> Below is a cache of http://www.usace.army.mil/publications/armytm/tm5-691/c-7.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
CHAPTER 7 HEATING, VENTILATING, AND AIR-CONDITIONING SYSTEMS

TM 5-691
7-1
CHAPTER 7

HEATING, VENTILATING, AND AIR-CONDITIONING SYSTEMS
__________________________________________________________________

7-1.

General heating, ventilating, and air-conditioning (HVAC) design

The DOD owns and operates many C4ISR facilities across the nation and abroad, ranging from small
computer rooms to large radar facilities. C4ISR facilities often house computer and communications
equipment, radar systems, printers, disk and tape drive, monitors, and system consoles all of which
generate substantial amounts of heat. To optimize performance and ensure continued operation, the
environment containing the electronic equipment must be maintained within stringent temperature and
humidity levels. In addition, the comfort needs of personnel required to operate the equipment must be
met and the airborne particulate matter effectively filtered from entering the C4ISR room airstream. The
electronic equipment and personnel must also be protected from electromagnetic pulse (EMP)
phenomena, biological contamination, and radiation. In the event of a power failure, a backup system
must be available to provide chilled water and cooling air to the water- and air-cooled electronic
equipment, respectively, for at least 15 minutes.

a. The information present here is not intended to replace the design analysis. Cooling systems must
be designed on a case-by-case basis, with consideration given to such factors as cost, location, and
mission criticality of the C4ISR facility.

b. With the rapid advancements in state-of-the-art electronics, existing equipment within C4ISR
facilities is constantly being replaced by new, efficient, and more powerful models. As a result, electronic
equipment with these facilities is continually relocated to enhance the performance of particular facility
functions. In addition to equipment relocation, the volume of equipment inside a C4ISR building often
increases over time. The cooling systems used to maintain environmental conditions with individual
C4ISR rooms must be designed to adapt to these changing circumstances, while still maintaining
adequate filtration, EMP protection, and backup. Conventional, commercially available HVAC
equipment is generally adaptable to use in C4ISR facilities whether aboveground or underground.

c. General guidelines for design of HVAC systems will be in accordance with TM 5-810-1
Mechanical Design Heating, Ventilating, and Air Conditioning. Criteria for the design of HVAC systems
in humid areas are also addressed in TM 5-810-1.

d. Policies, criteria, procedures, and responsibilities for operation, maintenance, repair, and
construction of facilities and systems for the efficient and economical management of Heating,
Ventilating, Air Conditioning & Refrigeration (HVAC&R) utility services shall be in accordance with
Army Regulation (AR) 420-49, Utility Services.

e. All mechanical systems shall have the capability of being remotely controlled from the control
room. Upon a loss of normal electrical power, HVAC systems serving mission critical areas or systems
shall be powered from the emergency generators. HVAC equipment and systems whose loss of power
would impact the facility mission (i.e., 15-minute chilled water backup pump, computer room air-
conditioning units, controls, and Chemical, Biological, and Radiological [CBR] blowers) will be operated
from the UPS system until the facility generators can restore power.
TM 5-691
7-2
f. The design conditions for the indoor and/or outdoor HVAC systems will be in accordance with the
design guidance in TM 5-810-1. Design conditions for electronic equipment areas will deviate from those
required for comfort conditioning only to the extent required to support the equipment to be housed
within the area.


7-2.

Heating, ventilating, and air conditioning (HVAC) equipment reliability and survivability

HVAC equipment installed in C4ISR facilities shall meet the following reliability and survivability goals.

a. Reliability. In mission critical facility applications, mechanical components, such as computer
room type air-conditioning units, chillers, and mechanical pumps, shall have N + 2 redundancy, where N
is the minimum number of units required and 2 is the required number of redundant units. If the mission
is less critical, one extra unit (N + 1) may be adequate. Three factors dictate the level of redundancy
required: (1) critical nature of the mission, (2) equipment reliability, and (3) equipment cost. The
authority having jurisdiction and the design engineer(s) shall assess these factors when determining the
level of redundancy required for a specific C4ISR facility. Redundant units will be designed to
automatically start and maintain the load should the operating unit fail. Due to the high degree of
reliability required for computer cooling systems, redundant components on the control system(s) may be
justified.

b. Survivability. HVAC equipment installed in C4ISR facilities will be of such design or otherwise
protected to withstand seismic effects as well as shock (ground motion) and overpressure effects of
weapons. A detailed dynamic analysis will be made of the supporting structure(s) of the HVAC
equipment to evaluate the magnitude of motion and acceleration established at the mounting points for
each piece of HVAC equipment. Where accelerations exceed the allowable limit of equipment available,
the equipment will be mounted on shock isolation platforms. The design will include, where feasible,
certain features which will enhance the survivability of the HVAC equipment. For example, double inlet
fans and double suction pumps are more likely to withstand shock forces generated by ground motion
with the fan wheel and impeller supported on both sides. Conversely, single inlet fans and pumps with
overhung wheels and impellers should not be used in C4ISR installations unless they are mounted on
shock isolated platforms.

7-3.

Mission critical areas and control room (CR)

Mission critical areas such as command centers, radar installations, and similar areas will generally
contain computers and ancillary equipment which are sensitive to extremes of temperature, humidity, and
the presence of duct. Computer and electronic equipment will deviate from indoor design conditions for
comfort cooling, including temperature, humidity, and level of filtration, to the extent required to support
the computers and equipment housed within the area. The HVAC system supporting the mission critical
areas and systems shall be dedicated to those areas and systems and shall not be shared with other non-
mission areas.

a. Air handling equipment. Computer room air-conditioning units located within the mission critical
areas of the C4ISR facility are typically chilled water type units served by a liquid chilling system. Where
practical, divide the area's cooling load between two or more smaller units to satisfy the required cooling
capacity. This will generally reduce the energy consumption at partial cooling loads and will also
increase overall system reliability. The use of multiple computer room type air-conditioning units (i.e., N
+ 2 rule) provides redundancy required for these facilities.

b. Air distribution systems. A variety of methods can be used to distribute conditioned air in
electronic equipment areas.
TM 5-691
7-3

(1) Most electronic equipment rooms use a raised floor as the underfloor plenum supply air system.
Perforated floor panels are used with the underfloor plenum for providing good flexibility in
accommodating potential relocation of equipment. The air distribution pattern can be modified by simply
exchanging standard floor panels with perforated floor panels to satisfy the changing locations of
concentrated cooling loads. The number and size of supply air outlets in the raised floor should be based
on delivering 80 percent of the total supply air. The remaining 20 percent of the supply air should be
routed to the room via cable cutouts in the raised floor. Raised floors using underfloor plenum and
perforated floor panels can also handle load expansion by accommodating additional air handling units.
Specify supply outlets suitable for installation in floors on which it is anticipated that equipment will be
moved.


(2) Other types of supply air distribution systems consist of ceiling plenum supply and overhead
ducted supply systems. These supply air systems are generally not as flexible as the underfloor plenum
supply system and