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HVAC Mechanical Room Safety

Chillers represent large, powerful machines filled with refrigerants. When chillers are placed in confined spaces, care must be exercised to provide safety to the equipment operator and the public at large.

ASHRAE Standard 15, Safety Standard for Refrigeration Systems and ASHRAE Standard 34, Designation and Safety Classification of Refrigerants, provide the designer with an excellent reference when designing a chiller mechanical room. In Canada, CSA–B52, provides similar information.

The following is a brief summary of the safety requirements covered by these documents as they apply to chiller mechanical rooms. This section is by no means a complete review of all requirements covered by these standards. It is recommended that the design have access to the most recent version of these documents. It is also recommended to be familiar with local safety codes that have jurisdiction in the area of construction.

1.0 Standard 15.0

The purpose of Standard 15 is to specify “safe design, construction, installation, and operation of refrigeration systems .” The following is a brief outline of the issues that affect chiller mechanical room design.

1.1 Occupancy Classification

Standard 156 identifies seven occupancy types that consider the ability of the occupants to respond to potential exposure to the refrigerant. An example is Institutional Occupancy where it is anticipated that the occupants may be disabled and not capable of readily leaving the building without assistance. A hospital is an institutional building.

1.2 Refrigeration System Classification

Refrigeration systems are based on how they extract or deliver heat. Chiller plants are considered indirect systems because they cool chilled water, which in turn cools the air. Indirect systems are subsequently subdivided by how the secondary fluid (chilled water) contacts the air stream. Assuming coils are used, the classification is an indirect closed system. If open spray coil systems are used then the classification becomes either indirect open spray system or double indirect spray system.

The refrigeration system classification is used to determine the probability that a refrigeration leak would enter the occupied space. Indirect closed systems such as chiller plants are generally considered Low-Probability systems provided they are either outside the building or in a mechanical room.

1.3 Refrigeration Safety Classification

Standard 15 uses the safety classifications listed in Standard 34. Table 4 of this guide is based on Table 1 from Standard 15. It shows the group, refrigerant name, formula and the minimum quantity of refrigerant allowed in an occupied area. Blends such as R-407C and R-410a are classified based on the worst-case fractionation of the refrigerant.

1.4 Restrictions on Refrigeration Use

The restrictions on where refrigerants can be used are based on results of occupancy, refrigerant system type and refrigeration type. With a high probability system (the refrigerant can enter the occupied space i.e. a VRV system) the maximum refrigerant level is defined in Standard 15. For example, R-123 can only have a concentration of 0.4 lb per 1000 ft³ occupied space. Once these levels are exceeded, the refrigeration equipment must be either outdoors or in a mechanical room. Refrigerant levels involved in chiller plants necessitate mechanical rooms or outdoor equipment.

An interesting issue occurs when an air handling unit that serves occupied spaces is in the chiller mechanical room. If a leak occurs, the refrigerant may be drawn into the air handling unit and circulated through the building. The best solution to this is to avoid air handling units in the chiller mechanical room. This may not be possible in existing buildings. Standard 15 does allow AHUs in the chiller mechanical room if they are sealed.

1.5 Pressure Relief Piping

One area that will involve the designer is pressure relief devices and piping. The pressure relief devices are typically part of the chiller. With field refrigerant piping, additional relief devices may be required. Medium to high-pressure refrigeration systems typically uses re-seating spring-loaded pressure relief valves. Negative pressure chillers often use rupture disks. Rupture disks are less expensive however, if they burst, the entire charge will be lost. Spring-loaded pressure relief valves will re-seat as soon as the pressure within the refrigeration system drops to a safe level. For negative pressure chillers, it is recommended that reseating pressure relief valves be used in addition to rupture disks for additional protection.

Pressure relief devices and purge unit discharges must be piped to the outdoors. The location must not be less than 15 ft above grade or 20 ft from a window, ventilation opening or doorway and the line size shall be at least the discharge size of the pressure relief device or fusible plug.

Multiple relief devices can be connected to a common header. The header size must be at least the sum of the discharge areas of the connected devices and designed to accommodate the pressure drop. Many chiller application catalogues provide tables for sizing relief piping.

ASHRAE Standard 15 also includes tables for sizing relief piping. The size of the relief piping is dependent upon the length of the pipe between the pressure relief device and the outdoors. Appendix H of Standard 15 provides equations to determine the allowable length of piping as a function of its diameter.

The allowable equivalent length of relief piping is determined from the following equation:

2.0 Standard 34

Standard 34 lists refrigerants and provides a safety classification as shown in Figure 359. Refer to Standard 34 or to Daikin Applied Application Guide AG 31-007, Refrigerants for further information on common refrigerants and their safety properties.

8 Moody friction factors are available in ASHRAE Standard 15 and in Moody, L.F. (1944) “Friction factors for pipe flow”, Transactions of the ASME 66(8):671-684. 9 ASHRAE, 2010. ANSI/ASHRAE Standard 34-2010, Designation and Safety Classification of Refrigerants. Atlanta, Ga.: ASHRAE

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