Exchangers, Coolers, Furnaces & Boilers
Heat is the primary driving force of any processing plant, it is used to initiate and maintain the chemical reactions that enable raw products to be processed into refined products.
Heat is either produced directly by Furnaces or Boilers or as a chemical reaction of another process. Whichever way it is produced it is a valuable resource and must be managed, conserved and recovered in the most efficient and economic way possible. This is usually achieved by heat transfer or exchange between fluids at different temperatures.
While the term heat exchanger could be applied to any of the equipment named above, it is usually taken to mean heat exchange between liquids. The most common form of liquid heat exchanger is the Shell & Tube Exchanger.
The exchanger consists of a cylinder (Shell) usually no more than 1.5 meters diameter and no more than 10 meters long. At one or both ends of the Shell there is a Channel bolted to the shell flange. The Channel contains the tube sheet through which the tubes are connected, the tubes run the length of the shell and either connect with rear Channel (single pass) or U turn back to the single front Channel (multi pass) which is the most common form. The many combinations of Shell and Channel can be found in the Tubular Exchanger Manufacturers Association (TEMA) documentation
For multi pass the Channel contains horizontal compartments to separate the inlet from the outlet liquid. For a 2 pass there will be two compartments for more pass’ there will be more compartments to force the liquid back down the U tubes until it reaches the outlet nozzle compartment. The Shell also has vertical baffles along its length, they have alternate gaps top and bottom to force the Shell liquid up and down for maximum contact with the tube liquid as it travels the length of the Shell. The Shell fluid usually only has one pass but can have two. Using the TEMA chart above the most common 2 pass configuration described here would be AEU
Whenever possible the two liquids should always flow in opposite directions* for maximum heat exchange efficiency. While the P&ID will show which liquid is ‘Tube Side’ and which is ‘Shell Side’, it is often the Piping designer who will chose the actual location of the nozzles. The higher temperature fluid, often steam, will usually be tube side and the lower temperature shell side but this is the choice of the process/mechanical engineer and can vary.
Inlet and Outlet of both Tube and Shell nozzles are always top and bottom (not side). So in a multi pass exchanger as shown above, if you were to chose the Tube inlet nozzle to be on the bottom of the Channel then the Shell inlet will be on the top towards the Channel end of the Shell, while the shell outlet will be on the bottom of the shell at the opposite end. The Tube outlet is obviously on the top of the Channel.
If any of these locations change then the others must also change to maintain opposing flow between the shell and tube liquids.*
For instance if the tube side fluid is steam the flow will always be from the top channel nozzle down to condensate out at the bottom channel nozzle
The layout of Shell & Tube Exchangers is critical as their maintenance and cleaning is time and labour intensive. When placed at grade exchangers are often laid out in groups along the side of a pipe rack with their Channels facing out.
All multi pass exchangers are designed so that their Channel can be unbolted and the whole ‘tube bundle’ removed from the Shell for cleaning. (Single pass double channel exchanges usually have a fixed tube sheet that cannot be removed and are cleaned in situ) This means that the Piping designer must model a cylindrical reserved space envelope in front of each exchanger to ensure that no other designer places any object that would impede tube removal or steam cleaning lances.
As the Channel needs to be unbolted it is important to not only provide removable tube side piping spools but to also ensure that the non removable tube side piping is well away from the Channel flange. This is often achieved by angling the tube inlet or outlet piping back at forty five degrees before dropping or rising past the side of the exchanger.
Like all horizontal cylindrical equipment, exchangers are always supported by two saddles. It is the Piping designers responsibility to decide which end is the fixed or anchored saddle and which is sliding. When exchangers are adjacent to a Pipe rack the anchored , fixed saddle is always closest to the rack and opposite end to the Channel end.
This is to allow the Shell to expand in the same direction as the Piping running parallel to the exchanger from the rack rather than the Shell opposing the expansion of the piping **
Sometimes process conditions will dictate that the exchanger or group of exchangers must be elevated above grade. The Piping designer will have to lay out an exchanger structure to accommodate the elevated and possibly stacked exchanger group. At this point it is important to know or to ask what the client’s maintenance philosophy will be. Does the client intend to use a crane mounted tube pulling device or should you the designer allow tube pulling space in your exchanger structure layout?
