Drop impingement, heat transfer

The reformer tubes typically operate at maximum temperatures of 1,600°F to 1,700°F and are designed for a minimum stress-to-rupture life of 100,000 operating hours. A 35/25 Ni/Cr alloy is used that is modified with niobium and microalloyed with trace elements such as titanium and zirconium. Smaller tube diameters provide better heat transfer and cooler walls. This reduces tube and fuel costs and increases tube life. But more tubes increases the pressure drop. The optimum inside tube diameter is 4 to 5 in. The wall thickness may be as low as 0.25 inch with a length of 40 to 45 ft. The lane spacing between tube rows must be enough to avoid flame impingement from the burners. Typical spacing is 6 to 8 feet. 

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... 

Keywords Atomization Correlations DepositiOTi E>rop Interaction Heat transfer Impinging spray Liquid deposition Multiple drop impacts Secondary thermodynamics Spray Spray-wall impact Statistics... 

In the case of spray impact without heat transfer, the deposited mass fraction and the mass ratio of secondary to impinging drops are related as... 

Jang and Kim [20] investigated experimentally fluid flow and heat transfer characteristics of a microchannel heat sink, again subject to an impinging jet. This type of heat sink retains the high heat transfer coefficient associated with a typical microchannel heat sink and experiences a low pressure drop compared to the microchannel heat sink with parallel flow. 

During the operation of the tower, water is lost by evaporation, water droplets entrained in the outgoing air, and in a water purge, called blowdown. To reduce carry-over of water droplets the air flows across drift eliminators. The water droplets impinge on the drift eliminators and then flows down to the bottom of the tower. The droplet water loss is about 0.2% of the incoming water [11]. After leaving the drift eliminators, air flows up and out of the tower. Evaporation of water into air transfers heat from the water to the air. Cooling the water requires about 1.0 % evaporation for every 5.56 C (10.0 "F) drop in the water tempera-ture[l 1]. To reduce scale formation in the tower because of dissolved calcium or... 

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