Sizing of Steam Injection Heat Pump
Project and Objectives
L’analisi The project involved the modeling of an air/water heat pump using R-32 with steam injection technology, to verify its performance under steady-state conditions and its dynamic response to typical disturbances the system may encounter.
With the air and water inlet conditions to the evaporator and condenser respectively set, and the control system of the machine implemented, the client requested the calculation of the optimal charge to ensure a certain level of subcooling of the refrigerant at the condenser outlet once steady-state operation was reached.
1. All system components were modeled using a 0D approach, employing Simcenter Amesim software from Siemens Digital Industries Software, This resulted in a lumped parameter system that is solved in a time-varying regime.
The heat exchangers were modeled in detail: for the finned coil, the tubing circuit through which the refrigerant flows was accurately reproduced, as well as the air flow sequence through the various ranks. For the plate heat exchanger, the parallel channels were modeled as a single equivalent channel, which was then discretized along its length. The heat transfer coefficients and pressure losses were calculated using correlations found in the literature for similar applications.
Compressors and valves within the system were modeled as stationary components since their dynamic response is much faster than that of the heat exchangers, using curves provided by the manufacturers.
2. The models of the individual components were then interconnected to create the complete system model, following the plant layout provided by the client.
To provide the most accurate estimate of the refrigerant charge within the system, the connecting pipes between the different devices were also modeled based on the length and internal diameter provided by the client. The model requires the state of the fluids (flow rate, temperature, pressure, etc.) at the evaporator (source) and the condenser (sink) as INPUT..
The control system implemented in the model will regulate the compressor speed and the valve opening degree to ensure a specific water outlet temperature from the condenser and a certain level of superheating of the refrigerant at the evaporator outlet and at the intermediate pressure side of the economizer, respectively.
3. After completing the physical model of the heat pump and the control system model, steady-state simulations were run, simulating the typical test these machines undergo in a climatic chamber.
Following the same procedure, the refrigerant charge was gradually increased, and for each steady-state condition, the level of subcooling of the refrigerant at the condenser outlet was observed.
Conclusions
BSIM developed a lumped parameter 0D model of a heat pump, which accurately simulated the machine’s operation under varying boundary conditions, characterizing it in both steady-state and variable regimes.
With the steady-state simulations, it was possible to reproduce the typical tests conducted in a climatic chamber to calculate power, efficiency, working pressures, the necessary refrigerant charge, etc. Meanwhile, with the transient simulations, it was possible to verify the operation of the control system and the response of the plant to a disturbance.
In this way, through a numerical model, BSIM was able to test different system configurations and verify their performance as the components varied.
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