M.S.

The gain in popularity of freezing as a means of food preservation has created a need for additional refrigeration on the farm. Even with the wide selection of commercial equipment to choose from, many of the units purchased have proven to be inadequate in storage and freezing capacity. A practical solution to the problem of ample capacity and economy of installation and operation, in many cases is the walk-in type farm refrigerator which may be constructed in part by farm labor using locally available materials. Such a unit designed, constructed and extensively tested in the V.P.I. Agricultural Engineering Department. One of the major items in the initial cost of such a farm refrigerator is the installation of the refrigeration equipment. The nature of this type of installation is conductive to very inefficient use of labor, equipment and facilities. If shops equipped for refrigeration work were used to prefabricate the refrigeration equipment the labor could be used more efficiently because, (a) costly time involved in travel to and from the installation would be reduced to a minimum and (b) the assembly would be made in familiar surroundings with all of the necessary tools, equipment and supplies available. Furthermore, the supplier should be able to finish a more trouble-free installation because the assembly could be made under better working conditions and the equipment could be thoroughly inspected and adjusted before being released from the shop. All these advantages for prefabrication should make it possible for the farm operator to ger a more satisfactory installation at less cost. Also, he would have a unit that would be structurally separate from the refrigerator in which it was installed. In case of emergency such a unit could be removed for repair or replacement or for use in other refrigerators. Manufacturers of refrigeration equipment offer integral units for air conditioning installations, for use with milk coolers and in similar applications which have proven highly satisfactory. These units are designed for operation at an evaporator temperature considerably higher than that required in a walk-in refrigerator. While the gasoline powered units designed for operation on refrigerated transport trucks came closer than any of the other units studied to meeting the requirements for use with the walk in farm refrigerator, they were not considered to be fully satisfactory. In all cases commercial equipment currently available would require considerable modification before it could be used. Since the integral refrigeration units used for other purposes have proven satisfactory it seemed feasible that such a unit could be designed and used on a walk-in type farm refrigerator. / Master of Science

During World War II, with the armed forces waging war on several fronts and with the development of high altitude aircraft, the necessity for a great deal of low temperature testing was brought about. The production testing of various instruments and appliances, such as radios, cameras, clocks, and meters, which would be subjected to low temperatures in arctic climates or at high altitudes, was found necessary. This subject had been given very little consideration prior to the war. When it became necessary to test equipment at a low temperature, it was normally transported to a cold climate and there the tests were conducted. With the increased demand for low temperature testing, the expense and time delay encountered in such movements became prohibitive. During and since the war many low temperature test chambers have been built. The design problems encountered, such as load determination, defrosting, lubrication, refrigerant selection, proper staging, and control selection, have been similar, but the solutions have been different. Moore, Gellalles, and Young state: "Experience has shown that research requirements change so quickly and so radically that the original design specification cannot be much more than a hopeful guess.”(22) Mr. Coy W. Brown states: "Any systems requiring evaporating refrigerant temperatures below -40° F. should be attempted only by engineers experienced in this presently special field."(5) Since definite data on equipment for low temperature test rooms seems to be limited, it is believed that from a design of such a machine much valuable information can be gained. Two low temperature testing rooms have been constructed in the Mechanical Engineering Laboratory at The Virginia Polytechnic Institute. One will operate at temperatures down to 0° F. and the other will operate at temperatures down to -40° F. In addition to being used for low temperature research, it is anticipated that the refrigerating equipment for these rooms will be used by students taking Mechanical Laboratory in routine experiments. The object of this thesis is to design suitable refrigeration equipment for the two rooms. The design will include specifications, bill of material and equipment, detail and assembly drawings, and operating instructions. Available data on the design, control, and operation of low temperature refrigerating equipment will be assembled and correlated. This information, with interpretation thereof, will be used in solving a practical engineering problem which is, at the present, in a special field. / Master of Science

Abstract not available

A two year experiment demonstrated electrical energy savings through evaporator fan cycling in part of a full-sized refrigerated controlled atmosphere apple storage facility in Yakima, Washington. Room cooling demand and evaporator fan operation was controlled by the bulk fruit temperature. Refrigerant temperature was regulated by a computer controlled ramping sequence and a weighted average of three thermistors rather than a room thermostat. Evaporator fans remained off 60-65% of the time during periods of fan cycling operation. Seasonal average fan energy savings approached 50-55% when the product remained in storage for a typical length of time. Room environment conditions, product mass loss, and fruit quality were not compromised. Bulk fruit temperature was exceptionally stable as compared to fruit stored in non-fan cycled rooms controlled by traditional means. Less fan motor heat input in the fruit storage space also results in compressor energy savings. Total electrical energy savings were projected to an 18 room fruit storage facility implementing computer controlled evaporator fan cycling. Projected electrical energy savings achievable through evaporator fan cycling for the state of Washington were made based on reported apple storage capacity. / Graduation date: 1993

