What are headers?
Headers are the boxes at the ends of the tubes which distribute the fluid from the piping to the tubes.
How are headers constructed?
Almost all headers on air-cooled exchangers are welded rectangular boxes. A vast majority of the headers are of the plug type. This means that there is a shoulder plug opposite each tube which allows access for inspection and cleaning of individual tubes. They can also be used to plug a leaking tube.
The plug holes are used in the manufacturing process for access to roller expand the tubes into the headers.
The other common type of header is the cover plate or bonnet type. These are usually used in low pressure applications (say below 150 PSIG) where complete tube access is desired. This usually means applications where fouling is a potential problem and the tube bundle may require occasional internal cleaning. As the name implies, these have a removable plate on the back side of the header opposite the tubes. The cover plate is attached to the header by a set of studs or through-bolts to a flange around the perimeter of the header. A bonnet header is similar, but opposite in construction. The whole header or bonnet bolts to the tubesheet and comes off. Bonnet headers are sometimes used where the corrosion potential of the process fluid is very high and the tubesheet material is some kind of expensive exotic alloy, such as titanium.
Headers are usually constructed of carbon steel or stainless steel, but sometimes more exotic alloys are used for corrosion resistance. The selection of materials is usually made by the customer.
Why are some coolers forced draft and some induced draft? Which is better?
It depends. The majority of air-cooled exchangers is of forced draft construction. Forced draft units are easier to manufacture and to maintain. The tube bundle is mounted on top of the plenum, so it can be easily removed and replaced. The fan shaft is short, since it does not have to extent from the drive unit through the tube bundle and plenum to the fan, as in an induced draft design. Forced draft units require slightly less horsepower since the fan are moving a lower volume of air at the inlet than they would at the outlet. If the process fluid is very hot, the cooling air is hot at the outlet. This could cause problems with some fans or fan pitch actuators if the fan is exposed to very hot exhaust air. Since forced draft coolers do not have the fans exposed to hot exhaust air, they are a better choice in such cases. (API 661 par. 4.2.3.15&16 offer some guidelines for this.)
However, induced draft units have some advantages, too. A common problem with forced draft coolers is accidental warm air recirculation. This happens when the hot exhaust air is pulled back in to the fans. Since a forced draft cooler has a low air velocity at the exhaust from the bundle and a high velocity through the fan, a low pressure area is created around the fan, causing the hot air to be pulled over the side or end of the bay. For this same reason, there should never be a small space between the bays of a bank of forced-draft cooler. Induced draft cooler have a high exhaust air velocity through the top-mounted fan, and a lower velocity into the face of the tube bundle below. This tends to minimize the probability of accidental air recirculation. Also an induced draft plenum does not have to support the tube bundle so some weight can often be saved in this area.
Painted or Galvanized?
This is usually a matter of customer preference. However, the costs are roughly the same if a multiple coat paint system is specified. Often the painted units are more expensive. There seems to be a trend toward more galvanized structures because they require virtually no maintenance. Painted structures require touch-up after installation and they often rust anyway.
We recommend galvanized units wherever possible.
Plenums, dispersion angle, and fan coverage:
The API specification includes a number of paragraphs about fan coverage and dispersion angle. This is for a very good reason. The actual air coming from a fan does not distribute itself evenly at first. The most air flow is seen around the fan tip area. If you measure the air flow across the face of a tube bundle, it is often very different around the fan blade tip as opposed to the center of the fan or the corner of the bundle. However, as the plenum becomes deeper, this localized effect is diminished as the air becomes more evenly distributed. All of the heat transfer programs assume that the air is distributed perfectly evenly.
The fan coverage is the ratio of the fan area to the bundle face area. The higher this ratio, the better the fan coverage. The API minimum is 40% with a 45 degree maximum dispersion angle from the fan ring to the middle of the tube bundle at the middle of the sides or the middle of the ends of each fan chamber. More fan coverage or a lower dispersion angle can improve the air distribution. (See Figure 6 on Page 14 of API 661for a sketch of this.)
A few manufacturers actually improve on this idea one step more, by using rounded and eased fan rings. Rounded and eased rings offer two advantages compared to the conventional fan rings. First, they enhance the distribution of the air. Secondly, they reduce the air pressure drop through the fan ring, slightly reducing the fan brake horsepower. When designing their coolers, some cooler manufacturers base their fan designs on the use of rounded and eased rings, even though they don't build them this way.
What kinds of controls are used?
As one might expect the best kind of control scheme depends on the application. Does the process require a very tight control on the process outlet temperature, or is it better to allow the process temperature to go down with the ambient air temperature. Is there a possibility of freezing the process? Is there a pour-point problem? Is the cost of operating the fan motors a significant factor?
The following is a list of some of the commonly used control devices for air coolers, but in no particular order.
1. Manually operated louvers.
2. Electrically or pneumatically operated louvers.
3. Pneumatically actuated automatic variable-pitch fans.
4. Variable-frequency fan drives.
5. Warm-air recirculation systems for freezing/pour point control in cold climates.
6. Steam coils.
How can I get additional information about air cooled heat exchanger ?
Please visit here to know more about air cooled heat exchanger.
