Needed Information

A plant survey requires information in two basic areas: plant operation and boiler room.

Detailed information on plant operation is needed to determine heat balance and required equipment. Particularly difficult to obtain is the actual operating pressure of process equipment. If, for instance, a boiler operating at 150 psig supplies steam to a process with no pressure reduction or temperature control, the operating pressure is 150 psig. If a pressure-reducing valve is included, the operating pressure will be the downstream pressure-relief-valve set pressure. A temperature control located at the steam inlet of the process equipment functions as a variable pressure-control system. Process pressure can vary between full line pressure and negative pressure, depending on the process load, temperature, etc. Sometimes, the only way to determine the average operating pressure of a temperature-controlled process is to log the pressure at the inlet to the trap on the equipment in question.

Also important to know when performing a plant survey is where components are located in relation to one another and the boiler room, the steam consumption of each piece of equipment, and the height of the condensate outlet above the floor. Boiler-room information is a must if new condensate equipment is to be incorporated into an existing operation. Boiler operating pressure, feedwater regulation, hours of operation, and cost of fuel is other information necessary for proper planning of an integrated closed boiler feedwater system.

Figuring Heat Balance

Heat balance is not difficult to figure. What one needs to know is the average steam load and how it is distributed percentagewise to the various users.

Consider, for example, a typical steam load of 50,000 lb per hour (1,500 hp) at an operating pressure of 150 psig, of which 20 percent is lost to process or flashing, 60 percent is returned at full pressure, and 20 percent is returned from a temperature-controlled system with an average pressure of 65 psig. Suppose that all condensate is being saved.

As can be seen in Table 2, the average feedwater temperature is 181.6 F. After the cycle is closed, it is 328.30 F. An increase in feedwater temperature of approximately 10 F results in fuel savings of 1 percent. This can be taken as an increase in boiler capacity or a reduction in firing. For instance, the calculation in Table 2 indicates:

TABLE 2. Calculations for a typical steam system.

• A near-15-percent savings in fuel at 50,000 lb per hour, or, in other words, that the same amount of production could be accomplished at 42,500 lb per hour.
  
  
• An increase in boiler capacity to nearly 59,000 lb per hour.

If the plant wasted all of its condensate to drain (no returns), the total heat lost would be 31.5 percent instead of 15 percent.

Fuel Savings

From the above results, calculating actual dollar savings is easy.
Assuming a cost of 80 cents per therm ($8 MBtu), the savings per hour would be:

Input (British thermal units per hour) × percent savings × cost per therm ÷ 100,000 Btu per therm

Input = output ÷ boiler efficiency

(50,000,000 ÷ 0.80) × 0.15 × 0.80 ÷ 100,000 = $75

Assuming the average 3,000 hr of operation per year, annual savings amount to $225,000.

Summary

Closing a condensate cycle to capture every available British thermal unit typically results in double-digit fuel savings. Proper selection of condensate-system components is vital to the maximization of savings. If applied properly, efficient condensate drainage, along with the continuous removal of gases, can increase production and result in savings beyond calculable fuel savings. Good trapless systems result in faster startup, reduced batch times, hotter surfaces, and measurably improved heat transfer. Review plant heat balance first, as it holds the greatest potential for double-digit fuel savings.

About the Authors

Martin "Mike" Bekedam founded Industrial Steam in 1952. Soon after, he was joined by James F. Williams. Employing a hands-on approach, the pair introduced many innovations to the boiler, deaerator, and condensate-recovery markets, refining designs and systems that remain in service. Bekedam died March 16 at the age of 94. Williams remains with Industrial Steam, playing an integral role in mentoring and teaching.