1.0 Introduction to Air Cooled Heat Exchanger
This kind of heat exchanger are used in petrochemical and refineries to utilize the atmospheric air to cool the hydrocarbon, process and utility fluids by means of indirect heat transfer from fluid (within the tube) to be cooled by air being circulated by means of forces / induced draft fan. In order to increase the heat transfer area, fins are also attached to the periphery of tubes. These heat exchangers are generally designed, inspected and tested as per API661 standard.
These exchangers are similar to the radiator of a motor car. The air cooled heat exchangers are mainly used where the heat load is very high and conventional heat exchanger becomes extremely big with large water requirement. By using air cooled heat exchanger, water requirement reduces drastically and cooling tower size comes down. The air coolers are of very large size as the heat transfer co-efficient with air is very low. Due to the large size, they are mounted on top of the pipe rack to save space.
2.0 Types of Air Cooled Heat Exchanger
There are three types of air cooled heat exchanger:
a) Forces Draft
b) Induced Draft
c) Natural Draft (used for applications like transformer oil cooling)
The outline sketch and components are as identified in figure 1A & B.
3.0 Definitions used for Air Cooled Heat Exchanger
The general terms used for air cooled heat exchangers are defined below:
A bank is one or more bays including one or more items arranged on a continuous structure.
b) Bare tube surface
Total area of the outside surface of the tubes in sq. meter.
A bay is one or more tube bundles serviced by two or more fans and that include structure, plenum and other attendant equipment.
d) Finned Surface
The finned surface of a tube is the total area of the outside surface of fins exposed to air.
e) Forced Draft exchanger
This is the one designed with the tube bundles located in the discharge side of the fan.
f) Induced Draft exchanger
This is the one designed with the tube bundles located on the suction side of the fan.
g) Tube bundle
This is an assembly of headers, tubes and frames.
Refer figure 1A, 1B, 2, 3, 4, 5 and 6 to understand the above definition.
4.0 Content of Enquiry Documents (Special Requirements)
The enquiry documents shall specify any special requirements concerning location of airflow, any obstruction to airflow and other heat sources.
5.0 Vendors Responsibilties
The vendor’s proposal shall include the following:
a) A proposal drawing showing major dimensions in plan, elevation, nozzle size and orientation
b) The proposal shall state whether the vertically mounted electric motors are to be shaft up or shaft down.
c) It should show the supporting arrangement of the tube bundle / plenum, fan and fan drive assembly.
d) It should show the relation of the tube bundle part of the bay with respect to the plenum to depict the movement of the nozzle installed on inlet and outlet headers.
e) It should show the platform and ladder for accessibility, operatibility and maintenance of fan, drive assembly and piping hooked up to the nozzles located on the inlet and outlet header.
f) It should show the weight of components, part of fan and drive assembly to derive the lifting beams, monorail and hoist arrangement to be permanently installed for maintenance of such components.
g) The supporting column shall be located such that it matches the supporting structure to house the air cooled heat exchanger e.g. pipe rack or building structure.
h) Allowable nozzle loading standard or data must be furnished to meet the same during detailed engineering.
i) The fixed point of the tube bundle shall be defined such that the inlet nozzles (hot fluid side) experience minimum movements in comparison to (cold fluid side) outlet nozzle to maintain the nozzle loading within allowable limits.
j) In order to achieve the smooth movement of the tube bundle headers and nozzles over plenum, a friction between the tube bundle and plenum must be means of providing smooth surface such as Teflon pads, stainless steel plate, graphite pads or similar material which can withstand the system temperature and compressive load of component at support.
6.0 Approvers Responsibilities
The approver shall approve following information received from vendor :
- Maximum and minimum design temperature.
- Overall dimensions.
- Dimension and location of supports.
- Nozzle size, rating, facing, location, projection, allowable movements and loading on nozzle (forces and moments).
- Weight of components for erection and maintenance.
- Drive mounting details.
- Screen platform and ladders.
a) Tube bundle design
i) A tube bundle shall be rigid, self contained and designed for handling as a complete assembly.
ii) Provision is required to be made by vendor to accommodate thermal expansion of tubes.
iii) All tubes are supported by vendor to prevent sagging or deformation of fins.
iv) A hold-down member (tube keeper) is provided at each tube support, hold-down members are attached to side frame by bolting.
v) Tubes of single pass cooler are sloped towards the outlet header.
vi) Tubes of multiple pass coolers may not be sloped.
vii) The exchanger may be designed for an internal steam out operation at the temperature / pressure specified by the process licensor.
b) Heating coils
i) Heating coils are provided to protect the process tube bundle against freeze up and are provided in the bundle separate from the process bundle.
ii) Heating coils to cover the full width of process tube bundle.
iii) Heating coils are normally single pass type.
