Learn How EES Absorption Cycle Works

Table of Contents

1. Introduction
2. Understanding YE-ES and EES
3. Using NH3 H2O in EES
4. Simulating an Absorption Cycle
5. Information from "Absorption Chillers and Heat Pumps" by Rademacher and Client
6. Components of a Simple Absorption Cycle
7. Operating Conditions of the Generator and Absorber
8. Function of the Condenser
9. Expansion Valve in the Cycle
10. The Evaporator's Role in the Cycle
11. Pumping System in Absorption Cycle
12. Calculation of State Points in the Cycle
13. Energy Balances in the Cycle
14. COP (Coefficient of Performance) Calculation
15. Modifications for Improved Performance
16. Using the Diagram Window in EES
17. Diagram Window as a Control Panel
18. Executable EES Programs
19. Conclusion

Introduction

In this article, we will explore the use of YE-ES (pronounced EES), an acronym for Engineering Equation Solver, and its application to simulate an absorption cycle using the fluid NH3 H2O. We will discuss the components of a simple absorption cycle, operating conditions of the generator and absorber, and the function of each component in the cycle. Additionally, we will delve into the calculation of state points, energy balances, and the coefficient of performance (COP) for the cycle. Finally, we will explore modifications that can be made to improve the performance of the cycle and highlight the use of the diagram window in EES as a control panel. So, let's dive in and understand the intricacies of simulating an absorption cycle using YE-ES and NH3 H2O.

Understanding YE-ES and EES

YE-ES, also known as Engineering Equation Solver, is a powerful software tool used for solving complex engineering equations. It allows engineers and researchers to model various fluid mixtures, including NH3 H2O. Unlike traditional solvers that require two properties to fix the state, YE-ES requires three properties for fluid mixtures. Additionally, YE-ES uses the indicator X to represent the mass fraction of ammonia, unlike the typical indicator Q used for quality. Furthermore, YE-ES offers a new function, "mass fraction," which provides the mass fraction given any combination of three other properties to fix the state.

Using NH3 H2O in EES

NH3 H2O, a mixture of ammonia and water, is commonly used in absorption cycles. In this simulation, we will focus on a simple absorption cycle in which the generator operates at 90 degrees Celsius and 1.6 mega Pascals. The generator produces 50 grams per second of saturated vapor, nearly pure ammonia, drawn from state 3. This vapor passes through the condenser, expansion valve, evaporator, and finally enters the absorber.

Simulating an Absorption Cycle

To simulate the absorption cycle using NH3 H2O in EES, we need to fix the state points of each component in the cycle. State 3, representing the flow leaving the generator, is fixed by the generator pressure, temperature, and being saturated vapor. The mass fraction and enthalpy at this state provide valuable information. Moving on, state 4, located after the condenser, does not undergo any pressure loss, and the mass balance on ammonia dictates that the mass fraction remains unchanged. The temperature leaving the condenser is fixed by the approach temperature difference and therefore, state 4 is fixed by pressure, mass fraction, and temperature.

Information from "Absorption Chillers and Heat Pumps" by Rademacher and Client

For a deeper understanding of simulating absorption cycles and their various types, we can refer to Harold Rademacher and Client's book, "Absorption Chillers and Heat Pumps." This resource provides detailed information on different types of absorption cycles and their simulation using YE-ES and NH3 H2O. It serves as an invaluable reference for engineers and researchers.

Components of a Simple Absorption Cycle

A simple absorption cycle consists of several components, including a generator, condenser, expansion valve, evaporator, and absorber. Each component plays a crucial role in the cycle's operation. The generator operates at a high temperature and pressure, creating vapor that is further utilized in the refrigeration cycle. The condenser and evaporator perform heat exchange with ambient temperature and the desired space, respectively. The expansion valve regulates the flow of the refrigerant, while the absorber ensures the generation of saturated liquid ammonia to maintain steady-state operation.

Operating Conditions of the Generator and Absorber

The generator, an integral component of the absorption cycle, operates at 90 degrees Celsius and 1.6 mega Pascals. This high-temperature environment facilitates the production of saturated vapor, which is then utilized in the refrigeration cycle. On the other hand, the absorber operates at 32 degrees Celsius and 400 kilo Pascals to maintain the generator and absorber at steady-state operation. This ensures the production of saturated liquid ammonia from the absorber, which is required for the absorption process.

Function of the Condenser

The condenser plays a vital role in the absorption cycle by rejecting heat to the ambient environment. It operates at 25 degrees Celsius and has an approach temperature difference of 4 degrees Celsius. This temperature difference indicates the efficiency of the heat exchange process and ensures the removal of thermal energy from the system. By liquefying the vaporized ammonia, the condenser facilitates its passage through the expansion valve, enabling further cooling in the evaporator.

