Figures 2.12b AnD 2.12c Show the arrangement of natural and forced circulation HRSGs. In the natural circulation unit the differential head between the cold water in the downcomer circuit and the hotter, less dense mixture in the riser tubes drives the steam-water mixture through the evaporator tubes. The circula­tion ratio (CR), which is discussed in Q7.29, is typically on the order of 8-20 depending on the system, the layout, and the size of downcomers, evaporator tubes, and risers. The forced circulation units are sized for a particular CR, typically 3-6. The circulation pumps provide the additional differential head to ensure flow through the evaporator tubes. The following are some of the features of these types of HRSGs.

1. Natural circulation units do not require a pump for maintaining circulation through the evaporator tubes. The circulation is ensured through natural gravity principles. The use of circulating pumps in forced circulation units involves an operational and maintenance cost, and their failure for some reason such as power outage or pump failure could shut down the HRSG.

2. The water boils inside vertical tubes in natural circulation units, and the steam bubbles formed move upward, which is the natural path for them; hence the tube walls are completely wetted by water. As a result, tube failures are rare, whereas with horizontal tubes there is a

Difference in temperature between the top and bottom portions of the tubes, which could cause thermal fatigue. Also, if the steam-water mixture velocity is not high enough, the vapor can separate from the water inside the horizontal tubes, leading to steam blanketing and possibly overheating the tubes. This is a possibility when the heat flux inside the evaporator tubes is high, for example, in fired conditions, particularly when a high fin density is used for the evaporator tubes.

3. Natural circulation units can tolerate higher heat flux, generally 50­80% more than horizontal tube designs due to the vertical configura­tion of the tubes. Also, in the event of nonuniform gas temperature or heat flux across the cross section (which is often likely due to maldistribution of gas flow), the tube receiving the higher heat flux in a natural circulation unit has a higher circulation ratio or higher steam-water mixture flow. This is due to the greater differential in fluid densities between the more dense fluid in the downcomer circuit and the less dense fluid inside the evaporator tubes, which is helpful and evens out flow imbalances. In a forced circulation unit, all the evaporator tubes receive the same steam-water flow, irrespective of their location, unless special efforts are taken to design the orifice in each tube as in controlled circulation utility boilers. Therefore severe gas-side flow and temperature maldistributions can lead to the possi­bility of tube failures or overheating in some tubes.

4. Natural circulation units require more real estate than forced circulation units, because heating surfaces are laid out one behind the other. The floor space occupied often runs into a few hundred square feet, particularly with multipressure units with catalysts for NOx and CO reduction. In forced circulation units the floor space may be small but the height of the HRSG will be large, requiring a large amount of supporting structural steel, ladders, and platforms.

5. During warm starts, the vertical, readily drainable superheater-reheater arrangement in natural circulation designs eliminates concerns over condensate carryover and impingement on hot headers and piping, which would result in thermal stresses at the headers.

6. The horizontal gas flow configuration of natural circulation HRSG provides an easy way to water wash the highly soluble ammonia compounds formed downstream of the SCR when operating with a sulfur-bearing fuel. A major deficiency of forced circulation or once — through units with their vertical gas path arrangement is the lack of a procedure to water wash deposits from heat transfer surfaces down­stream of the SCR without damage to the SCR catalysts.

7. During start-up and low load periods, steam bubbles generated in the economizer section have to flow down in the counterflow direction in

Once-through and forced circulation units, which is not their natural path. To overcome steaming concerns, the feedwater control is some­times located between the economizer and the evaporator. This increases the design pressure of the economizer. A safety valve is also required at the economizer.

8. The casing design for forced circulation units is typically ‘‘hot,’’ that is, it is insulated on the outside. Hence the designer is required to use alloy steel material for the casing, and one has to evaluate the impact of thermal expansion.

Despite their differences and the pros and cons, all three types of HRSGs are used throughout the world. Selection is generally based on the experience of the plant managers, their consultants, and the end users.