CHAPTER 6. HEATING PRODUCTION EQUIPMENT AND SYSTEMS

Transcription

1 CHAPTER 6. HEATING PRODUCTION EQUIPMENT AND SYSTEMS 6.1 Types of Heating Systems 6.2 Heating Energy Sources 6.3 Furnaces and Air Heaters 6.4 Boilers 6.5 District Heating 6.6 Selection of Medium- Steam vs Hot-Water 6.7 Auxiliary Systems 6.8 Sizing Boiler Capacity 6.1 Types of Heating Systems Heating systems can be classified according to energy source and energy transport medium. Energy source: Coal, electric, gas, oil, biomass, district heat energy Energy transport medium: Steam: Low-pressure (below 1 kg/cm 2 ) Medium-pressure (1 to 7 kg/cm 2 ) High-pressure (7 to 25 kg/cm 2 ) Water: Low-temperature (below 120 C) Medium-temperature(120 to 170 C) High-temperature(over 170 C) Air: Gas furnace, oil furnace, electric furnace, in-space heaters Direct radiation : Gas or electric infrared heater Refrigerant: Heat pump systems

2 The basic components of a heating system: Steam systems: Steam boilers, heat transfer equipment(exchangers, coils), combustion air supply and preheating, makeup air supply and preheating, flue gas venting(flue stack or chimney), condensate(return, deaeration), water(makeup, deaeration, treatment, and pumping), fuel(supply, pumping, heating, burners, and storage), control and safety devices, etc. Water systems: Hot-water boilers and circulating pumps. Other components are the same as in the steam system, except that equipment for condensate return and deaeration is not required. Air systems: Furnaces, in-space air heater, ductwork, fuel, combustion air, and flue gas components are similar to those in steam or hot water systems. Electric furnaces do not require flue gas removal or fuel systems. Infrared systems: Heaters(electric, gas). Flue and gas venting for gas heaters. Heat pump systems: Air-to-air, air-to-water, water-to-water and air-to-refrigerant systems, pumps and compressors. Compressor 6.2 Heating Energy Sources Heat for HVAC systems is produced by combustion of fuels or electric resistance. Heat pumps and heat reclaiming from processes are alternative methods of producing heat that can conserve fuels and electricity. Fuels vary in cost and convenience. Generally, hydrocarbon fuels are classified according to physical state (gas, liquid, or solid). Different types of combustion equipment are usually needed to burn fuels in the different physical states. Gaseous fuels can be burned in premix or diffusion burners. Liquid fuel burners must include a means for atomizing or vaporizing fuel and must provide adequate mixing of fuel and air. Solid fuel combustion equipment must (1) heat fuel to vaporize sufficient volatiles to initiate and sustain combustion, (2) provide residence time to complete combustion, and (3) provide space for ash containment.

3 Major fuels for HVAC systems are natural (LNG) and liquefied petroleum gases (LPG), fuel oils, diesel and gas turbine fuels (for on-site energy applications), and coal. Coal and heavy oil are economical only for large-volume users. Electric resistance can be used to produce heat directly in convectors

4 Fuel choice is based on one or more of the following factors: 1) Fuel factors: - Availability, including dependability of supply - Convenience of use and storage - Economy - Cleanliness, including amount of contamination in unburned fuel - affecting (1) usability in fuel-burning equipment and (2) environmental impact 2) Combustion equipment factors - Operating requirements -Cost - Service requirements - Ease of control

5 6.3 Furnaces and Air Heaters Application: small buildings and residences, or small decentralized systems in large buildings Fuel: natural gas, oil, and electricity Furnaces is not practical to transfer heat by air for long distances, owing to the physical space required for ductwork and the cost of electricity to run fans Type of Furnaces Type of fuel or energy: Gas, oil, gas-oil, or electricity Process of combustion: Open combustion chamber or a sealed chamber (impulse) Design and construction: - Design: Cabinet configuration (vertical or horizontal), airflow (up or down flow), air delivery (ducted or free delivery) - Construction: indoor or outdoor, pad-mounted or rooftop-mounted Services: Heating only combined / Heating and cooling combined Combustion Efficiencies Annual Fuel Utilization Efficiency (AFUE): - Open chamber Gas or Oil, AFUE =75 % - 80 % - Sealed chamber (impulse) : Gas only, AFUE = 92 % - 95 %

6 Cutaway section of the sealed combustion chamber of an impulse type of furnace Cutaway view and installations of a typical open-chamber type gas-fired furnace

7 6.4 Boilers Type of Boilers Source of energy: Gas, oil, gas-oil, electric, coal, biomass, etc. Heat transfer surface: - Combustion type: fire tube or water tube boilers - Electric type: resistance or electrode boilers Design of combustion chamber: - high-firebox type - low-firebox (Scotch marine) type Construction: - Cast-iron Sectional or packaged - Steel or copper tubes Fire Tube Boilers The combustion gases travel through the firebox and through the insides of steel tubes, usually in two or more passes, before being exhausted to the stack. The water is contained in the boiler shell, which completely surrounds and submerges the fire tubes. Hot-water boiler: The shell is completely filled with water. Steam boiler: The water level is lowered to allow a space for evaporation. Fire tube boilers are suitable for producing large amount of hot water or steam.

