Natural versus Forced Circulation HRSGs

Natural versus Forced Circulation HRSGs

In the United States, the trend is to use natural circulation HRSGs, while in Europe, the forced circulation units are the norm. Some thoughts on these designs are outlined in the following text:

1. Natural circulation units do not require a pump for maintaining circulation

through the evaporator tubes. Static head available, steam pressure, and system resistance caused by size and length of downcomer, riser pipes determine the CR. The use of circulation 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 move upward, which is the natural path for them; hence, the tube walls are completely wetted by water (if heat flux is nominal). With horizontal tubes, there is a difference in temperature between the top and bottom portions of the

192 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

(a)

Furnace

Evaporator Burner

(a) Furnace-fired HRSGs in operation. (b) Furnace-fired HRSG plan view. (c) Dual-pressure natural circula- tion HRSG. (Courtesy of Cleaver Brooks, Thomasville, GA.)

Waste Heat Boilers 193

Air out

Water in

Water out Air in

FIGURE 4.23

Pressurized hot air water preheater. (Courtesy of Cleaver Brooks, Thomasville, GA.)

tubes, which could cause thermal fatigue. Also if the selection of streams is not proper and if the velocity of the steam–water mixture is not high enough, the vapor phase can separate from liquid phase leading to steam blanketing and pos- sibly overheating of tubes. This is a possibility when the HRSG is fired and the heat flux inside the tubes is high, particularly when high fin density is used and also if the gas temperature profile at the tube bundle is nonuniform.

3. Natural circulation system with vertical tubes can tolerate higher heat flux gener- ally 50%–80% more than horizontal tube designs. If there is a nonuniformity in gas temperature or heat flux at any cross section, the tube receiving the higher heat flux will have a higher CR due to the greater differential in fluid densities between the more dense fluid in the downcomer circuit and the less dense mix- ture in the evaporator tubes, which is helpful to even out the imbalances. In forced circulation units, the flow resistance of all tubes is nearly the same as well as the flow of mixture and any severe variation in gas flow and temperature at a cross section can cause some tubes to generate more steam and hence be vulnerable to tube failures.

4. Natural circulation units require more real estate than forced circulation units. The floor space can run into several hundred feet if there are multiple modules and catalysts for NO x , CO control. In forced circulation units, the floor space may

be small, but the height of the HRSG will be large requiring more support steel, ladders, and platforms.

5. The horizontal gas flow configuration of natural circulation units provides an easy way for water washing the highly soluble ammonia compounds formed downstream of the selective catalytic reduction (SCR) when operating with a sulfur-containing fuel. When a vertical gas flow unit is water washed, the slush and corrosive products are likely to deposit over the SCR below the low-temperature surfaces.

194 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

6. The economizer water flows downward in a forced circulation unit with gas flow- ing from bottom to top. Hence, if steam bubbles are formed at low loads, there is

a possibility of flow stagnation inside tubes. One way to overcome this is to place the feed water control valve between the economizer and the evaporator so that the operating pressure of the economizer is increased and some approach tem- perature (difference between saturation temperature at that pressure and water exit temperature) is available.

7. The casing of the forced circulation unit is typically hot as it is easier to contain the tube bundles inside the casing and support them. However, this requires exter- nal insulation, and the hot casing requires alloy steel depending on gas tempera- tures. Thermal expansion issues between casing and tube bundles also have to be reviewed.