Finned tube heat exchangers are commonly used to transfer heat between a gas and liquid. The tubes used in these units are equipped with fins that extend outward from the tubes as shown below.
The fins on the tubes allow for a much larger surface area to be packed into a small volume. This is especially important when transferring heat to or from a gas as gasses have extremely low heat transfer coefficients (meaning that large amounts of area are required).
Fin-fan heat exchangers are designed to use air to cool process fluids. Think of them as a giant radiator. The process fluid is passed through the coils and a fan helps pull air over the outside surface to promote cooling. These units again must provide a very large surface area to make up for the poor heat transfer of the air.
Aerial coolers are heat exchangers composed of a series of tubes, containing a hot process fluid exposed to air moving across the tubes.
The tubes usually have fins on their outer walls to increase contact surface area. A chamber, called the plenum, directs air across the tubes by means of a motor-driven fan.
The movement of air across the tubes is referred to as draft. There are two types of draft: forced draft and induced draft.
Forced-draft aerial coolers have the fan mounted so that air is blown or pushed across the tubes.
The tubes are designed for at least two passes of the hot fluid through the tubes before leaving the heat exchanger. Many draft coolers have six to eight passes.
Induced-draft aerial coolers have the fan mounted to draw or pull the air across the tubes instead of pushing it.
For certain service, induced-draft is more desirable than forced-draft because it reduces the chance for warm exhaust air being drawn across the tubes.
Typical applications for aerial coolers include: removing heat from compressor cooling water, cooling compressor discharge or inter-stage cooling, and cooling compressor lubricating oil.
Air coolers have the following advantages:-
- An air piping system is not required and there is no limit to the air supply volume.
- There is practically no fouling on the outside surfaces of the tubes.
- They are economical and easily maintained.
One disadvantage is that high ambient temperatures limit cooling.
One of the major costs incurred by the process industries is for unscheduled shut downs. If a piece of processing equipment is shut down and production is lost, the costs can be enormous – and far greater than during planned shut downs. Two things are therefore very important to plant managers:
- Ease of maintenance.
Since many process conditions and cooling-water circuits are corrosive, heat exchangers are particularly susceptible to breakdown because of the large surface areas that are exposed. In certain critical cases heat exchangers are mounted in tandem with a reserve unit, constantly on standby. Flow can be redirected instantaneously through the reserve exchanger and the other removed for maintenance without incurring downtime.
Oil & gas production, refinery and petrochemical processes are often subject to fouling and HEs require regular cleaning in order to retain good heat transfer. Shell-and-tube HEs with a removable tube bundle are well suited to this, also making inspection a simpler process. Plate HEs on the other hand must be entirely dismantled, plate by plate, to facilitate cleaning and inspection.
Heat exchangers are often designed with a little thermal overcapacity in case problems occur during operation. This means that in the case of tubular exchangers individual corroded tubes may be plugged thus extending the life of the exchanger, possibly to a scheduled shutdown when a more thorough remedial action can be carried out.
The removable tube bundle allows a great deal of flexibility during scheduled shut downs. If necessary the bundle can be retubed (retaining the existing tube sheet, baffles, tie rods, etc.) before being put back into the original shell.
We shall now briefly cover shell and tube heat exchanger maintenance.
For intrusive work on a heat exchanger a Permit to Work (PTW) is always required.
The permit must be linked to a company Isolation Certificate. The PTW will specify any PPE requirements.
The isolation certificate must ensure process flows to the shell and the tubes are mechanically and electrically isolated, de-pressurised, flushed and vented and if necessary gas and oxygen tested.
The plant history records of the particular exchanger must be checked. The work area must be inspected and the task assessed. A risk assessment must be made and a method statement written.
Before work commences it is important to ensure that all necessary spares are available. All manufacturers drawings must be checked to enable any lifting or craneage requirements to be assessed, safe-working loads must then be adhered to.
Craneage must be arranged in advance as must any scaffold requirements. If the exchanger is lagged this must be removed by the appropriate personnel.
The area should be barriered off and signs posted, especially if the work is overhead.
The work team must briefed by a supervisor before the job commences. The PTW must be read by all relevant personnel and posted at the workplace. All isolations must be checked and a gas tester standing by if specified.
It may be on larger exchangers that hydraulic tensioning equipment may be required to loosen the channel bolts.
Tubes can be fixed to a tube-sheet in any of the following ways:-
Expanded:-On this type of tube attachment the tube is expanded into the tube-sheet using a parallel roller tube expander. Normally, during assembly of the heat exchanger in the factory, automatic torque control expanding equipment is used.Depending on tube size, hand tube expanders may be used during on-site maintenance.
The tubes extend 3mm past the tube-sheet face and are “rolled” over. This type can also use expanded annular collars for additional strength.
This type of tube uses packing. The packing slides over the tube and is compressed by a screwed ferrule. The compression causes the packing to expand and secure the tube within the tube-sheet. The normal practice is to use metallic fibre packing round the tube ends, this allows movement in the tube plate while the other tube end is locked by a ferrule.
This method of fixing is expensive and, because of space required for welds, the distance between the tubes (pitch) is increased. Thus less tubes can be accommodated than with other joints. However, the joint can be leak free. This method is normally applied to ferrous materials only. Maintenance can be expensive because of the additional cost involved in reaming out tubes and re-machining the weld preparation before installing new tubes.
Exchangers subject to fouling or scaling should be cleaned periodically as slight sludge or scale forming on the surface greatly reduces the heat transfer efficiency.
Tubes must be cleaned periodically. If the unit is scheduled for a shutdown, it is a good time to take the exchanger apart for cleaning.l
Circulating hot wash oil or fresh water will often remove sludge or soft deposits from the exchanger. Should this fail, cleaning compounds are available. Hard scale can be removed by circulating a hydrochloric acid solution then washing out with an alkaline solution. High-pressure hydro jetting is now the most common method of cleaning fixed both fixed tube-sheets and non-U tube floating bundles. As a last resort mechanical cleaners such as wire brushes may be used. Great care must be taken when using mechanical aids as these brushes or scrapers may be sharp enough to cut the tube metal.
Pressure and leak testing can be used to check the tubes in a bundle. A pressure test consists of filling the exchanger with liquid or air to a pressure higher than atmospheric pressure. A loss of pressure after a period of time will indicate a leak.
Leaking tubes may be re‑rolled if they are leaking at the tube sheet. If only a few tubes are leaking somewhere in the middle of the tube bundle, they may be plugged. In this case, a tapered metal plug is driven into each end of the tube.
The plugging of a few tubes in a bundle and immediate re-commissioning is not uncommon. This may be repeated until the number of tubes plugged interferes with efficient exchanger operation. Usually no more than 15% of tubes can be plugged.