There is one variation on the normal Shell & Tube exchanger called a Kettle Re-boiler, AKU. The only physical difference is that rather than the Shell being a straight cylinder after the Channel the Re-Boiler Shell expands eccentrically upwards to create a vapour collection space above the straight tube bundle below. Kettle Re-Boilers are usual used at the bottom of fractionation towers to ‘re boil’ the tower bottoms liquid back to a vapour to be re-circulated to the bottom of the tower. They are different in that the exchanger shell side usually remains liquid whereas re-boilers create a shell side vapour with steam being the tube side heating fluid.
While Plate Exchangers are thermally efficient, smaller and easier to maintain they cannot equal the output of a large Shell & Tube exchanger, are more expensive than and therefore not as common as Shell & Tube.
They consist of a series of plates which are contoured on both sides so that when they are bolted together they form 2 unconnected but adjacent liquid pathways which are the equivalents of shell side and tube side.
They are rectangular in shape usually are supplied in a support frame which incorporates a mono rail for sliding and separating the plates for cleaning.
They do not therefore require the large reserved space maintenance envelope of the Shell and Tube, but removable spools need to be supplied on the movable plate nozzles to allow separation and movement of the plates.
Hairpin or U Tube Exchangers
Hairpin Exchangers are a very basic and simple form of Shell and Tube exchanger usually consisting of a single tube inside a shell in a vertical U configuration. The outer shell is rarely larger than 300mm diameter with the tube150mm diameter max.
They do not have a Channel as such, but flanges for disassembly, the tube nozzles are horizontal one above each other at the front with the shell nozzles vertically top and bottom just behind. Flow direction of tube side and shell side liquids are still in opposite directions
They have very low duty and are fairly uncommon.
*Where multiple exchangers are stacked vertically and connected in series it is not possible to consistently maintain these shell side nozzle locations. The shell outlet of the lower exchanger must match the shell inlet of the upper while keeping all the Channel nozzles inline vertically.
** An exception to this anchoring philosophy is where cooling water is the tube side liquid and the large diameter (36”+) cooling water mains are run underground because they are too large and heavy for the rack. In this case the cooling water flow and return branches leaving the ground adjacent to the Channel nozzles create very rigid connections and therefore the anchored, fixed end should be closest to the Channel.
The term cooler is usually used to describe equipment that uses ambient air to cool a process liquid. Because the heat is lost to the atmosphere they are not an efficient form of heat exchange but are used when the high cooling load required cannot practically use any other form of heat exchange.
The most common type are Aerial Coolers, they consist of multiple bays often mounted above the pipe rack to save plant space at grade. If this is the case it is important that the steel columns of the pipe rack are fireproofed. The pipe rack is supporting a large equipment load in addition to the piping and the collapse of a rack supporting coolers can cause considerable damage in the event of a major refinery fire
The length of each bay is approximately the same as width as the pipe rack and consists of a single or double fan to move air across horizontal finned tubes containing the process fluid to be cooled, the tubes are fed from header boxes which run the full width of the each bay on both sides. Each bay is usually between 3 to 4 meters wide and it is not uncommon to see anything from 10 to 20 bays mounted adjacent to each other and fed in parallel.
The fans can be mounted below the tubes (Forced Draught) or above the tubes (Induced Draught ), this has no influence on inlet or outlet piping configurations
Each header box usually has 1 or more inlet and outlet nozzles, both on the same side for 2 pass tubes, on each side for single pass tubes. The inlet nozzles face up the outlet nozzles face down.
The most important consideration for the piping designer is to ensure equal liquid distribution across the whole bank of 10 to 20 cooler bays. This distribution will usually be shown on the P&ID and is dictated by the process engineer or sometimes the process licensor.
The simplest and most common distribution is to use a single continuous size header for the inlet and another for the outlet with straight branches to each nozzle. To achieve equal distribution the inlet header is run from one end of the cooler bank and the outlet header is run from the other end. This ensures that the cooler bay that receives its inlet liquid first is the last to outlet to the outlet header. This helps to balance the bank.
This method is the most economic and easiest to support although it can lead to some stress engineering problems due to so many nozzles being directly and rigidly connected to the headers.