i) Provisions are made in the design of header, to prevent excessive warpage of tube sheets and leakage at tube joints. Any alternative operating conditions including low process flow at low ambient air temperature, freezing of fluids in tubes, steamout, stoppage of fan due to power failure, any cyclic conditions must be included by the vendor in the analysis if specified by the designer.
ii) When the fluid temperature differential between inlet of one tube pass and the outlet of adjacent tube pass is higher than 110degC then split header construction with ‘U’ tubes or other method of restraint relief shall be employed.
iii) If the fluid temperature difference between inlet and outlet of multipass bundle exceed 110degC, the need of restraint relief shall be insisted.
iv) The cover plate header designer shall permit removal of cover without disturbing header piping connections. This helps in providing access to tubes during maintenance and repairs.
v) The bonnet header design shall permit removal of bonnet with minimum dismantling of header piping. This helps in access to the tubes during maintenance and repairs.
vi) Plug header are equipped with threaded plug holes provided opposite to the ends of each tube for access. This helps in providing access to the tubes during maintenance and repairs without disturbing the header as well as piping.
d) Nozzle and other connections
i) All connection 1 ½” and larger shall be flanged
ii) In hydrogen service all connections shall be flanged and slip on flange shall not be used.
iii) Where design conditions require class 900 or higher flange rating, all connections shall be flanged.
e) Maximum allowable moments and forces for nozzle and headers
i) In corroded condition each nozzle shall be capable of withstanding the simultaneous application of the forces of the forces and moments as defined in Annexure A.
ii) The sum of nozzle on a single header will consist of components that do not exceed Mx of 4500 ft.lbs, My of 6000 ft.lbs, Mz of 3000 ft.lbs and Fx of 2250 lbs, Fy of 4500 lbs and Fz of 3750 lbs. The application of the forces and moments as per Annexure A will cause movement that will tend to reduce the loads to the values given above.
iii) The total of all nozzle loads on one multi bundle bay shall not exceed 3 times allowed for a single header.
f) Air side design
The environmental factors such as weather, terrain, adjacent building and equipment will influence the air flow and hence performance of an air cooled heat exchanger for fan dispersion angle refer figure 3.
g) Drive arrangements
Refer figure 3 and 7 for typical drive arrangement for air cooled heat exchanger.
h) Design loads
i) Thermal forces shall include all forces due to partial or complete anchorage of piping or equipment, friction from sliding or rolling of equipment, and forces from expansion or contraction of structure.
ii) Nozzle load shall include all forces and moment applied to the nozzle face including the dead weight of pipe, thermal forces and the weight of fluid in the piping.
i) Mechanical Access facilities
i) The number and location of header access platform interconnecting walkways and ladder shall be specified in enquiry specification during detailed engineering.
ii) Maintenance platform shall be provided beneath each drive assembly for easy access to drive and for removal and replacement during maintenance of all drive components. The platform shall be provided all around the drive assembly.
iii) Ladder, Railings, toe plates and safety chains with safety bolts etc. shall be provided for platform as per good engineering practice.
iv) Header shall be provided with toe bone or knee railing on the side next to the exchanger.
8.0 Design Features of Air Cooled Heat Exchanger
Air cooler consist of tubes, header boxes, fan, motor and sometimes louvers. Tubes are all placed in number of horizontal layers and both ends of the tubes are welded in the rectangular header box. This is similar to tube sheet of conventional heat exchanger. The piping gets connected to nozzles of this header box. Typical drawing of above arrangement is attached here with (refer fig. 2, 5, 6) these header boxes are supported on steel frame. The fan is provided below the tube bundles in force draft arrangement. The fan is provided above tubes in induced draft arrangement.
Following different type of constructions are generally available:
a) Singlepass Cooler.
b) Multipass Cooler.
c) U tube Coolers
In a single pass and multipass cooler with odd number of passes, the fluid enters from one end of the header box and leaves at the other end of the header box.
In multipass with even number of passes and ‘U’ tube, the fluid enters and leaves from the same end of the header box.
The following general design featured shall be taken into account from equipment layout/piping layout point of view.
8.1 The tube bundle has a provision to move in lateral direction + 6 mm or 13mm in one direction. This movement is required for accommodating piping header movement. In case additional movement is required due to piping, needs to be specified during enquiring stage. This is shown in figure 1.
8.2 The tube expands in longitudinal direction and normally a provision is made such that the inlet header side is a fixed side and tube expands in the other direction.
9.0 Consideration from Equipment Layout Point of View
9.1 This equipment requires smooth air flow for cooling purpose hence the location of the equipment should be such that it is not closely surrounded by equipment or structure which blocks the air flow path.
9.2 To give a better air flow, this is installed on the top of the pipe rack, or structure so that there is no obstruction to reduce air flow. At the same time by installing on top of the rack, the space on the ground can also be saved and the plant becomes more compact.
