Principles of Heat Transfer: Gas Furnaces - Research Paper Example

Principles of Heat Transfer: Gas Furnaces A furnace is a machine or a device that produces or acts as a source of heat. In the United States and other cold countries, the device acts as the source of warmth in many homes and houses. Furnaces are also widely used in industries in heat-treating of metals to change their structure and in the manufacture of steel. The furnace as a heating device stretches to centuries back; it was an invention of the Romans, and was used as heat provider. The technology then allowed the use of furnaces in brick and stone homes as a form of under-floor heating. Although the flues of the furnaces were connected to chimneys that enabled exhausting of smoke, the use of furnaces was hazardous due to the possibility of fire and suffocation (Mullinger 90). Furnaces generated heat through combustion of wood during the early days of invention. Wood was replaced by coal as a form of fuel in the 17th century. This was replaced by gas in the 1940’s and finally in the 1970’s electric furnaces replaced the gas furnaces. Wood and coal furnaces were dispensed way with because of their heat inefficiency and constant attention required in the feeding of fuel (Mullinger 100). Today, modern furnaces are manufactured using aluminum, stainless steel, fiberglass and copper. The modern furnaces are designed to avoid heat inefficiencies. The heat exchangers are made out of stainless steel in order to avoid rusting, while the blowers, burners, and stand are made out of aluminized steel. Brass and copper are used for the valves and wiring respectively, while the fiberglass insulates the hood of the furnace. The most valuable part of a furnace is the heat exchanger. This component is responsible for the efficiency of any furnace; therefore, its design and mode of functioning is an integral element when purchasing a furnace. This paper focuses on furnace performance as a function of heat exchanger design, and it tackles the principles that involve the efficiency of different standard furnaces. Furnace heat exchangers As aforementioned, the heat exchanger is a critical component of a furnace. The condition and functioning of a furnace depends on the condition of this critical component. The heat exchanger functions to separate the exhaust gases and flames from the air circulating inside the house. The component makes sure that the hot gas does not directly meet air on the other side of the heat exchanger. A heat exchanger has a life expectancy of 20 years. After this period, the heat exchanger may fail in two ways; it can crack or rust. Either way, the heat exchanger, will lose its efficiency because it will allow the escape of combustion gases into the house air supply. These cracks in the heat exchanger are not easily identified during a home inspection, and it advisable that, after an average lifespan of 20 years, a furnace should be replaced in order to ensure heat efficiency (Dunlop 150). Furnace efficiency In this current economic times, the efficient a furnace is, the lower the power used. A furnace that is efficient will burn less gas and consume a small amount of electricity in running its controls and blowers. The efficiency of a furnace is measured as a percentage that is reflected in the annual fuel-utilization efficiency rating (AFUE). If the percentage rating of furnace in AFUE is high, then it means that the furnace utilizes less gas and its emissions have a lower impact on the environment. Through the years, furnaces have become more efficient because of the ingenuity in design; currently the most efficient model has an AFUE of 98% compared to an AFUE of 64% in the 1970’s (Silberstein 200). Furnace efficiency is a function of heat exchanger design. Design considerations of heat exchangers The design of heat exchangers influences the performance of a furnace. The objective of a heat exchanger is to transfer heat from one media to another at a cheap cost. Heat exchangers are designed to transfer heat through conduction and convection, which makes it, imperative for the following conditions to be met during the design of an efficient heat exchanger: 1. Total amount of heat that should be transferred 2. The total flow of the principal gas 3. The mean temperature difference between the principal and secondary gases 4. The coefficient of heat transfer between the secondary and principal gases 5. The required temperature rise of the principal gas (Kreith 220) These factors listed above are vital considerations that must be considered during the designing of heat exchangers. The gas velocity is not considered as because it is a dependent variable. This is because it depends on heat exchanger face area, total flow and the ration between face area and total area. For an efficient heat exchanger, the system will direct the principal gas from side to side. This directed flow has the effect of ensuring that the transfer coefficient is high along the fixed axes. The pattern of the heat exchanger tubes influences the heat transfer coefficient and the turbulence present in the tubes. The design of the pitch impact on turbulence with triangular pitches exhibiting 30% heat transfer coefficient as compared to a square pitch. Principles of heat exchangers The functioning