The most sophisticated method is the Cascade. In this method the full size inlet and outlet headers run above and below to and from the center of the bank nozzles, they then tee vertically, reduce and elbow each way to each half of the bank, they then tee and reduce again to each quarter of the bank. This continues until each pair of bays is feed by a separate tee followed 2 reducers and elbows with connection to the 2 nozzles.
The Cascade method gives better distribution but is obviously very expensive and should only be used if the P&ID specifically requires it. It is difficult to support but does have the advantage of being easier to stress.
There are many configurations between simple and cascade the objective is always to achieve balanced flow to all coolers in the bank.
It is worth discussing a compromise of the two methods with process before deciding on either.
There are many types of Cooling Tower, they can be wet or dry, mechanical or natural draft. The most common type in petrochemical plants are Induced draft, wet, multi cell type with adjustable air inlet louvers, each cell has its own induced fan on top of the cell. Their main support structure is often wood which is both economic and corrosion resistant. Less common are the concrete, natural draft, evaporative, hyperboloid shaped, wet cooling towers often seen on power stations. Both of these types of cooling tower use evaporation to cool the returned condensate or hot water they are very wasteful in terms of heat recovery but are much more economic than aerial or shell and tube heat exchangers.
They both consist of an open top tower with hundreds of internal nozzles which spray hot condensate into a rising ambient air stream. In the hyperboloid type the air naturally rises from inlets around the base and is accelerated by the hyperboloid shape of the tower.
In the multi cell induced draft the air is drawn in from the side louvers by the induced draft fan. In both types the evaporation of the hot condensate results in massive clouds of pure water vapour often mistaken by the media as evidence of air pollution. The resulting cooled liquid droplets fall into the basin below the tower(s) called the Forebay. From there is pumped back to into the boiler feed water system by large pumps taking suction from the Forebay.
From a Piping Design point of view there is very little internal or external piping to be laid out as the Tower is usually a vendor supplied package so there are no particular design parameters to be taken into account other than good pump suction/discharge layout.
From a Plant Layout point of view The Cooling Towers should always be located downwind of the main plant to prevent the clouds of water vapour condensing and possibly freezing on plant structures and equipment.
As opposed to boilers which produce hot water and steam, the term furnace is usually used to describe large gas fired heaters which heat process fluids as part of the with thermal ‘cracking’ cycle in the production of petroleum products.
Furnaces can be single free standing units often circular, but any plant requiring major process heating, such as ethylene production, will usually have a large furnace area consisting of 8 or more furnaces adjacent to its own pipe rack.
These furnaces are usually rectangular and arranged in pairs. As each furnace has its own radiant section each pair is connected above the radiant sections by breaching and share a common convection section and stack.
The radiant section is used to heat the process fluid. As the name implies the process piping coil inside the radiant box receives radiant heat from the multiple burners that are mounted in the floor and walls of the radiant box.
From a piping design point of view connecting the vertical inlet and outlet flanges of the radiant coil is fairly simple and straightforward. From a stress point of view it can be complex. The radiant coil sees very high temperatures and therefore experiences high thermal growth.
The radiant coil(s) consists of vertical risers and drops connected top and bottom by U bends and is arranged to hang vertically from the top and guided at the bottom of the radiant box. The hanger arrangement involves springs or sometimes a complex pulley and weight system outside the box to absorb the vertical coil growth.
The Convection section above the radiant has no burners but uses the waste heat from the radiant section to pre heat the process fluids and also produce steam. This section is sometimes referred to as the Economiser. The process coils are usually at the bottom of the section where the flue gas is hotter, the steam coils at the top where the gas has become relatively cooler
The coils in the convection section are horizontal with many passes connected by external U bends front and back with the external inlet and outlet piping connections at the front. Some of these connections are crossovers from one coil to the next and the confined space between the coils can make this piping complex.
Another problem is that due to the internal temperatures the internal coil materials are more exotic than the external piping specs so a material change is often required at the coil connections. As space is limited there is sometimes no room for flanges so the connection has to made by dissimilar material welds which are undesirable and expensive.
This problem continues up the coil bank. Although the material integrity of each internal coil can be reduced as the flue gas gets cooler the external piping will always require a lesser quality material than the internal.
Finally one of the most challenging areas of piping design on a large furnace is the fuel gas piping to the burners, particularly the floor burners.
