A typical university campus can have as many as several hundred buildings. While these buildings may vary greatly in terms of type and utilization, they all need thermal energy for space heating, humidification, domestic-water heating, and the like. With most university-campus buildings in close proximity to each other, district steam systems are a viable thermal-energy source, as confirmed by the hundreds of universities across North America that utilize them.

A district steam system consists of at least one steam boiler, a fuel system, a feedwater system, a flue-emissions-control system, a boiler-control system, a condensate-return system, and a steam-distribution system. This article will discuss steam-distribution-system design outside of a boiler plant, focusing on data collection, preliminary engineering, and detailed engineering. A university replacing building- dedicated steam boilers with a campuswide district steam system will be used as an example.

Data Collection

The long-term success of a district steam system is dependant upon the gathering of all relevant site physical data (site survey, soils report, environmental testing, current steam usage) and owner requirements (future steam usage, steam requirements).

Site survey. For a district steam system, hiring a state-licensed land surveyor to develop survey drawings is essential. These drawings must include:

• Topography profiles, with contours every 2 ft of elevation, for the entire steam path.
• Building locations, with main- and basement-floor elevations.
• All roads and walkways.
• All road and utility easements.
• All underground utilities (water, sewer, electrical, teledata, steam, etc.). The surveyor's contract should include pot holing so that exact utility inverts can be determined.
• All underground man-made obstructions (culverts, tunnels, mines).
• All power/telephone poles, street lights, signs, and monuments.
• Grave sites.
• Historical and archaeologically significant sites. * Trees and shrubs.
• Underground springs or streams.

Soils report. In plotting a steam-distribution-piping path, a good understanding of subsurface soil conditions (rock, silt, sand, previously disturbed earth, water level) is vitally important. With a couple thousand of extra dollars spent on soil borings--and the subsequent moving of a planned steam-distribution path by 30 or 40 ft or the raising of buried steam-distribution piping by a foot--the expense of several hundred thousand dollars on the blasting of rock, the replacement of bad fill material, or construction dewatering can be avoided.

In addition to information on subsoil conditions, a soils report should provide information on soil compressive strength.

Environmental testing. Most mechanical-engineering firms do not perform environmental testing, their liability insurance not covering environmental claims. Therefore, universities are encouraged to hire an independent environmental consulting firm for all environmental testing and remediation-plan development. Designers should discuss with these firms a campus' history. Does a proposed steam-distribution path go through an area from which a chemistry building or boiler fuel-oil tank was removed? Does the soil need to be tested for contaminants? Does existing steam piping have asbestos insulation or fittings?

Current steam usage. Utilize available steam-flow data. If individual-building steam-flow data is not available, install temporary flow sensors. If temporary flow sensors are impractical, the university's facility department may have steam-flow values it wants used, or it might be necessary to perform heating-load calculations to determine each building's maximum steam-flow requirement.

Future steam usage. Most universities have a 10-, 20-, or 30-year strategic capital plan noting the location, size, and utilization of future facilities. Based on existing-building steam usage, determine future-building steam usage on a square-foot basis. Beyond that, determine with the university the excess steam capacity. (Typically, excess steam capacity is in the 10-to-30-percent range.) Once all steam usage is determined, develop a table with building name, maximum steam usage (pounds per hour), and required steam pressure (Table 1). Note any maximum steam usage that does not occur during normal high-steam-usage months (December, January, and February). Do this on a monthly or quarterly basis.

As can be seen in Table 1, maximum steam usage on our example campus is 258,500 lb per hour.

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