8 Scotch marine-type boiler: - Originally designed for use on board ships. - Low height is an advantage in building spaces that are restricted in height. Typical configuration of a low-firebox Scotch marine-type boiler Maintenance on a fire-tube boiler

9 6.4.3 Water Tube Boilers Water is on the inside of the tubes and the combustion gases pass around the outside of the tubes. Water tube boilers are available in all sizes ranging from small residential units to very large units used for generating electric power. The water tube boiler generally contains less water than a fire-tube boiler of equivalent capacity. This type boiler, therefore, responds quickly to load fluctuations, such as those experienced in a hospital or residence.

10 Typical large high-firebox water tube boiler Cast-Iron Sectional Boilers Cast-iron sectional boilers are units consisting of a series of vertical cast-iron sections filled with water. When the sections are connected, the void inside the U forms the furnace or fire box. These boilers are used primary in residential and small to medium-size commercial buildings. They can be assembled in the field, section by section. Cast-iron boilers are not applicable to high-pressure systems owing to limited strength. They are rated for 1 kg/cm 2 steam and a maximum of 7 kg/cm 2 for water.

11 6.4.5 Electric Boilers Electric boilers are made for either hot water or steam. The most common type is the immersion resistance boiler, with capacities up to 1500 kw of input. Larger boilers may be of the electrode design, which utilizes water as the currentconducting medium to generate steam by electrolysis. Typical resistance-type electric boiler

12 Typical electrode-type steam boiler 6.5 District Heating Many metropolitan areas offer district heating. District heating plants are large boiler facilities producing steam for distribution through pipes under the streets. The plants use large boilers fired by coal, oil, gas, biomass, and waste heat. The pressure of steam is 5 to 10 kg/cm 2 (80 to 150 psig)

13 A district heating plant in Vienna, Austria Typically, condensate is wasted to sewer to avoid the expense of pumps and piping for its return to the central plant. The building s energy usage is determined by metering incoming steam or wasted condensate. Eliminating boilers results in less investment for mechanical systems and space for boilers and their auxiliary systems. Eliminating boilers also eliminate shaft space for flue stack which penetrates all floors. Eliminating boilers also reduces maintenance and eventual replacement costs. Steam heat exchangers using district heat are a compact, low-maintenance solution compared with boilers

14 Typical steam-to-water U-tube heat exchanger 6.6 Selection of Medium Steam vs Hot-Water Hot water is preferred heating medium owing to its simplicity of design and lower maintenance when compared with steam. If the building supported by the boilers requires heating only, the most logical choice is hot water If the building requires steam as well as hot water, it is desirable to produce steam for the specific requirements and use steam-to-water heat exchangers to produce hot water. Hot water systems can operate at different temperatures: Low temperature system: 80 C C. Medium temperature system: 120 C C High temperature system: over 170 C

15 Steam system uses steam as the heating medium with an operating temperature ranging from 105 C C. The steam can easily be transported through the piping system by its own pressure without the need of the pump. Its condensate must be pumped, except where it can be returned by gravity. Pipes for condensate must be properly pitched to facilitate the return of medium and to avoid water hammer and noise. Steam systems can operate at different pressures: Low-pressure system: below 1kg/cm 2 Medium-pressure system: 1 to 7kg/cm 2 High-pressure system: 7 to 25kg/cm 2 Radiators for hot-water and steam heating systems Supply gate valve Supply gate valve Elbow(90 ) Steam trap Hot water radiator Steam radiator

16 Steam trap A steam trap is a device used to discharge condensate and non condensable gases with a negligible consumption or loss of live steam. The three important functions of steam traps are: - To discharge condensate as soon as it is formed. - To minimize steam loss to return pipe. (i.e., to maximize release of latent heat stored in the steam) - To have the capability of discharging air and other non-condensable gases contained in the steam. Types of steam trap (Thermostatic / Mechanical / Thermodynamic) Thermostatic (operated by changes in fluid temperature) The temperature of saturated steam is determined by its pressure. In the steam space, steam gives up its enthalpy of evaporation (heat), producing condensate at steam temperature. As a result of any further heat loss, the temperature of the condensate will fall. A thermostatic trap will pass condensate when this lower temperature is sensed. As steam reaches the trap, the temperature increases and the trap closes.