For ease of operation the underside of the furnace is usually between 2 and 3 meters from grade with multiple burners projecting down from the underside. The network of fuel gas piping must be accessible for each burner valve adjustment while at the same time cause no obstruction to safe operator access or egress and allow unobstructed view of the many peep holes used by operations to observe and adjust each burner flame.
Any open flame equipment is a potential danger in a petrochemical plant. For this reason a manual quick acting gas shut off valve must be placed on the main fuel gas header a safe distance (at least 50ft.) away from the nearest burner.
The design of the fire suppression hydrant system around furnaces must take this potential asset loss into account.
The primary purpose of a boiler in a petrochemical plant is to produce high pressure superheated steam from treated water to power turbines and to be used as part of the refining process. While steam can also be produced as a by -product of other process’ such as a furnace boilers are usually required to supplement this ‘waste steam’ production. As the steam loses its superheat qualities it is used for other duties such as heating and steam tracing. Once the steam turns to condensate it is recovered, cooled and returned to the boiler feed system.
The water is treated and deaerated to mitigate the effects of corrosion and scaling inside the boiler. It is an expensive process requiring a dedicated water treatment unit and is the justification for recovering and recycling the resulting condensate whenever possible.
The boilers are usually the water tube type consisting of a series of many tubes vertically connecting a ‘mud’ or water drum at the bottom and a steam drum on the top. The boiler shell is often a horizontal cylinder with a horizontal oil or gas burner mounted at the front with the flue stack at the back.
Discharge from the boiler feed pumps enters the boiler via the economiser which, like the convection coils of a furnace, is mounted in the ‘cooler’ flue gas stream before the stack. This takes advantage of ‘waste heat’ and gives some preheating to the boiler feed water.
From the economiser it enters the mud drum and natural thermal circulation circulates the warming water between the mud and steam drums until the steam drum contains saturated steam ( The term saturated steam means pure steam in direct contact with, and at the same temperature as, the liquid water from which it was generated) This steam can still be partially ‘wet’ and contain water droplets which can severally damage turbine blades.
To remove any water droplets from the steam to produce dry superheated steam the saturated steam must pass from the steam drum through the superheater tubes. These tubes are separate from the drum circulation and pass through the relatively hot flue gas stream to raise the steam temperature and pressure to superheat condition.
Piping connections to boilers are relatively simple. Primary connections are boiler fed water inlet and superheat steam outlet, no special requirements other than normal good practice, support and stress considerations.
Secondary connections are PSV connections, fuel gas or oil and condensate blowdown.
Unlike process PSV’s, which must discharge to a flare header system, boiler PSV’s are only discharging water vapour so they can discharge directly to atmosphere. They should do this as directly as possible while taking sound attenuation and operator location into consideration. The PSV ‘s must be accessible for annual inspection and maintenance but their piping discharges must be remote and away from any area where there is a potential for operator hearing damage. Silencers are often installed in the PSV discharge.
The Boiler vendor will usually provide a fuel gas or oil piping skid containing meters, filters etc. The piping designer only has to locate the skid and pipe up to the inlet.
Even though the boiler feed water is treated the mud drum and internal steam tubes can still build up solids and sludge, this needs to be drained or blown-down intermittently or sometimes constantly. This involves connecting the many small bore (1”-2”) blowdown connections on the underside of the boiler and piping them into a header system that runs to the blowdown sump. As these connections all need access to their angled blowdown valves. Blowdown piping can be the most complex part of boiler piping design.
Once Through Steam Generators
OTSG’s are a simple form of boiler usually used on Steam Assisted Gravity Feed (SAGD) heavy oil recovery projects. They produce low quality wet saturated steam for injection into heavy oil bearing sands for the purpose of heating and emulsifying the oil, enabling it to be pumped to the surface. A SAGD facility will often have 10 or more OTSG’s in parallel.
There are no steam or mud drums in an OTSG. Their cylindrical shells contain only a continuous U bended coil running along the whole of the inside surface of the cylinder with the burner front and center. As their name implies the boiler feed water enters at one end of the coil and leaves as wet steam at the other end after a single pass.
The boiler feed is still treated but as the raw water used is already non potable and poor quality it is not treated to the same degree as in a conventional super heat steam boiler.
The piping connections are no different to a conventional boiler with the exception that if there is a blowdown at all it is likely to be a single connection.