hot water or steam) to lift the solution.Heat source can be reasonably utilized by using bubble pump in absorption refrigeration, reducing the dependence on high-quality energy (Liu et al., 2003).In recent years, many scholars, at home and abroad, have done a lot of researches about single-stage bubble pump, including the establishment of the mathematical model of the bubble pump operation and the construction of the single-stage bubble pump test setup to do some experimental studies on many factors affecting the bubble pump performance (Saravanan and Maiya, 2003;Pfaff et al., 1998;Deng and Ma, 1999).Peng and Xiao (1989) gave the theoretical formula on bubble pump.Gu and Wu (2006) and Gu et al. (2008) designed and used second generator in the bubble pump refrigeration cycle.But few researches on the double-stage bubble pump have been done, especially the pump-up phenomenon of the smalltype double-stage bubble pump.
The solution in the high-pressure generator is directly heated and concentrated, thus the refrigerant vapor is generated.The solution in the low-pressure generator is heated by the refrigerant vapor from the high-pressure generator to generate bubbles to drive second-stage bubble pump, meanwhile the solution is concentrated again.The experiment shows that the heating of refrigerant vapor is only one of the elements of pump-up phenomenon.Another is that the intermediate solution flashes to vapor and becomes bubbles.In this study the pressure difference between the two stages bubble pump is presented, which will be helpful for further study of the double-stage bubble pump.

EXPERIMENTAL SETUP
The test setup includes high and low pressure generators, first and second stage gas-liquid separators, absorber, condenser, needle valve pressure regulators and the interconnected pipelines.The system is shown in Fig. 1.This experimental setup is designed to observe the performance of the bubble pump in the pump-free lithium bromide absorption refrigeration system.In order to analyze the influence of each parameter to the performance more clearly, two 1 m length see-through glass tubes are used in the first and second stage bubble pump respectively in this experiment.To simplify the experimental setup, the system eliminates the evaporator and other components.The heating bar is fixed in the high-pressure generator and it is electrified through additional power source.The heating input is controlled by changing the heating bar voltage and the heating input ranges from 0 to 4000 W. The driving height is adjusted of the range of 300-450 ~ 450-450 mm (the first and second stage bubble pump, respectively).The lift height is kept at 1250-1400 mm (the first and second stage bubble pump, respectively).The lithium bromide solution concentration ranges from 45.5 to 59.5%.Due to high requirement of air-tightness in the experimental operating, the connection of glass tubes and steel components as well as between steel components needs to be done very well.
Table 1 gives experiment measurement equipment specifications.

The process of bubble pump cycle:
The solution in the high-pressure generator is heated by the heating bar to generate bubbles.The gas-liquid mixture is transported into the first-stage gas-liquid separator under the action of the pressure difference between the absorber and the generator.The refrigerant vapor goes into the low-pressure generator to exchange heat with the solution and then be transported into the condenser.The intermediate solution goes into the low-pressure generator.After pumping up of the second-stage bubble pump, the concentrated solution goes into gas-liquid separator and then into the absorber to be sprayed to absorb the refrigerant vapor from the second-stage gasliquid separator.In order to enhance the absorption effect, the absorber is cooled.Theoretical cycle is shown in Fig. 2.
As shown in Fig. 2, the pressure of the highpressure generator is Pr.The weak solution from the absorber flows into high-pressure generator, the inlet solution is in the sub-cooling state 2 and then reaches the point 5 H and begins to generate bubbles through the heating exchange, so the density of mixture in the bubble pump decreases and the bubble pump begins running, the outlet solution is concentrated to intermediate solution at the point 4 H .The intermediate solution goes through the pressure reducing valve into low-pressure generator and its pressure is

Table 1 :
The experiment measurement equipment specifications