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A zero ODP replacement for R12 in centrifugal compressor: an experimental study using R134a
No full text
An analytical and experimental analysis of a very fast thermal transient
Full text linkAccording to some international standards, some products, developed for use under heavy thermal conditions, have to be tested by subjecting them for a short time to a particular heating and cooling thermal stress to allow them an acceptable future operative life. It is possible to obtain these fast thermal gradients in
con"ned environments, called climatic chambers where the air is heated by an electrical resistance and is cooled with a "nned evaporator which is linked to a vapour compression system subjected to a particular control system of the refrigerating power. In particular, in this paper the air and object tested thermal transients are studied from an analytical and experimental point of view. The study of the mathematical model is realized assuming simpli"ed hypotheses about the air, the object and the air cooled evaporator temperature. The most complex circumstances are related to a very fast temperature decrease because under this working condition the mathematical model is characterized by a nonlinear di!erential system. The nonlinear term is represented by the refrigerating power that varies in a de"nite range with the evaporator temperature according to a sinusoid trend. For this power a suitable analytical expression, derived by the control system performance and by the compressor characteristic, has been found. The analytical}experimental comparison during a cooling thermal stress of typical products subjected to international standard tests as the electronic boards, has been carried out showing acceptable results. The model presented is useful to foresee the climatic chamber performances in the presence of a speci"c refrigerating power trend; this is the start-point for the design of the vapour compression plant and its control system
An Air Cooled Tube-Fin Evaporator Model For An Expansion Valve Control Law
No full textFor control purposes, a mathematical model of a tube-fin evaporator of a vapour compression plant running with R22 is analyzed. The refrigerant behavior in an evaporating region is described by a homogeneous model. The balance equations, together with the constitutive equations, determine a differential system which makes explicit the mechanism of dynamic behavior. At first, the numerical solution of the steady state is obtained, both in evaporating and superheated regions. Moreover, the numerical analysis allows us to evaluate the transition phase and to locate the interface. Subsequently, various analytical aspects are discussed. For the nonlinear two-phase flow, the dependence of the solution on the boundary data is estimated by means of a qualitative analysis. Then, a linearized model for the single-phase flow is deduced and solved explicitly. The analytical solution is compared with the numerical results and the degree of superheating is estimated in terms of the model parameters
Energy resource management of the Italian army
No full textThis article analyses the several initiatives that the Italian Army has launched over the last years
to ensure more efficient management of energy resources. It has produced a special handbook
to outline the necessary rules for better energy efficiency and has developed an algorithm comparing
the amount of energy theoretically required to this aim to the quantity really consumed
in order to assess the energy data of the Army. Additionally, it has carried out two audits and
a study about the energy requalification of a swimming-pool located in Rome. In this context,
several proposals have been presented. Finally, this article discusses the possibilities for increasing
energy savings
Transcritical CO2 refrigerator and sub-critical R134a refrigerator: a comparison of the experimental results
No full textExperimental analysis of a transfer function for an air cooled evaporator
Full text linkA transfer function to model a direct expansion air cooled evaporator, inserted in a vapor compression refrigeration plant, is deduced by means of experimental analysis. For inlet air temperatures onto the evaporator and refrigerant mass flow rate variable in appropriate ranges, the evaporator dynamic behavior is simulated by a linear model with delay. The results of transfer function are compared with experimental data, obtained by applying both step inputs and periodic changes to the refrigerant mass flow rate. The influence of the hunting, typical of a thermostatic expansion valve, is also estimated experimentally and then validated by the transfer function, obtaining a good agreement. These results could be applied to obtain a control algorithm for the refrigerant mass flow rate feeding the evaporator, by varying the speed of the compressor motor
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