17 Mechanical (operated by changes in fluid density) This range of steam traps operates by sensing the difference in density between steam and condensate. These steam traps include 'ball float traps' and 'inverted bucket traps'. In the 'ball float trap', the ball rises in the presence of condensate, opening a valve which passes the denser condensate. With the 'inverted bucket trap', the inverted bucket floats when steam reaches the trap and rises to shut the valve. Both are essentially 'mechanical' in their method of operation. Thermodynamic (operated by changes in fluid dynamics dynamic and static pressures) Thermodynamic steam traps rely partly on the formation of flash steam from condensate. The thermodynamic trap is an extremely robust steam trap with a simple mode of operation. The trap operates by means of the dynamic effect of flash steam as it passes through the trap. The only moving part is the disc above the flat face inside the control chamber or cap. Control chamber Disc Inlet Outlet

18 (1) On start-up, incoming pressure raises the disc, and cool condensate plus air is immediately discharged from the inner ring, under the disc, and out through three peripheral outlets (only 2 shown in the figure). (2) Hot condensate flowing through the inlet passage into the chamber under the disc drops in pressure and releases flash steam moving at high velocity. This high velocity creates a low pressure area under the disc, drawing it towards its seat. (3) At the same time, the flash steam pressure builds up inside the chamber above the disc, forcing it down against the incoming condensate until it seats on the inner and outer rings. At this point, the flash steam is trapped in the upper chamber, and the pressure above the disc equals the pressure being applied to the underside of the disc from the inner ring. However, the top of the disc is subject to a greater force than the underside, as it has a greater surface area. (4) Eventually the trapped pressure in the upper chamber falls as the flash steam condenses. The disc is raised by the now higher condensate pressure and the cycle repeats. 6.7 Auxiliary Systems Burners Gas, oil, and gas-oil combination burners. Feedwater systems Provide treated water to boilers. Fuel supply systems Transfer liquid fuels from storage tanks to the burners. Combustion air supply A large quantity of air is need to proper fuel combustion. The air should be heated to improve the efficiency of the boiler. Flue gas discharge Flue gas must be vented through one or more stacks. The stack(chimney) must extend vertically above the highest part of the building or any adjacent taller buildings. Water treatment systems Consist of the water softeners and chemical feeders to control the corrosive and scaleforming characteristics of water.

19 Water Treatment All heating systems involve a temperature change that affects the solubility of solids (minerals) and oxygen in water. The precipitation of dissolved solids and oxygen released from water can cause these symptoms: (1) a reduction in the heat transfer rate - due to scale. (2) reduced water or steam flow - due to reduced diameter of pipe by scale. (3) corrosion or destruction of the equipment due to oxidization. Scale Corrosion in steel pipe Corrosion in copper pipe Principles of Water Treatment Removal of solids in water: Scale is a hard substance deposited on the interior surface of pipes and heat exchangers. It is caused by the precipitation of calcium (Ca) ions in water. The calcium ions in water can be replaced by sodium (Na) ions through the zeolite ion exchange process. Removal of oxygen: Oxygen is the primary cause of corrosion, particularly in a steam condensate system. Deaeration by heating boiler feedwater is an effective way to remove oxygen. Reduction of acidity in water: Acidity is another cause of corrosion. A ph of 10.5 (alkalinity) is desired to stop corrosion. Inhibition (decreasing the rate of chemical reaction): Chromate has been a very effective agent for inhibiting corrosion, however it is classified as cancer-causing substance. Sodium nitrite (NaNO 2 ) is used as a substitute.

20 6.8 Sizing Boiler Capacity Step 1: Calculate heating loads for sensible heat and latent heat losses. Assume that the heating loads are calculated to maintain design indoor temperature and relative humidity at t a and RH a, respectively. Step 2: Determine the temperature of supply air (SA) which is usually 10 to 20 higher than the room design temperature. t s = t a + (10 to 20) [ ] Step 3: Determine the mass or volume of supply air. Step 4: Determine boiler capacity for sensible heat and latent heat due to humidification. q q SH LH 0.24G( ts tm)[kcal/h] 597G( xs xm)[kcal/h] Q G 0.24( t Q V 0.29( t SH a SH a [kg/h] t ) s 3 [m /h] t ) s Step 5: The total capacity of boilers should include the load of hot water supply and heat loss from pipe, which is usually 20% of sensible heat load. Step 6: When the humidification is provided by water spray inside the AHU, the mass of water for humidification can be determined by: L G( x )[kg/h] s xm