Natural draft installations

Fan power consumption is the major operating cost and can be counterbalanced in part by greater investment in natural draft construction. In the majority of process applications, fen-operated towers are preferred. Very large installations such as those in power plants employ chimney assisted natural draft installations. A limited use of atmospheric towers is made in areas where power costs are especially high. 

Perhaps the most common environmental requirement in modem cooling tower installations is that of noise. The fan equipment and the falling water generate cooling tower noise. In large mechanical- or natural-draft cooling... 

Figure 2. The natural air draft system installed inside the aerobic tank of the constructed wetland.

Cooling towers are classified according to the method by which air is introduced to the tower. The principal types are atmospheric spray, natural-draft, mechanical-draft, deck-filled, spray-filled, coil shed and hyperbolic towers. Most industrial cooling tower installations are field-erected units designed for specific thermal characteristics. 

In an atmospheric spray tower the air movement - is dependent on atmospheric conditions and the aspirating effect of the spray nozzles. Natural-draft cooling tower operation depends on a chimney or stack to induce air movement. Mechanical-draft cboling towers utilize fans to move ambient air through the tower. Deck-filled towers contain tiers of splash bars or decks to assist in the breakup of water drops to increase the total water surface and subsequently the evaporation rate. Spray-filled towers depend only on spray nozzles for water breakup. Coil shed towers are comprised of a combination structure of a cooling tower installed on top of a substructure that contains atmospheric section coils. Hyperbolic natural-draft cooling towers are typically large-capacity systems. 

Applications Economics favor mechanical-draft towers over hyperbolics, except in very large installations. Considering the climatic and load conditions, natural draft has its best application in the power industry. Selected when (a) operating conditions consist of low wet-bulb temperature and high relative humidity (b) a combination of low wet-bulb and high inlet and exit water temperature exists and (c) heavy winter load is possible. 

All that exists for these facilities67 are a few documents and contradictory, partly unbelievable eyewitness statements.68 According to Pressac, the two western, heated, unidentified rooms, as well as their anterooms, served as gas chambers . There is no evidence for any ventilation facility for these rooms prior to May 1944. For this reason Pressac suggests that up to that date ventilation was achieved by means of natural draft. In crematorium IV, but not in crematorium V, a ventilation facility is said to have been installed in May 1944, when the extermination of the Hungarian Jews allegedly began 69 Pressac shows a blueprint for this, but fails to cite a source for it. According to his blueprint, the ventilation shaft would have opened into an additional chimney of crematorium IV. How-... 

Air Preheating. The majority of the gas burners installed in cracking furnaces are natural draft burners. As combustion air is introduced at ambient conditions, a part of the heat released from the fuel has to be used to heat up the combustion air to the fire box temperature. Therefore, preheated air reduces the fired duty. Figure 6 shows the fuel consumption as a function of combustion air temperature. For example, preheating combustion air from 0° to 100 °C reduces the fired duty from 100 to 95.5%, which means savings of 4.5% fuel. 

Combustion air supply for inductive burners is similar to combustion air supply for natural suction burners. However, the draft in a combustion chamber is created by flue gas fan. This method is used whenever there is insufficient natural suction or if a pressure drop disables suction of a sufficient amount of combustion air (e.g., fhrough convective heat exchangers, filters, etc.). Installation of burners with an inductive supply of combustion air as opposed to natural draft burners is less common. 

The above SRU, designed on the basis of modified Claus process, is equipped with a tail gas Incinerator of natural draft, sub-atmospheric pressure type thermal incinerator with air flow controlled using burner air registers. Tail gas inlet line to the incinerator is installed with a Tied Universal type metallic bellow expansion joint to accommodate line expansion and movements. This expansion joint made out of Inconel-65 metallurgy was in operation since Year 2005 which subsequently during May-2011 had developed a crack on the metallic bellow element and started leaking toxic tail gas to atmosphere. 

Atmospheric Cooling Towers. When there is a need for larger coohng ranges and closer approaches, the natural-draft or atmospheric towers might be considered. Fill installed in... 

ISO, 1999. Petroleum and Natural Gas Industries—Offshore Production Installations—Guidelines on Tools and Techniques for Identification and Assessment of Hazardous Events. Draft International Standard ISO/DIS 11116, International Organization for Standardization, Geneva. 

The advantages of forced draft air cooler are 1) tube bundle is easier to be removed for service, 2) fan and motor are easier to be installed and accessible for maintenance, 3) easier for warm air recirculation ftireold climate service, 4) use slightly less horsepower, since air to fan is cooler. Its disadvantages are 1) poorer air distribution over tube bundle, 2) tOip of tube bundle is exposed to ixmbient weather, 3) bottom of tube bundle is hard to reach for cleaning, 4) more chance for hot exit air recirciilation, due to lower exit air velocity and no stack, 5) less cooling in case fans are failed due to tow natural draft. 

---