Selection and balancing of components

The four main components of a vapour compression refrigeration circuit - the evaporator, the compressor, the condenser and the expansion valve - must be selected to give a balanced system. 

Function as required in conjunction with the other components 

The system designer must consider these components and examine the options which may be available in order to determine a best selection with reference to first cost, installation, operation, running cost, maintenance and expected life. The following factors are some of those affecting the final decision  

If the initial capital cost is the deciding factor, then the plant will almost certainly be more expensive to operate. 

Installation of a new plant may cause serious disruption of the user s ongoing business, and the extent of this disruption should be determined before it is too late. Apart from the installation of the plant itself, there is the associated builders work and the temporary disconnection of other services. The use of factory-built packaged equipment helps to reduce this nuisance. 

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The selection and sizing of auxiliary devices, also called balance of plant (BOP) components, depends on their interactions with the stack and on all other possible inter-connections inside the overall system. 

In electrochemical cells we often find convective transport of reaction components toward (or away from) the electrode surface. In this case the balance equation describing the supply and escape of the components should be written in the general form (1.38). However, this equation needs further explanation. At any current density during current flow, the migration and diffusion fluxes (or field strength and concentration gradients) will spontaneously settle at values such that condition (4.14) is satisfied. The convective flux, on the other hand, depends on the arbitrary values selected for the flow velocity v and for the component concentrations (i.e., is determined by factors independent of the values selected for the current density). Hence, in the balance equation (1.38), it is not the total convective flux that should appear, only the part that corresponds to the true consumption of reactants from the flux or true product release into the flux. This fraction is defined as tfie difference between the fluxes away from and to the electrode ... 

The total balances were selected in lieu of the component water balances because fewer variables were involved in the equations. The energy balances of Table VII form Eqs 6-12, in the same order, in Table XI. Eqs. 13-18 are the enthalpy relationships for the NaOH solution in each effect and the water... 

Hence, water molecules enhance the acidic properties of the zeolite s Bronsted acids. Adsorbate-adsorbent interactions and, therefore, adsorbent selectivity and adsorbate mass transfer rates are altered due to water polarization. When developing an adsorbent to be used in a commercial adsorptive separation process, the water content of the adsorbent is adjusted to balance adsorbent selectivity and component mass transfer rate. 

Figure 4.8 Concentration gradients that form adjacent to the membrane surface for components (a) rejected or (b) enriched by the membrane. The Peclet number, characterizing the balance between convection and diffusion in the boundary layer, is the same JvS/Di = 1. When the component is rejected, the concentration at the membrane surface r, cannot be greater than 2.72 c,., irrespective of the membrane selectivity. When the minor component permeates the membrane, the concentration at the membrane surface can decrease to close to zero, so the concentration polarization modulus becomes very small...

Interaction is unavoidable between the material and energy balances in a distillation column. The severity of this interaction is a function of feed composition, product specification, and the pairing of the selected manipulated and controlled variables. It has been found that the composition controller for the component with the shorter residence time should adjust vapor flow, and the composition controller for the component with the longer residence time should adjust the liquid-to-vapor ratio, because severe interaction is likely to occur when the composition controllers of both products are configured to manipulate the energy balance of the column and thereby "fight" each other. 

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