EJECTOR REFRIGERATION SYSTEM (ERS)

SUBMITTED BY Ankit Panghal

Introduction Refrigeration and air-conditioning systems are widely used in air-handling and

cooling applications. Improved system performance will reduce energy consumption as well as reduce CO2 emissions.

Ejector air-conditioning or refrigeration system powered by low-grade energy that can be obtained from industrial waste and solar energy.

An ejector based cooling system offers several advantages, such as no moving parts in the ejector, efficient utilization of the waste heat and low cost.

The main part of jet ejector refrigeration system is ejector . In conventional refrigeration system we use compressor that utilize the electrical energy coming from fossil fuels.

Ejector work like a mechanical compressor and can be used to increase the fluid pressure without any moving part.

3

Ejector Refrigeration System

Working of ejector

Schematic diagram of a typical supersonic ejector

a steam ejector consists of four principal parts the primary nozzle , the mixing chamber , the ejector throat and the subsonic diffuser.

High pressure primary fluid(P) expands and accelerates through the primary nozzle , it fans out with super- sonic speed to create a very low pressure region at the nozzle exit plane and hence in the mixing chamber.

The high-velocity primary stream draws and entrains the secondary fluid(S) into the mixing chamber.

The combined streams are assumed to be completely mixed at the end of the mixing chamber and the flow speed is supersonic.

Types of ejector1. Constant Area Mixing Ejector :- Exit of the primary nozzle lies just at the inlet of the constant area section. Mixing of the primary and the entrained streams takes place in the constant

area section at a constant pressure.

2. Constant Pressure Ejector :- Exit of the primary nozzle lies in the suction chamber. Mixing of the primary and the entrained streams takes place in the mixing

at a constant pressure.

Constant pressure ejector is better than the constant area mixing ejector.

Performance Parameters

Entrainment Ratio (ω): Ratio of the secondary mass flow to the primary mass flow. It shows the energy efficiency of a refrigeration cycle.

Pressure Lift Ratio or Compression Ratio: Ratio of the static pressure at the diffuser exit to the static pressure of the secondary flow at the suction port.

Expansion Ratio: ratio of the primary pressure to secondary pressure

High entrainment ratio and the high possible discharge pressure are the desired characteristics of the ejector.

Mode of Operation Double choking or critical mode: Primary and entrained flows both are

choked and entrainment ratio (ω) is constant. Single choking or subcritical mode: Only the primary flow is choked and

ω changes with back pressure. Back flow or malfunction mode: Both the flows are not choked and the

secondary flow is reversed i.e. primary flow enters into the secondary flow port and ω < 0

Keenan’s Theory Mixing of the primary and secondary fluid takes place at constant pressure at the exit of

the primary nozzle. Both models presumes that the shock waves occur within the mixing section due to the

high pressure region in the downstream. This 1-D constant pressure mixing theory is unable to analyze the choking of the

entrained flow at critical mode operation In practice two choking phenomenon exist in the ejector performance: one in the

primary flow through the nozzle and second in the entrained flow

Munday & Bagster Theory Postulated: After exhausting from the primary nozzle, the primary fluid fans out without

mixing with the entrained flow and induces a converging duct for the entrained flow. This duct acts as a subsonic nozzle such that the entrained flow is accelerated to a sonic

velocity at hypothetical throat. After that mixing of the two streams takes place at constant pressure. Mixing of the

primary and the entrained streams occur in the constant area section with a constant pressure.

Huang Model

1 D model to predict the ejector performance at the critical mode operation by Huang et. al.

LITERATURE REVIEW

Keenan in 1950 was the first who made 1-D model to design the ejector. However, this model cannot predict the constant capacity characteristic of the ejector.

Keenan model was later modified by Munday and Bagster with the fact that primary fluid fans out the nozzle without mixing with the secondary flow and induces a converging duct for the secondary flow.

Eames et al. studied a small-scale steam-jet refrigerator and presented a theoretical model that included irreversibilities associated with the primary nozzle, the mixing chamber and the diffuser. This model was based on constant-pressure mixing process, but without considering the choking of the secondary flow.

Yapici and Ersoy derived a local model based on constant-area mixing process. The ejector consisted of a primary nozzle, a mixing chamber in cylindrical structure and a diffuser. Compared with a similar model designed by Sun and Eames under same operating temperatures, Yapici’s model showed better COP.

Rogdakis and Alexis improved the model proposed by Munday and Bagster by using the thermodynamic and transportation properties of real gases. When considering the friction losses, a constant coefficient was assumed to simplify the model.

Selvaraju and Mani developed a model based on Munday and Bagster’s theory for critical performance analysis of the ejector system. This model applied an expression to describe the friction losses in the constant area section.

One dimensional study of the ejector is carried out by Huang et al. considering ideal gas flow of R141b and they determined the aerodynamic throat area by constant pressure mixing ejector theory. An analysis is performed for the prediction of ejector performance at critical-mode operation assuming the constant-pressure mixing inside the constant-area section of the ejector and the entrained flow at choking condition.

A 1-D model avoiding the ideal gas assumption was proposed by Grazzini et al. Heat exchanger irreversibilities were taken into consideration, and real gas behavior was simulated.

One dimensional fanno flow is considered by Kumar et al. to take account the friction in the ejector with the refrigerant R141 b. They included the variation of heat capacity ratio for real gases which was not consider by Huang et al. and enhanced the performance of ejector model.

A simulation was done by Khalil et al. to design the ejector for vapor jet refrigeration system. They predicted the performance of system using low temperature energy source working with R134a as a refrigerating fluid and concluded that the temperature range of waste heat sources should be 60-85˚C for optimum operation.

References Keenan H, Neumann EP, Lustwerk F. An investigation of ejector design by analysis and

experiment. J. Appl. Mech.,Trans. ASME 1950;72:299–309. Munday, John T, Bagster DF. A new ejector theory applied to steam jet refrigeration. Ind.

Engng Chem., Process Des. Dev. 1977;16:442–449. Huang BJ, Chang JM, Wang CP, Petrenko VAA. 1-D analysis of ejector performance.

International Journal of Refrigeration 1999;22:354–64. Eames IW, Aphornratana S, Haider H. A theoretical and experimental study of a small-scale

steam jet refrigerator. International Journal of Refrigeration 1995;18:378–86. Zhu Y, Cai W, Wen C, Li Y. Simplified ejector model for control and optimization. Energy

Conversion and Management 2008;49:1424–32. Khalil A, Fatouh M, Elgendy E. Ejector design and theoretical study of R134a ejector

refrigeration cycle. International Journal of Refrigeration 2011;34: 1684–98. Rogdakis ED, Alexis GK. Design and parametric investigation of an ejector in an air-

conditioning system. Applied Thermal Engineering 2000;20: 213–26. Grazzini G, Milazzo A, Paganini D. Design of an ejector cycle refrigeration system. Energy

Conversion and Management 2012;54:38–46. Selvaraju A, Mani A. Analysis of an ejector with environment friendly refrigerants. Applied

Thermal Engineering 2004;24:827–38.