ASHRAE Journal - July 2012 - 22

along with a re-optimized piping layout, Maximum RCL Factor for Net System Refrigerant “Critical” perhaps the total system charge will now Example R-410A from Std 15 Concentration Limit Room Size (ft3) (lb/1,000 ft3) (lb) be in compliance. 2. Another allowable “fix” would be 1a – Commercial 864 26 22.4 to use one common ducted evaporaOffice Building tor to condition the two smallest rooms 1b* 1,296 26 33.7 together, allowing both rooms to be summed for purpose of compliance. 2 – Hotel 2,516 26 65.4 3. In lieu of an in-room console unit, 3 – Hospital 1,700 13 22.1 another option is to use an above-ceiling evaporator unit ducted to one or *Similar to Example 1a with the smallest room removed from the variable refrigerant flow system. more of the smaller rooms, while drawing unducted return air through the Table 1: Summary of three example calculations. ceiling cavity. Therefore, the ceiling void space could now be counted as part of the room volume Since VRF evaporator units are often 100% recirculating (§7.3.2.2). types, a separate system for delivering outdoor air, such as a 4. Finally, one could use two completely separate VRF sys- dedicated outdoor air system (DOAS) is often provided. An tems: one for each side of the corridor, and avoid the corridor example is a hotel room continuously ventilated with outdoor altogether, particularly if the corridor is a means of egress. air delivered to the sleeping area and exhaust air in the bathroom. Can the parallel ventilation system be relied upon for Example 2: Hotel Guest Rooms dilution of a hypothetical refrigerant leak? Formal InterpreNext, imagine a guest room floor of a hotel featuring a se- tation IC 15-2007-38 stated that increasing the allowable reries of identical units of 350 ft2 (32.5 m2) with an 8.5 ft (2.6 frigerant limits for R-410A due to dilution by supply and/or m) ceiling, for a volume of 2,975 ft3 (84.2 m3). A VRF system exhaust air ventilation should not be considered. using R-410A is proposed, with a 1 ton (3.5 kW) evaporator per guest unit. What is the maximum refrigerant capacity per- Example 3: Hospital Patient Rooms mitted in one VRF network? The RCL is 26 lb/1,000 ft3 (420 Finally, consider the application of VRF to the in-patient room g/m3) for R-410A in a Residential occupancy. Therefore, the wing of a hospital. At first glance, this may seem to be very maximum permissible total refrigerant charge is 77.3 lb (35.1 similar to the previous hotel room example. However, recall kg) including the condensing unit, all evaporator units, and the that for Institutional occupancies, the RCL is reduced by 50%. field refrigerant piping. A typical hospital patient room (again excluding bathroom as However, system designers should consider the ramifica- for the hotel) may be 200 ft2 (18.6 m2) with an 8.5 ft (2.6 m) tions of the bathroom in a typical hotel guest room. Some hotel ceiling, for a volume of 1,700 ft3 (48.1 m3). A VRF system usguests close the door of their bathroom before sleeping. With ing R-410A is proposed, with one 0.75 ton (2.6 kW) evaporathe bathroom door closed, the volume of space to which the re- tor per guest unit. Now the RCL is 13 lb/1,000 ft3 (210 g/m3) frigerant may disperse now would exclude the bathroom. For- and the maximum permissible total system refrigerant charge mal Interpretation IC 15-2007-28 implies that the bathroom of is 22.1 lb (10.0 kg). Again, assuming a refrigerant charge of 4.5 a typical hotel guest room should not be counted in the room lb per ton (0.6 kg/kW), the refrigerant charge limit would allow volume calculation. If, for example, the bathroom is 54 ft2 (5.0 a 4.9 ton (16.6 kW) system, or a maximum of 6 rooms with a m2), the effective volume of the guest room proper is reduced 0.75 ton (2.6 kW) capacity requirement per room. to 2,516 ft3 (71.2 m3). In this case, the maximum permissible total system refrigerant charge is 65.4 lb (29.7 kg). Ambiguities How many of these guest rooms can be networked on one A key component of the previous example calculations VRF system? For illustrative purposes only, let us assume is the determination of the volume of the smallest occupied an R-410A refrigerant charge of 4.5 lb per ton (0.6 kg/kW). space not connected to other spaces through permanent openIn the example above, a 65.4 lb (29.7 kg) refrigerant charge ings (§7.3.1). If two or more rooms are connected by permalimit would allow a 14 ton (49 kW) system, or a maximum nent openings, the volume of those rooms may be combined to of 14 rooms with a 1 ton (3.5 kW) capacity requirement per find the RCL. A potential ambiguity exists in the evaluation of guest room. So a VRF system in this hypothetical hotel would what constitutes a permanent opening. Does an undercut door meet the RCL limit, as long as guest rooms are grouped into or a transfer opening qualify? If so, how large an undercut or separate VRF systems of not more than 14 rooms/each. The transfer opening would be needed? These questions are not actual capacity, room size, load calculation, refrigerant type specifically addressed in Standard 15. and refrigerant charge of each system must be evaluated on a Clearly, undercut doors or transfer openings would eventucase-by-case basis and may differ from this example, but these ally permit a large leak of refrigerant in one small room to disfigures lend some order-of-magnitude to the discussion. perse to adjacent rooms. However, without detailed study or
22 ASHRAE Journal ashrae.org July 2012

ASHRAE Journal - July 2012

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