9.3 Normally tube bundle length is fixed based on the width of the piperack or structure so that the supporting legs of air cooler bundle comes on the main beams, which can simplify the pipe rack design. Also it is preferable to adjust piperack / structure longitudinal column spacing based on the width of the air cooler bundle so that legs of bundle straight away sit on top of the column. This may not be possible to adjust some times as each tube bundle may have varied width depending on service condition and adjusting piperack columns for different width may not be feasible from structural design and detailing point of view.
9.4 It is required to provide walkways between two sets of air coolers. This means say one cooler may consist of 10 bundles and other one of 5 bundles then walkways should be provided between, after tenth bundle and before starting of next five bundles. This walkway shall be minimum 1.5 to 2.0 m wide as this will be the only place at that elevation to store tools and parts during maintenance.
9.5 The air coolers on the piperack shall be located such a way that at least from one side the bundles are accessible with crane.
9.6 Air cooler should have access platform mounted on the air cooler structure at least on the operating side. Platform all around is better for maintenance.
9.7 Air coolers have motors hanging at the bottom of the cooler. It is required to provide access platform underneath the cooler for motor & for maintenance. This platform can be a localized also.
9.8 To access the air cooler platforms or motor maintenance platform, a regular staircase is required to be provided.
9.9 Inlet piping of air cooler has a symmetrical distribution and loops as explained later in this article. This is required to be supported hence either air cooler structural columns need to be extended upwards to support piping or piperack / structure columns. This data is required to be given very early in the project as it is to be considered in piperack design.
10.0 Consideration from Piping Point of View
The air coolers are mainly used where very large quantity of vapor is required to get condensed or very large quantity of gas/ liquid is required to be cooled. The application is very common in case of column overhead vapor condensation. Following points need to be taken care while laying air cooler piping.
10.1 Piping distribution to air cooler should be symmetrical from centre line of complete air cooler assembly.
The typical configuration of inlet piping is shown in Figure 8, 9, 10, 11.
10.2 In case of supply line having very low pressure, care shall be taken to keep no. of bends to minimum without sacrificing functional and stress requirement. Line sizing during the distribution should be sufficient, if required check with process department.
10.3 Length of each branch pipe for all bundles from its header shall be more or less same to keep pressure drop same and equal distribution of fluids to all bundles.
10.4 Inlet side header box shall be considered as a fixed point (in tube direction) for piping connection. But the bundle can move in transverse direction of tubes + 6 mm or if it is fixed at one edge then it can move by 13 mm in the other direction. This movement is required for piping header expansion compensation. If air cooler is required to be mounted in eccentric position i.e. to get 13 mm movement in one direction, vendor shall be informed early enough.
10.5 The transverse movement of bundle can occur only when piping connected to nozzles generate enough force to overcome friction at the bundle support point. That is why normally at the support point vendor provides S.S plate, PTFE plate or ball bearings to ease the movement.
10.6 The force due to thermal expansion of piping created on the bundle nozzle shall be less than the limits given by API 661.
10.7 While doing stress analysis, the following considerations shall be given.
10.7.1 It is ideal to simulate the complete air cooler with tubes, header boxes and support points in computer program. But most of the time it is difficult, then model air cooler bundle as a rigid element with total weight of bundle with supports and friction co-efficient depending on type of supports.
10.7.2 After modeling piping along with each bundle as explained above, the piping stress analysis to be carried out. During this analysis all the nozzles in longitudinal & transverse direction to be considered as rigid i.e. anchor. After the analysis, check the loading on each nozzle. If these loading are within the limits of API 661 there is no problem. By modeling the air cooler either as rigid element or as a normal equipment, with weight & support friction, the nozzle loads shown by computer in operating condition will be taking care of bundle movement. In case nozzle loads on some nozzles exceed API 661 limits, the configuration of such pipe needs to be modified to reduce nozzle loads.
10.7.3 In case it is difficult to model air cooler due to some reason, the following method should be adopted.
Consider all nozzles as anchor points and model complete piping system as usual. Now do the analysis & find out which all nozzle loads exceeds, the nozzle where load is exceeding the values of API 661, feed 1 mm nozzle movement to that nozzle and carry out analysis. This 1 mm movement shall be fed in the direction in which header will try to move the bundle. If it still does not meet go on adding movement of 1 mm & check till results are satisfactory. In the first case where nozzle is considered as anchor point, find out the difference between actual loads & API loads, that will tell whether the differential loading will allow bundle to move or not with friction factor on support point. Of course this is a very crude method of analysis and as far as possible should be avoided unless it is a very small air cooler and nozzle load is not governing the design.
10.7.4 The outlet piping when analyzed, the bundle movement due to inlet piping should be modeled if analysis for inlet & outlet is not done together. Again nozzle loading criteria for outlet piping to be met as per API 661.