of heat exchangers is pegged on the fact heat flows from a point of high concentration to a region of low temperature concentration; therefore, if a hot gas is separated from a cold gas by a surface that conducts heat, the heat can be transferred from the hot to the cold gas through conduction. The principal gas is normally cold while the secondary gas is hot and consists of combustion gases. The heated gas is then transferred to warm the house through convection currents. This works because of the fact that warm air is less dense than cold air, and so it rises up while cold air flows down (Kreith 250). The rate of heat transfer (kW/ m2) in a heat exchanger depends on the heat transfer coefficient (U), and the difference in temperature between the principal and the secondary gas. The heat transfer coefficient (U) is a function of the gas velocity, construction materials, gases involved, cleanliness and shape of the heat exchanger (Kreith 178). The principle behind total heat transfer (Q) depends on three factors namely: 1. Heat transfer surface area (A), the larger the area the higher rate of heat exchange 2. Heat transfer coefficient 3. Average temperature difference between the streams, strictly the log mean (DTLM) Thus, the total transfer of heat is determined using the equation Q = UADTLM. (al) Features of different types of gas furnaces Gas furnaces are the most popular types of furnaces that are utilized across many homes in the US. This is because of their efficiency and the cost of gas as the primary fuel. Gas furnaces are distinguished into three levels depending on the features and the AFUE rating. Old or Standard efficiency furnace, which has a, continuous pilot light, a heavy heat exchanger, a natural draft that ensures the flow of exhaust gas. This gas furnace has an AFUE rating of 68%-75%. The mid efficiency gas furnace has more features compared to the standard gas furnace. These features include an exhaust fan, an electronic ignition, a small diameter flue, and it is designed to a light, small size, which reduces losses related to recycling. The AFUE rating of this furnace is 80% -85%. The last type is the high efficiency gas furnace that has an AFUE of 92%-98%. The gas furnace has sealed combustion and has condensing flue gases that are present in a second heat exchanger; this feature makes it extra efficient (Silberstein 300). Flue gas temperature A furnace has flue that acts as the conduit of directing the exhaust gases to the chimney. The flue is responsible for some of the energy losses that are present in a furnace; both sensible and potential energy losses. The flue is considered as the main cause of gas energy loss due to the high temperatures of the gases that are lost. A furnace that has a high level of excess gas incurs sensible heat loss as a percentage of the heat generated while potential energy losses are a result of poor combustion of fuel and products that result from this state. It is vital to maintain a correct combustion temperature in order to avoid flue energy loss. Maintaining a low air-fuel in the burner reduces the possibility of air leakage into the furnace. The combustion temperature in the furnace should be kept at a level that will ensure complete combustion of fuel; 1-2% of oxygen should be maintained to enable full combustion and minimize sensible heat loss in the flue. (Dunlop 180). The flue temperature can be increased through preheating the air supply in the burner or through installing thermal regenerator or flue gas recuperator. Air Filtration Strategies A furnace may act as a source for potentially harmful gases that may result in suffocation, fire or death of humans. Many energy efficient residential furnaces have an efficient ventilation system that allows for circulation of air supply and exhaust of combustion gases. In America, ventilation systems for furnaces are designed to suit the climatic condition of the residential place of the user, a perfect example being the systems that are designed for colder climates do not work well in the south of America. Old furnaces utilized a ventilation system that is outdated and inefficient. This system of ventilation has been replaced with new systems that utilize direct ventilation. This system is efficient because it is not subject to heat loss and is efficient for zone heating. Ventilation systems are fully covered reduce the incidences of heat loss due the covering of the tubes with non-conducting material (Silberstein 470). Conclusion The efficiency of a furnace depends on a number of factors, but the vital one is the design of its heat exchanger. The heat exchanger is a vital component of any furnace or device that is involved in heat transfer. Maintenance and cleaning of the heat exchanger is highly recommended in order to prolong the lifespan of a furnace (Dunlop 220). Works Cited Kreith, F., et al. Principles of Heat Transfer. New York: Cengage Learning., 2010. Print. Dunlop, Carson. Principles of Home Inspection: Chimneys & wood heating. Chicago: Dearborn Real Estate., 2003. Print. Mullinger, Peter. Industrial and process furnaces: principles, design and operation. London: Butterworth-Heinemann., 2008. Print. Silberstein, Eugene. Residential construction academy: HVAC. New York: Cengage Learning., 2004. Print. Read More