Headers are the boxes at the ends of the tubes which distribute the fluid from the piping to the tubes.
How are headers constructed?
Almost all headers on air-cooled exchangers are welded rectangular boxes. A vast majority of the headers are of the plug type. This means that there is a shoulder plug opposite each tube which allows access for inspection and cleaning of individual tubes. They can also be used to plug a leaking tube.
The plug holes are used in the manufacturing process for access to roller expand the tubes into the headers.
The other common type of header is the cover plate or bonnet type. These are usually used in low pressure applications (say below 150 PSIG) where complete tube access is desired. This usually means applications where fouling is a potential problem and the tube bundle may require occasional internal cleaning. As the name implies, these have a removable plate on the back side of the header opposite the tubes. The cover plate is attached to the header by a set of studs or through-bolts to a flange around the perimeter of the header. A bonnet header is similar, but opposite in construction. The whole header or bonnet bolts to the tubesheet and comes off. Bonnet headers are sometimes used where the corrosion potential of the process fluid is very high and the tubesheet material is some kind of expensive exotic alloy, such as titanium.
Headers are usually constructed of carbon steel or stainless steel, but sometimes more exotic alloys are used for corrosion resistance. The selection of materials is usually made by the customer.
Why are some coolers forced draft and some induced draft? Which is better?
It depends. The majority of air-cooled exchangers is of forced draft construction. Forced draft units are easier to manufacture and to maintain. The tube bundle is mounted on top of the plenum, so it can be easily removed and replaced. The fan shaft is short, since it does not have to extent from the drive unit through the tube bundle and plenum to the fan, as in an induced draft design. Forced draft units require slightly less horsepower since the fan are moving a lower volume of air at the inlet than they would at the outlet. If the process fluid is very hot, the cooling air is hot at the outlet. This could cause problems with some fans or fan pitch actuators if the fan is exposed to very hot exhaust air. Since forced draft coolers do not have the fans exposed to hot exhaust air, they are a better choice in such cases. (API 661 par. 4.2.3.15&16 offer some guidelines for this.)
However, induced draft units have some advantages, too. A common problem with forced draft coolers is accidental warm air recirculation. This happens when the hot exhaust air is pulled back in to the fans. Since a forced draft cooler has a low air velocity at the exhaust from the bundle and a high velocity through the fan, a low pressure area is created around the fan, causing the hot air to be pulled over the side or end of the bay. For this same reason, there should never be a small space between the bays of a bank of forced-draft cooler. Induced draft cooler have a high exhaust air velocity through the top-mounted fan, and a lower velocity into the face of the tube bundle below. This tends to minimize the probability of accidental air recirculation. Also an induced draft plenum does not have to support the tube bundle so some weight can often be saved in this area.
Painted or Galvanized?
This is usually a matter of customer preference. However, the costs are roughly the same if a multiple coat paint system is specified. Often the painted units are more expensive. There seems to be a trend toward more galvanized structures because they require virtually no maintenance. Painted structures require touch-up after installation and they often rust anyway.
We recommend galvanized units wherever possible.
Plenums, dispersion angle, and fan coverage:
The API specification includes a number of paragraphs about fan coverage and dispersion angle. This is for a very good reason. The actual air coming from a fan does not distribute itself evenly at first. The most air flow is seen around the fan tip area. If you measure the air flow across the face of a tube bundle, it is often very different around the fan blade tip as opposed to the center of the fan or the corner of the bundle. However, as the plenum becomes deeper, this localized effect is diminished as the air becomes more evenly distributed. All of the heat transfer programs assume that the air is distributed perfectly evenly.
The fan coverage is the ratio of the fan area to the bundle face area. The higher this ratio, the better the fan coverage. The API minimum is 40% with a 45 degree maximum dispersion angle from the fan ring to the middle of the tube bundle at the middle of the sides or the middle of the ends of each fan chamber. More fan coverage or a lower dispersion angle can improve the air distribution. (See Figure 6 on Page 14 of API 661for a sketch of this.)
A few manufacturers actually improve on this idea one step more, by using rounded and eased fan rings. Rounded and eased rings offer two advantages compared to the conventional fan rings. First, they enhance the distribution of the air. Secondly, they reduce the air pressure drop through the fan ring, slightly reducing the fan brake horsepower. When designing their coolers, some cooler manufacturers base their fan designs on the use of rounded and eased rings, even though they don't build them this way.
What kinds of controls are used?
As one might expect the best kind of control scheme depends on the application. Does the process require a very tight control on the process outlet temperature, or is it better to allow the process temperature to go down with the ambient air temperature. Is there a possibility of freezing the process? Is there a pour-point problem? Is the cost of operating the fan motors a significant factor?
The following is a list of some of the commonly used control devices for air coolers, but in no particular order.
1. Manually operated louvers.
2. Electrically or pneumatically operated louvers.
3. Pneumatically actuated automatic variable-pitch fans.
4. Variable-frequency fan drives.
5. Warm-air recirculation systems for freezing/pour point control in cold climates.
6. Steam coils.
How can I get additional information about air cooled heat exchanger ?
Please visit here to know more about air cooled heat exchanger.
