Furnace The furnace known either as a boiler or a heater, and it uses in oil reneries. Fuel gas is burned with air by a combustion device to generate the heat. The heat. The furnace is of cabin type with horizontal tubes designed for a heat duty of 21 !"#$. %fter combustion, the resultant hot &ue gas &ows up through the stack and discharges into the atmosphere. 'ecoverable heat from the &ue gas is further removed in the super heater section before the &ue gas leaves the furnace through the stack. Furnaces can be classied into four general categories, based on e(ciency and design. )atural draft* The draft* The rst category would be natural draft, atmospheric burner furnaces. These furnaces consisted of cast+ iron or riveted+steel heat echangers built within an outer shell of brick, masonry, or steel. The system was simple, had few controls, a single automatic au tomatic gas valve, and no blower. blower. They have been bee n operated with wood, coke, coal, trash, paper, natural gas, and fuel oil. Forced+air* The second category of residential furnace is the Forced+air* The forced+air, atmospheric burner style with a cast+iron or sectional steel heat echanger. echanger. This style furnace fu rnace was used to replace the big, natural draft systems, and was sometimes installed on the eisting gravity duct work. The heated air was moved by blowers which were belt driven and designed for a wide range of speeds. Forced draft* The draft* The third category of furnace is the forced forced draft, mid+e(ciency furnace with a steel heat echanger and multi+ speed blower. blower. These furnaces were physically much more compact than the previous styles. These furnaces have multi+ speed blowers and were designed to work with central air+ air+ conditioning systems. -ondensing* The fourth category of furnace is the high+e(ciency, -ondensing* or condensing furnace. $igh+e(ciency furnaces can achieve from /0 to /0 fuel e(ciency e(cie ncy.. This style of furnace includes a sealed combustion area, combustion draft inducer and a secondary heat echanger. echanger. ecause the heat echanger removes most of the
heat from the ehaust gas, it actually condenses water vapor and other chemicals. urners* The combustion device consists of four &oor mounted burners. ach burner has fuel gas and pilot gas lines, with a block valve on the pilot gas line and an ad3ustable valve on the fuel gas line. ach burner also has an igniter and a secondary air register. 4raft 5ystem* The air intake into the furnace bo is by natural draft mechanism. The tall structure provides the necessary draft at the base of the furnace for air to &ow through the burner assembly. % damper is provided at the furnace stack for controlling the pressure in the furnace bo. The process feed is heated through 2 passes in the furnace. 6t is important to maintain a uniform &ow over the two passes. The &uid &ow rate at each pass is controlled two override controllers, Fuel !as 7ow 8ressure -ontroller 98-:17; and Fuel !as $igh 8ressure -ontroller 98-:1<$;, are provided to protect the furnace from low and high fuel gas pressures respectively. The purpose of these overrides is to ensure that the fuel gas pressure always stays in the stable operating region of the burner &ame. by its individual Flow -ontroller 9F-11= > F-12=;. Furnace %rch 8ressure -ontrol 98-?=;* 6t is important to maintain the furnace pressure to ensure that ade@uate amount of air 9ecess air; is provided for a given fuel &ow rate. )ormally, the opening of the air registers is ed and the air &ow rate is determined by the furnace pressure. %s the furnace pressure increases, the air &ow rate decreases. 5imilarly, as the furnace pressure decreases, the air &ow rate increases. 6nitial -onditions* Two initial conditions are provided. ach of them when loaded starts the simulation at a particular operating condition of the furnace unit. The initial conditions are* a. -old 5tart+Ap -ondition* 6n this initial condition the furnace unit is in a complete shutdown state. The unit is at ambient conditions with no inventory in any vessels. The startup eercise of the furnace unit can be performed with this initial condition.
b. 4esign Bperating -ondition* This is the normal initial condition. 6n this initial condition, the furnace unit is at a steady state with all burners in service. The operating conditions are the same as the design operating conditions. This initial condition can be used to study the eCect of changes in operating conditions on the performance of the unit. %lso this initial condition can be used for studying the eCect of malfunctions and for performing the shutdown eercise of the unit.
