nooter eriksen hrsg advantages

7/28/2019 Nooter Eriksen HRSG Advantages

1/3

The Product Page 1 of 3

http://www.nootereriksen.com/hrsgs_advantages.cfm 2001/06/20

NATURAL CIRCULATION TECHNOLOGY

While in Europe and the Far East most HRSGs have assisted circulation, in North America nearly allHRSGs use natural circulation. Up till recently, the choice of whether to use a natural or assistedcirculation design has generally been determined by the end-user's past experience. Today, the trend istoward natural circulation as the most cost effective and reliable design.

True natural circulation HRSGs are usually configured for a horizontal exhaust gas path with the heatexchanger tube banks arranged vertically. The vertical tubes are usually top supported to provide forunrestricted downward expansion. Circulation of the water-steam mixture in the evaporators is achievedby the natural buoyant forces of the water-steam mixture in the tubes. Circulation ratios vary from 8:1to 15:1. The external "cold" casing is internally insulated and lined with a liner system that allows forfree thermal growth. The exhaust stack is self-supporting.

Assisted circulation HRSGs (also called forced circulation) usually are configured for a vertical exhaustgas path with the heat exchanger tube banks arranged horizontally. The horizontal tubes are supportedby drilled tubesheets, hung from the steel structure which also supports the boiler casing. Circulation ofthe water-steam mixture in the evaporators is achieved by circulation pumps at ground level. Circulation

ratios are usually less than 3:1 to 5:1, although some suppliers go as low as 1.5:1. The "hot" casing isusually externally insulated and lagged although some assisted circulation suppliers use an internallyinsulated casing. The exhaust stack is located above the tube banks.

The key differences between the two designs are discussed below:.

Thermal Performance:

The thermal performance of natural and assisted circulation HRSGs should be similar. Each can bedesigned to the same pinch and approach temperatures.

Horizontal Tubes:

Forced circulation boilers normally use horizontal tubes. Horizontal evaporator tubes have the potentialfor "dry out" at the top of the tubes if the wall is not continuously wetted. Solids and/or harmfulchemicals can accumulate at this point and cause either overheating of the tube or corrosion. Vertical orinclined tubes in natural circulation boilers are more uniformly wetted and less susceptible to "dry out".

Pumps:

Assisted circulation boilers require pumps to overcome the resistance in the tubes and circulate thesteam-water mixture in the evaporators. These consume power, require maintenance and decrease thesystem's reliability. To increase the reliability, a minimum of two pumps are installed and ongoingmaintenance is required. Circulating pumps are not necessary in a natural circulation boiler.

Physical Arrangement:

From the diagrams below it can be seen that assisted and natural circulation HRSGs usually occupysimilar plot plans. Even though the tube bundles are stacked above one another on the assistedcirculation design, the long tube lengths, outboard structural steel, pumps and piping take up anequivalent area.

Natural vs. Assisted Circulation


2/3



In applications where visual impact or height restrictions apply, natural circulation HRSGs present alower profile. Apart from contributing to a reduction in far field noise levels, this also helps wheresevere seismic loads apply.

If exhaust stacks of multiple HRSGs are to be collected to a common area, it is generally easier to routeand support the ductwork at ground level than up in the air above an assisted circulation HRSGs.

Operational Characteristics and Start-up:

The rate at which a HRSG is started up and pressurized is dictated by the thickness of the steam drumfor either natural or assisted circulation HRSGs.

Natural circulation HRSGs may respond to steam pressure changes a bit slower than assisted circulationHRSGs, due to the greater mass of water, but this is generally viewed as an advantage because itresults in higher operational stability.

In the event of a non-uniform gas flow, gas temperature or heat flux, the circuits in a natural circulationHRSG receiving the highest heat flux generate the most steam. In an assisted circulation HRSG these

circuits will receive the lowest water flow, because each circuit has a constant pump head available. Thebuoyant forces in a natural circulation boiler are greatest in these tubes, since the higher steamgeneration provides a greater static head. The water flow in a natural circulation boiler is thus greatestin areas where it is needed most.

Auxiliary Equipment:

Natural circulation HRSGs are more suitable for supplemental firing due to the higher circulation ratiosand their ability to adapt to non-uniform heat flux.

Horizontal natural circulation HRSGs can also easily accommodate the inclusion of catalyst systems. Forassisted circulation designs, the vertical support system makes it more difficult to install a catalyst

system between the tube bundles.

Economizer Steaming:

The water in assisted circulation HRSGs flows downward through the horizontal tubes in a serpentinefashion. As a result any steam bubbles generated, during start-up or a load swing, want to flow upwardagainst the downward water flow. Vertical tube natural circulation HRSGs are usually arranged so thatany bubbles generated in the hottest tubes of the economizer will be freely vented to the steam drum.In the assisted circulation design, this issue can be addressed by placing the feedwater control valvedownstream of the economizer coil, however, this adds cost to the economizer and the feedwatercontrol valve.

Casing:

The casing is unrelated to the circulation but a distinction is generally found between the two designconcepts. Externally insulated casing (hot casing) has typically been used on assisted circulation designsand internally insulated casing (cold casing) on natural circulation designs. With a vertical gas flowHRSG, the hot casing can be hung from the support steel with the interface at a low gas temperature;whereas, with a horizontal HRSG it is difficult to separate hot casing from the support steel at ambienttemperatures.

Care must be taken on internally insulated designs to ensure that the liner system is suitable for high


3/3



turbulence areas and allows for free thermal growth. Externally insulated designs usually require higheralloy materials for high exhaust gas temperature areas. Care must be taken on hot casing designs toensure that there are allowances for thermal expansion. In addition, areas with adjacent thick and thinsections will expand at different rates and fail.

Summary:

Both natural circulation and assisted circulation designs are proven technologies although assistedcirculation designs have a more limited range of application. Natural circulation HRSGs are morereliable, more stable, have lower maintenance costs and are less expensive to supply and erect.

nooter eriksen hrsg advantages

Documents