DESCRIPTION
Convection is the heat transfer from one place to another by the movement of fluids (liquids and gases).There are two types of convections, natural convection and forced convection:
NATURAL CONVECTION
In natural convection, the fluid receives heat with surrounding a heat source and becomes less dense and then rises. As that condition, the surrounding cooler fluid will move to replace the fluid that has been raised. This process will continue when the heat source keeps producing heat and warm air rises up and cooler fluid goes to replace it as always. This process is forming convection current.
FORCED CONVECTION
In forced convection, the fluid is forced to flow over a surface by an external source such as fans, by stirring, and pumps to create convection current. This method is to transport heat energy very efficiently with using external source. For example, the radiator needs steam or hot water that come from pumps and pipes to heat up the core of cast iron in order to produce the thermal heat to the air. The external source have to be applied for heat transfer in the forced convection.
The convective heating is kind related to the passive solar. Two systems both using the circulation of the convection. Since the air gets warm and then rises, the cold air will replace to where the warm air rises. However, the differences between them is the passive solar uses the natural solar as an external source and convective heating is applied for movement of fluids (liquids and gases).
The most common convective heating user is the radiator. Radiators are heat exchangers used to transfer thermal energy from one place to another for the purpose of cooling and heating. Radiators are basically using to heat buildings in a central heating system. The hot water or steam are circulated by pumps and generated in a central boiler through the radiators in the building. Radiators can be made with different shapes and sizes but they all should have a series of pipes that run a hot fluid through them. There are two types of pipes: Single-pipes work for the steam and double pipes work with steam and hot water. Those two types of pipes both can work with radiator very well. The hot fluid heats up the cast iron or the large series of fins to produce the heat to the air immediately. The warm air will go up because of the less density depends on the convection heating, and also the cool air will replace the space after warm air goes up. This will be a circulation happens to the radiator.
NATURAL CONVECTION
In natural convection, the fluid receives heat with surrounding a heat source and becomes less dense and then rises. As that condition, the surrounding cooler fluid will move to replace the fluid that has been raised. This process will continue when the heat source keeps producing heat and warm air rises up and cooler fluid goes to replace it as always. This process is forming convection current.
FORCED CONVECTION
In forced convection, the fluid is forced to flow over a surface by an external source such as fans, by stirring, and pumps to create convection current. This method is to transport heat energy very efficiently with using external source. For example, the radiator needs steam or hot water that come from pumps and pipes to heat up the core of cast iron in order to produce the thermal heat to the air. The external source have to be applied for heat transfer in the forced convection.
The convective heating is kind related to the passive solar. Two systems both using the circulation of the convection. Since the air gets warm and then rises, the cold air will replace to where the warm air rises. However, the differences between them is the passive solar uses the natural solar as an external source and convective heating is applied for movement of fluids (liquids and gases).
The most common convective heating user is the radiator. Radiators are heat exchangers used to transfer thermal energy from one place to another for the purpose of cooling and heating. Radiators are basically using to heat buildings in a central heating system. The hot water or steam are circulated by pumps and generated in a central boiler through the radiators in the building. Radiators can be made with different shapes and sizes but they all should have a series of pipes that run a hot fluid through them. There are two types of pipes: Single-pipes work for the steam and double pipes work with steam and hot water. Those two types of pipes both can work with radiator very well. The hot fluid heats up the cast iron or the large series of fins to produce the heat to the air immediately. The warm air will go up because of the less density depends on the convection heating, and also the cool air will replace the space after warm air goes up. This will be a circulation happens to the radiator.
TYPICAL USES
RADIATORS
Radiators can be used almost anywhere. They are the most common to pick for the heating equipment. They are always appearing in residential homes, apartments, workshops and schools. Many old radiators were made by the cast iron and old column locates in the old houses. Those materials on the radiators are inexpensive and no harmful to people. The new radiators are very popular used in the tall building because the steams will easily go up without pumping it. In the modern application, new radiators are accepting for a lot residential buildings because they are made by a large series of fins or copper (Light weight), which can heat surrounding immediately depends on the good conductivity. The radiators are easily to set up and have low profile. Radiators have most effectively temperature control in the small area, that's why people install the radiators in homes, apartments and schools, however they are not the best to use when trying to heat a large open space as they effective area they can heat is not very large. |
FIREPLACE
Fireplaces were the most common type of heating sources for homes and other smaller building types. In order to adjust the heat output from a fireplace, it was simply the matter of increasing or decreasing the amount of wood. Now that there are many other methods for heating that are more efficient, fireplaces have become more of an aesthetic. This is something typically found in residential homes that have also adopted a more efficient way of heating with the use of inserts installed to increase the energy efficiency of heating the whole house.
