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Multi-stage sampling (also known as multi-stage cluster sampling) is a more complex form of cluster sampling which contains two or more stages in sample selection. In simple terms, in multi-stage sampling large clusters of population are divided into smaller clusters in several stages in order to make primary data collection more manageable. It has to be acknowledged that multi-stage sampling is not as effective as true random sampling; however, it addresses certain disadvantages associated with true random sampling such as being overly expensive and time-consuming.
An innovative method for improving system efficiency by overfeeding evaporators arranged in parallel inR744 (CO2) refrigeration systems has been developed.It is well known that superheat control of evaporators introduces a significant energy loss, as the maximumevaporation temperature is limited by the temperature approach between air inlet temperature andrefrigerant temperature at evaporator outlet (Fig. 1). Moreover the overall heat transfer capability isworsened, in comparison with the same overfed evaporator, by the fact that part of the evaporatorcontains a single-phase vapour flow .ASSESSMENT OF NEW DESIGN - EXPERIMENTAL ACTIVITYA test rig was built to have a direct comparison betweenevaporator superheat control and overfed control obtainedwith liquid recirculation with “Enjector”. Refrigerant iscompressed by (1) and heat is rejected in the condenser/gascooler (2). High pressure fluid expands through the ejector (4)and energy recovered is used for pumping liquid - if any ispresent - from suction accumulator (7) into the intermediatepressure receiver - IPR - (5). A conventional high pressure valve(3) is used in parallel to Enjector for precise control of optimalhigh pressure. IPR is maintained by valve (8) at the setpressure, sufficient for fluid circulation through the 3evaporators (6A, 6B and 6C). Expansion valves are controlledby a signal representing either the evaporator load –air ON– orthe performance –air OFF– using a simple control algorithm.Suction pressure -SST- is automatically maintained by thecompressors controller at the maximum/optimal value at anyload condition.The control of each evaporator can be switched to a conventional superheat control for comparison.Due to a special design for oil separation evaporators are fed with pure CO2.Some tests, at different load conditions, were carried out.An Infra-Red camera was used to qualitatively assess air inlet side of evaporator 6B, comparing overfed andsuperheat –SH– mode. Temperature of tube bends of the evaporator was measured with thermocouples.Fig 1. SH control Fig 2. Overfed contro
multi stage refrigeration system contain 2 or more compressors while multi stage refrigeration system have more than 1 evaporators
