Nozzle arrangements, heat transfer

Refrigeration units modified for free cooling do not include the hq-uid-refrigerant pump and cooler spray header nozzles. Without the cooler refrigerant agitation for improved heat transfer, this arrangement allows up to about 20 percent of rated capacity. Expected capacities for both tnermocycle and free cooling are indicated in Fig. 12-21. 

The arrangement of baffle plates and nozzles. Figure 10-96C, are important to prevent (a) tube vibration, (b) maldistribution of the process boiling fluid, and (c) poor heat transfer coefficients due to uneven and stratified flow resulting in uneven and dry spot heat transfer from nonuniform tube wetting, and others. ... 

Liquid distribution. The liquid distribution requirement of kettle reboilers is similar to that of horizontal thermosiphon reboilers. The techniques for overcoming liquid maldistribution in horizontal thermosiphon reboilers (Sec. 15.5) also apply to kettle reboilers. The author is familiar with one case in which channeling in a kettle reboiler was evidenced by the shell surface being much warmer in the center (above the inlet) than near the shell ends. The problem was eliminated, and heat transfer largely improved, after a horizontal baffle (which directed liquid toward the sides) was installed above the inlet nozzle.. The baffle arrangement must not restrict inlet liquid flow, as this may back liquid up in the column. In one reported case (237), such liquid backup caused column flooding. 

The heat-transfer coefficient as a function of jet velocity, the pressure drop at the nozzle, and the power requirement are the key parameters in the design of impingement dryers. These parameters are inflnenced by various geometric parameters (e.g., arrangements of nozzles and exhaust... 

Martin [86] provides design correlations for multiple-slot and round jets besides recommendations for the spatial arrangement of jet nozzles on the basis of maximizing heat transfer per unit fan energy. The optimal ratio of the pitch of the nozzles compared with the distance above the surface (0.7), which is recommended by Martin, is close to the reported critical value at which jet-to-jet interactions start influencing the heat transfer at the stagnation point under the jet axis [83]. 

The fields of nozzles can be made from single nozzles, or hole channels, or from perforated plates with aligned or staggered arrangements, permitting a variety of geometric parameters. The heat transfer coefficient of nozzle arrays is therefore considered in more detail in the following. 

In Fig. 1.2a, the view of a nozzle array is shown with the impacted area. The nozzles may be arranged either in-line or staggered (in this figure the nozzles are in-line). In any case, each nozzle is influenced by the square area t of the nozzle pitch, and by using this area the average of the heat transfer can be determined. 

Fig. 1.11 Local heat transfer along lines A and B in (a) an in-line and (b) a staggered nozzle arrangement.

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