Fireplaces were the most common type of heating sources for homes and other smaller building types. In order to adjust the heat output from a fireplace, it was simply the matter of increasing or decreasing the amount of wood. Now that there are many other methods for heating that are more efficient, fireplaces have become more of an aesthetic. This is something typically found in residential homes that have also adopted a more efficient way of heating with the use of inserts installed to increase the energy efficiency of heating the whole house.
Types of Fireplace Fuels and Designs:
WOOD BURNING: Wood burning fireplaces are the oldest type. Their popularity is surely due to the easy availability of wood fuel. A combination of the type of wood used as fuel, and the design of the fireplace can have a great effect of its ability to heat a space. COAL: Coal is another fuel that has been in existence for a very long time. It is also more efficient when burned in a stove rather than an open fire. Coal stoves are more popular throughout the state of Pennsylvania because of its abundance in the area. GAS: Gas fireplaces are typically easier to operate and safer to use then solid fuels. Fires can be lit or put out by simply turning a knob or flipping a switch. Also, the combustion gases are considerably less then those of solid fuels, so smaller clearances can be used for the flue area. ELECTRIC: Electric fireplaces need no flue because no combustion is occurring therefore giving them the name, "vent-less" fireplaces. Heat is created using electric coils and blowers, while the ambiance is created with lights and blowers. PELLETS: Because pellets are very environmentally friendly, many people choose to use them as fuel. They are most often made from recycled saw dust and their high density and low water content allow for very high combustion efficiency. |
HEATING PANELS
Most typical uses are residential uses such as homes, retail and industrial, workshops, warehouses, medical facilities, multi-family homes. Since heating panels are primarily used for indoor heating, these examples are the most obvious. Heating panels also have been installed in assisted living centers, yoga studios, fitness centers, barns, and animal kennels. These examples demonstrate the use of energy efficient space heating in larger indoor spaces that can house a larger group of people/animals. The reduced amount of material and energy costs allow for a uniform distribution of heating in larger spaces and an easier method of temperature control on a room to room basis. My favorite use of heating panels are bedroom use on a cold morning.
Most typical uses are residential uses such as homes, retail and industrial, workshops, warehouses, medical facilities, multi-family homes. Since heating panels are primarily used for indoor heating, these examples are the most obvious. Heating panels also have been installed in assisted living centers, yoga studios, fitness centers, barns, and animal kennels. These examples demonstrate the use of energy efficient space heating in larger indoor spaces that can house a larger group of people/animals. The reduced amount of material and energy costs allow for a uniform distribution of heating in larger spaces and an easier method of temperature control on a room to room basis. My favorite use of heating panels are bedroom use on a cold morning.
LIMITATIONS OF THE SYSTEM
RADIATORS
Radiators do not work well in large open spaces unless many radiators are used spaced in a grid pattern. Radiators are difficult to maintain at a given temperature as many older models are not electronic, and this leads to problems such as a delayed effect when the temperature is increased or decreased, and it can be difficult to achieve an optimal temperature. Radiators in multistorey structures require additional water pipes leading to and from every radiator, and a large pump to move the water through the system. Old radiators are also very large and heavy and typically can’t be moved as they must be connected to the pipes, this can cause issues in a house environment where the placement of the radiators is not always ideal.
Electrical radiators often heat the element to a much high degree. This is due mostly to the fact that the electrical resistance is very high, and simply because portable radiators are smaller but are still used to heat a room. These increase temperatures make electric radiators dangerous as the can possibly ignite things that are too close to them.
FIREPLACE
Fireplaces should not be used in buildings where energy efficiency is an issue. It has been estimated that about 90% of the heat generated in a standard wood fireplace escapes through the chimney. With the use of a fan and venting system, cold room air is drawn into the firebox through a lower louvre where blowers push the air up and over your firebox where the air is heated as it travels and exits through the top louvre. The room size a fireplace can heat is dependent on the size of the fireplace. A 24"x30" fireplace could heat 150 ft² room however 400 ft² room would require a fireplace with at least a 4' opening, which is very impractical and is actually illegal in most populated areas. Cities passed these ordinances to cut down on pollution and the safety of others in densely populated areas. The selection of fuel for a fire place is very important as well because depending on the availability and prices, it would not be economically smart to use fire as a heating source.
