Central Recirculation System

FIGURE 8.1 Model of a central recirculating system used for calculating the connection between contaminant concentrations, airflow rates, contaminant source strength, q, and air cleaner efficiency, rj. Cj p is the concentration in the supply (outside) air, c is the concentration in the room, c is the concentration in the returned air, (JaMot the total flow rate through the room, ic is the ratio between recirculated airflow rate and total air flow rate, T is the time constant for the room, and V is the room volume. 

In the figure the two flows (supply—exhaust and recirculated) are separated for clarity. Normally they are merged on both the supply and exhaust side. 

The following differential equation (or something similar), derived from a mass balance for the room, is solved to find the correlation between flow rates, source rate, contaminant concentrations, cleaning efficiency, and time. 

It is possible to have a separate recirculating system in addition to the general ventilation system then there is no restriction on the flow rate. This case is the same as a recirculating local exhaust system (see below) 

By assuming the supply air concentration to be zero, since usually there are quite different contaminants in the outside air and from the source, and that the initial concentration also is zero, the time-dependent solution is the following, 

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Typically, when central recirculation is used the contaminant in the supply air is the main source. This is not the case for industrial use, where the main source is in the ventilated room. This usually results in the concentration being somewhat higher when using recirculation than when not using it. Figure 8.1 outlines the ventilation system, the contaminant source, and the cleaning system. 

Local recirculation systems differ from central systems in that all exhausted air is passed back to the room after cleaning and that the flow rate could be larger than the flow rate through the room. 

The room air cleaner consists of a fan and some kind of air cleaner for particles or gases or both, usually mounted together as one unit. This is a local recirculating system and the equation for the contaminant concentration in the room, derived with the same assumptions and in the same way as for central systems, is the following ... 

Zellars-Williams, Inc., "Water Recirculation System Balance of Central Florida Phosphate Mining. Mine I Calculations. 1974-1975 Rainfall Calculations." BuMines Open File Report No. 120-77. Tuscaloosa. AL. 1977. p, IV. 

Zellars-Williams, "Water Recirculation System Balance of Central Florida Phos 4iate Mining, BuMines OFR 120-77, NTIS PB 270 359. U.S. Bureau of Mines. Tuscaloosa, AL. Jan. 1977, 83 pp. 

They found fhat the same swirl generator can provide different velocity patterns as the relative furnace size dj/ di, is changed. At the same time, the size and the shape of the internal or central recirculation zone (CRZ) is primarily dependent on furnace diameter and not vane swirler diameter. Cristea [15,16] in 1984 and 1987, respectively, introduced, relative to a complete system swirler-quarl-furnace, a new modified swirl number in which the characteristic length is the exit diameter of the burner quad. 

Cristea, E. D. "Prediction of the Central Recirculation Zone Size for a Complete Bumer-Quarl-Fumace System." Institute of Aeronautics and Astronautics Journal 25, no. 3 (1987) 457-463. 

The gas disposal assembly conveys the excess gases to a point where they can be discharged safely into the atmosphere. Several methods in use include a nonrecirculating or recirculating ventilation system, a central vacuum system, a dedicated waste gas exhaust system, or a passive duct system. 

The following equations separately outline calculating contaminant concentration inside a room with central and local recirculation. The assumptions for the room are that it has one main ventilation system with supply and exhaust air and that the contaminant concentration is the same in the whole volume (except very close to the contaminant source or in the ducts, etc.). The contaminant source is steady and continuous. The model for local ventilation assumes also one main ventilation system to which is added one local exhaust hood connected to a local ventilation system (see Chapter 10) from which all the air is recirculated. In the central system the number of inlets and outlets could vary. The flow rates are continuous and steady. 

The dialysate solution is recirculated through the hemodyalyzer system. In hospitals where multiple patients are treated, central dialysate supply systems are normally used. The flow rates of blood and dialysate through a hollow-fiber-type... 

A variety of different reactor systems has been described for the use of SCCO2 in batch-wise and continues operation. In principle, all systems consist of a central high pressure autoclave which is connected to various additional components via valves and tubing. Commercial pumps and/or compressors can be used to deliver the CO2 and to add or recirculate fluids. Devices to measure, record, and control pressure and temperature (wall temperature and inside temperature) should be installed to allow at least a minimum of reaction control. Exploratory studies of chemical synthesis involve mostly batch-type procedures, and some typical reactors used in our laboratories are shown in Fig. 2. 

The dialysate solution is recirculated through the hemodialyzer system. In hospitals where multiple patients are treated, central dialysate supply systems are normally used. The flow rates of blood and dialysate through a hollow-fiber-type dialyzer are approximately 200-300 ml min-1 and 500 ml min-1, respectively. The more recently developed hemodialyzers have all been disposable that is, they are presterilized and used only once. Normally, a patient will undergo dialysis for 4—5 h per day, for three days each week. 

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