Expansion Valve in the Cycle

The expansion valve in the absorption cycle serves as a key component for regulating the flow of the refrigerant. Downstream of the valve, the pressure is equal to the absorber pressure. The mass and energy conservation on the valve require that the mass fraction and specific enthalpy remain unchanged as the refrigerant flows from state 4 to state 1. Thus, state 1 is fixed by pressure, mass fraction, and specific enthalpy.

The Evaporator's Role in the Cycle

The evaporator in the absorption cycle removes heat from the desired space, which is maintained at 5 degrees Celsius. This heat removal is essential for achieving the desired refrigeration effect. The evaporator operates with an approach temperature difference of 3 degrees Celsius, ensuring effective heat exchange. The refrigerant, in its liquid form, undergoes evaporation in the evaporator and is subsequently fed to the absorber for the absorption process to continue.

Pumping System in Absorption Cycle

In the absorption cycle, a pumping system is employed to maintain the generator and absorber at steady-state operation. A pump, with an efficiency of 62 percent, is responsible for increasing the pressure of the relatively ammonia-rich saturated liquid from the absorber to the generator. This incompressible fluid experiences an enthalpy increase across the pump, which is calculated considering the pump's efficiency. Furthermore, the pump ensures the removal of saturated liquid with lower ammonia content from the generator, which is then expanded through a valve back into the absorber.

Calculation of State Points in the Cycle

To accurately simulate the absorption cycle using YE-ES, it is crucial to fix the state points of all components in the cycle. The known information includes the flow leaving the generator (state 3), pressure, temperature, and saturation as vapor. These properties fix the state by pressure, temperature, and mass fraction. Similarly, the temperature leaving the condenser (state 4) is fixed by the approach temperature difference, pressure, and mass fraction. State 1 is determined by pressure, mass fraction, and specific enthalpy, while state 2 is fixed by pressure, temperature, and mass fraction. The liquid leaving the absorber (state 5) is saturated liquid and its state is fixed by temperature, pressure, and mass fraction. By accurately calculating these state points, we can better understand the operation of the absorption cycle.

Energy Balances in the Cycle

Energy balances are essential for understanding the heat rejection and refrigeration effect in the absorption cycle. The energy balance on the condenser allows us to determine the heat rejected to the ambient environment. In contrast, the energy balance on the evaporator helps us calculate the refrigeration effect, which is the heat removed from the desired space. By analyzing both balances, we can quantify the thermal performance of the absorption cycle.

COP (Coefficient of Performance) Calculation

The coefficient of performance (COP) is a vital parameter for evaluating the efficiency of an absorption cycle. It is defined as the ratio of the refrigeration effect to the thermal input to the generator. By calculating the COP, we can assess the thermal efficiency of the cycle. In the discussed simulation, the calculated COP is approximately 0.21, indicating the performance level of the absorption cycle under the given conditions. Modifications can be implemented to improve this value further.

Modifications for Improved Performance

To enhance the performance of the absorption cycle, several modifications can be implemented. These may include modifications to the generator and absorber operating conditions, change in fluid properties, or alterations in the overall cycle design. By making these modifications, engineers can achieve higher COP values and improved overall efficiency in the absorption cycle.

Using the Diagram Window in EES

EES offers a diagram window that provides a user-friendly interface for simulating the absorption cycle. By toggling between development and application mode, engineers can manipulate and control the diagram window. This window can display cycle diagrams, state points, temperature, and other relevant variables. It allows users to set values of variables and examine the impact on the cycle using the graphical user interface.

Diagram Window as a Control Panel

In application mode, the diagram window in EES can serve as a control panel for the simulation. Engineers can change values of variables, such as the generator temperature, and use the calculate button to obtain a solution. This provides a more intuitive and interactive way of examining the impact of different parameters on the absorption cycle. By incorporating indicators, controls, and a calculate button in the diagram window, engineers can efficiently control and analyze the performance of the cycle.

Executable EES Programs

EES also supports the creation of executable programs, which further enhance user-friendliness and ease of use. These programs allow engineers to execute specific calculations or simulations without the need for inputting values manually. By creating executable programs, engineers can streamline their work and improve the efficiency of their design and analysis processes.

Conclusion

In conclusion, YE-ES (Engineering Equation Solver) is a powerful tool for simulating absorption cycles using fluid mixtures such as NH3 H2O. By accurately fixing the state points and performing energy balances, engineers can evaluate the performance of the cycle and assess its efficiency. The coefficient of performance (COP) serves as a key parameter for measuring the thermal efficiency of the cycle. Modifying various components and operating conditions can lead to improved performance. The diagram window in EES allows engineers to visualize and control the simulation, enhancing usability and efficiency. With executable EES programs, engineers can further streamline their design and analysis processes. Understanding and utilizing YE-ES and NH3 H2O in simulating absorption cycles is valuable knowledge for engineers and researchers in the field.