CONSTRUCTION ISSUES
· Clearances
· Flammable materials may not be placed near fireplace (beds, curtains, furniture)
· Chimneys must meet minimum clearances
· If installed on an interior wall it must be contained all the way to the roof
· Wall Penetration
· A flue to expel combustion gas, most go through the roof or exterior wall
· Surrounding construction
· Flame retardant
· Firewall must be used between residences
· Non-flammable carpets must be used in case of sparks
HEATING PANELS
For larger spaces, several panels are required to keep the temperature at the desired temperature. Thermostats control the panels, which also requires there to be a thermostat in any room that contains panels. Installation can provide problems if the space has high ceilings or curved walls, the flat panels will not be able to be installed correctly and it's preferred that a qualified electrician do the wiring and panel installation. Panels tend to come in only a 2'x2' or 2'x4' sizes. In addition, if the ceiling height is greater than 10 feet, a higher wattage must be supplied to the panels to generate the proper amount of heat.
Radiators do not work well in large open spaces unless many radiators are used spaced in a grid pattern. Radiators are difficult to maintain at a given temperature as many older models are not electronic, and this leads to problems such as a delayed effect when the temperature is increased or decreased, and it can be difficult to achieve an optimal temperature. Radiators in multistorey structures require additional water pipes leading to and from every radiator, and a large pump to move the water through the system. Old radiators are also very large and heavy and typically can’t be moved as they must be connected to the pipes, this can cause issues in a house environment where the placement of the radiators is not always ideal.
Electrical radiators often heat the element to a much high degree. This is due mostly to the fact that the electrical resistance is very high, and simply because portable radiators are smaller but are still used to heat a room. These increase temperatures make electric radiators dangerous as the can possibly ignite things that are too close to them.
FIREPLACE
Fireplaces should not be used in buildings where energy efficiency is an issue. It has been estimated that about 90% of the heat generated in a standard wood fireplace escapes through the chimney. With the use of a fan and venting system, cold room air is drawn into the firebox through a lower louvre where blowers push the air up and over your firebox where the air is heated as it travels and exits through the top louvre. The room size a fireplace can heat is dependent on the size of the fireplace. A 24"x30" fireplace could heat 150 ft² room however 400 ft² room would require a fireplace with at least a 4' opening, which is very impractical and is actually illegal in most populated areas. Cities passed these ordinances to cut down on pollution and the safety of others in densely populated areas. The selection of fuel for a fire place is very important as well because depending on the availability and prices, it would not be economically smart to use fire as a heating source.
CONSTRUCTION ISSUES
· Clearances
· Flammable materials may not be placed near fireplace (beds, curtains, furniture)
· Chimneys must meet minimum clearances
· If installed on an interior wall it must be contained all the way to the roof
· Wall Penetration
· A flue to expel combustion gas, most go through the roof or exterior wall
· Surrounding construction
· Flame retardant
· Firewall must be used between residences
· Non-flammable carpets must be used in case of sparks
HEATING PANELS
For larger spaces, several panels are required to keep the temperature at the desired temperature. Thermostats control the panels, which also requires there to be a thermostat in any room that contains panels. Installation can provide problems if the space has high ceilings or curved walls, the flat panels will not be able to be installed correctly and it's preferred that a qualified electrician do the wiring and panel installation. Panels tend to come in only a 2'x2' or 2'x4' sizes. In addition, if the ceiling height is greater than 10 feet, a higher wattage must be supplied to the panels to generate the proper amount of heat.
NUMERIC PARAMETERS
The equation for convection is expressed as:
q = hc A dT (1)
Where:
q = heat transferred per unit time (W)
A = heat transfer area of the surface (m2)
hc= convective heat transfer coefficient of the process (W/(m2K) or W/(m2oC))
dT = temperature difference between the surface and the bulk fluid (K or oC)
The heat transfer coefficient is dependent upon the type of media, gas or liquid, flow properties such as velocity, viscosity and other flow temperature dependent properties. The convective heat transfer coefficient for some common fluids are within the ranges below:
· Free Convection - Air : 5 - 25 (W/(m2K))
· Free Convection - Water: 20 - 100 (W/(m2K))
· Forced Convection - Air: 10 - 200 (W/(m2K))
· Forced Convection - Water: 50 - 10.000 (W/(m2K))
· Boiling Water : 3.000 - 100.000 (W/(m2K))
· Condensing Water Vapor: 5.000 - 100.000 (W/(m2K))
RADIATORS
Radiators can vary greatly in size and strength, however using the following rules of the thumb, we can get an estimate on the general sizes of radiators.
Lounges and dining rooms: Multiply cubic feet by 5
Bedrooms: Multiply cubic feet by 4
Common areas and kitchens: Multiply cubic feet by 3
For rooms facing north: Add 15%
For French windows: Add 20%
For double glazing: Deduct 10%
Using these guide lines a small kitchen (12'x14'x8') requires a radiators of about 4,000 BTUs, where as a very large lounge (20'x20'x10') requires a radiators rated at 20,000 BTUs. Using a middle value of 10,000 BTUs per radiator, say 6 rooms in a small house that is 60,000 BTUs, adding an extra 10,000 BTUs just for the water means a 70,000 BTU boiler is needed. This boiler is about a 20.5Kw boiler. All of these number are estimates of course however because of the variability of radiators it is difficult to give exact numbers without some example number to start from.