Refrigeration systems employing the NH3-H2O absorption cycle provide cooling using a thermal energy input. This cycle relies on the zeotropic nature of the refrigerant - absorbent pair: because of the difference in boiling temperatures between NH3 and H2O, they can be separated through selective boiling in the desorber. Desorbers with counter-current flow of the solution and generated vapor enable efficient heat and mass transfer between the two phases, reducing the absorbent content in the generated vapor. Flow visualization experiments at temperatures, concentrations and pressures representative of operating conditions are necessary to understand the heat and mass transfer processes and flow regime characteristics within the component. In this study, a Flooded Column desorber, which accomplishes desorption of the refrigerant vapor through a combination of falling-film and pool boiling, was fabricated and tested. Refrigerant-rich solution enters the top of the component and fills a column, which is heated by an adjacent heated microchannel array. The vapor generated within the component is removed from the top of the component, while the dilute solution drains from the bottom. Flow visualization experiments showed that the Flooded Column desorber operated most stably in a partially flooded condition, with a pool-boiling region below a falling-film region. It was found that the liquid column level was dependent on operating conditions, and that the pool-boiling region exhibits aggressive mixing between the vapor and solution phases. Heat transfer coefficients were calculated from the data for the pool-boiling region, and were compared with the predictions of several mixture pool-boiling correlations from the literature. The correlations from the literature were in general unable to predict the data from this study adequately. It was found that the Flooded Column desorber yielded higher heat transfer coefficients within the pool-boiling region than those predicted by these correlations. Therefore, modifications to existing mixture boiling correlations are suggested based on the findings of this study. The resulting modified correlation predicts 33 of the 35 data points from this study within ±40%, with an average absolute error of 19%.

No description available.

The purpose of this thesis is to experimentally determine the feasibility of an ejector based, waste heat recovery driven refrigeration system applied to the data center environment in order to reduce operational cooling costs. A comprehensive literature review is detailed to determine the current state of the ejector refrigeration research and assess the initial direction of this thesis. A simplified model was created to perform preliminary performance estimations and system sizing before constructing an experimental system apparatus to evaluate the model predictions. The pressures and temperatures used in the model and instituted in the experimental system are based on the maximum temperatures typically observed in computing servers (50-75°C). Precision controlled heaters are used to simulate the computer server heat, and R245fa is used as the working fluid. Performance results ranged from 0.06 to 0.13.

Pulse tube refrigerators (PTR) are robust, rugged cryocoolers that do not have a moving component at their cold ends. They are often employed for cryogenic cooling of high performance electronics in space applications where reliability is paramount. Miniaturizing these refrigerators has been a subject of intense research interest because of the benefits of minimal size and weight for airborne operation and because miniature coolers would be an enabling technology for other applications. Despite much effort, the extent of possible PTR miniaturization is still uncertain. To partially remedy this, an investigation of the miniaturization of pulse tube refrigerators has been undertaken using several numerical modeling techniques. In support of these models, experiments were performed to determine directional hydrodynamic parameters characteristic of stacked screens of #635 stainless steel and #325 phosphor bronze wire mesh, two fine-mesh porous materials suitable for use in the regenerator and heat exchanger components of miniature PTRs. Complete system level and pulse tube component level CFD models incorporating these parameters were then employed to quantitatively estimate the effects of several phenomena expected to impact the performance of miniature PTRs. These included the presence of preferential flow paths in an annular region near the regenerator wall and increased viscous and thermal boundary layer thicknesses relative to the pulse tube diameter. The effects of tapering or chamfering the junctions between components of dissimilar diameters were also investigated. The results of these models were subsequently applied to produce successively smaller micro-scale PTR models having total volumes as small as 0.141 cc for which sufficient net cooling was predicted to make operation at cryogenic temperatures feasible. The results of this investigation provide design criteria for miniaturized PTRs and establish the feasibility of their operation at frequencies up to 1000 Hz with dimensions roughly an order of magnitude smaller than those that have recently been demonstrated, provided that challenges related to their regenerator fillers and compressors can be addressed.

published_or_final_version / abstract / toc / Mechanical Engineering / Master / Master of Philosophy