FIREPLACES
TRADITIONAL FIREPLACE
Dimensions:
3 feet wide by 2.5 feet high, with a depth of 1.5 feet.
Heating Efficiency:
Traditional fireplaces are notoriously inefficient. Up to 90 percent of the heat created can be lost through the flue.
Cost of Fuel:
Firewood is measured in the unit of a cord (128 cubic feet of stacked wood). This corresponds to a stack 8 feet by 4 feet by 4 feet. The cost can range between free to $200+ depending on the type of wood and the area.
ELECTRIC FIREPLACE
Dimensions:
The dimensions for an electric fireplace vary greatly but like any fireplace, it is suggested that the space is limited enough that it creates a direct laminar flow in the flue.
Heat Output:
Max BTUs: 4600 per hour
Cost of Fuel:
The cost of fuel is related to the price of electricity in your area. If listed as 1350 W, so 1.35 kW-hr can be multiplied by the price of electricity to give a results.
HEATING PANELS
q = hc A dT (1)
Where:
q = heat transferred per unit time (W)
A = heat transfer area of the surface (m2)
hc= convective heat transfer coefficient of the process (W/(m2K) or W/(m2oC))
dT = temperature difference between the surface and the bulk fluid (K or oC)
The heat transfer coefficient is dependent upon the type of media, gas or liquid, flow properties such as velocity, viscosity and other flow temperature dependent properties. The convective heat transfer coefficient for some common fluids are within the ranges below:
· Free Convection - Air : 5 - 25 (W/(m2K))
· Free Convection - Water: 20 - 100 (W/(m2K))
· Forced Convection - Air: 10 - 200 (W/(m2K))
· Forced Convection - Water: 50 - 10.000 (W/(m2K))
· Boiling Water : 3.000 - 100.000 (W/(m2K))
· Condensing Water Vapor: 5.000 - 100.000 (W/(m2K))
RADIATORS
Radiators can vary greatly in size and strength, however using the following rules of the thumb, we can get an estimate on the general sizes of radiators.
Lounges and dining rooms: Multiply cubic feet by 5
Bedrooms: Multiply cubic feet by 4
Common areas and kitchens: Multiply cubic feet by 3
For rooms facing north: Add 15%
For French windows: Add 20%
For double glazing: Deduct 10%
Using these guide lines a small kitchen (12'x14'x8') requires a radiators of about 4,000 BTUs, where as a very large lounge (20'x20'x10') requires a radiators rated at 20,000 BTUs. Using a middle value of 10,000 BTUs per radiator, say 6 rooms in a small house that is 60,000 BTUs, adding an extra 10,000 BTUs just for the water means a 70,000 BTU boiler is needed. This boiler is about a 20.5Kw boiler. All of these number are estimates of course however because of the variability of radiators it is difficult to give exact numbers without some example number to start from.
FIREPLACES
TRADITIONAL FIREPLACE
Dimensions:
3 feet wide by 2.5 feet high, with a depth of 1.5 feet.
Heating Efficiency:
Traditional fireplaces are notoriously inefficient. Up to 90 percent of the heat created can be lost through the flue.
Cost of Fuel:
Firewood is measured in the unit of a cord (128 cubic feet of stacked wood). This corresponds to a stack 8 feet by 4 feet by 4 feet. The cost can range between free to $200+ depending on the type of wood and the area.
ELECTRIC FIREPLACE
Dimensions:
The dimensions for an electric fireplace vary greatly but like any fireplace, it is suggested that the space is limited enough that it creates a direct laminar flow in the flue.
Heat Output:
Max BTUs: 4600 per hour
Cost of Fuel:
The cost of fuel is related to the price of electricity in your area. If listed as 1350 W, so 1.35 kW-hr can be multiplied by the price of electricity to give a results.
HEATING PANELS
The range of wattage varies depending on ceiling height, the size of the space, and the number of panels installed.
The below chart shows that the heat transfer is similar to the convective heat transfer coefficient in that they both depend upon the temperature difference. From the chart we can see that the heat transfer and convective heat transfer coefficient increase with increasing temperature difference.
The below chart shows that the heat transfer is similar to the convective heat transfer coefficient in that they both depend upon the temperature difference. From the chart we can see that the heat transfer and convective heat transfer coefficient increase with increasing temperature difference.