Consulting-Specifying Engineer - January 2009 - (Page 14) M/E/P Roundtable unit volume associated with each type of refrigerant and where refrigerant piping should not be routed. Design professionals may not fully understand how this standard relates to VRF systems and may misapply the refrigerant codes by not correctly calculating the available exposure volume associated with determining the maximum refrigerant charge per circuit. I have reviewed multiple projects, initially deemed problematic, in which the RCL is ultimately a non-issue. Smith: There has been a variety of scaremongering about refrigerant safety associated with VRV/VRF systems, as the perception is that they offer a greater risk than any other system. In reality, ASHRAE 15 applies to all systems. Also, similarly stringent standards exist in Asia and Europe where VRV/VRF systems are well established and are applied with sound engineering. In addressing ASHRAE 15 specifically, it is important to ensure the refrigerant volume of a system doesn’t exceed the RCL (25lbs/1000cu. ft as defined in ASHRAE Standard 34) in the smallest occupied space. It is also important to review the classification of occupancy and the routing of the pipe work so that it does not hinder occupants’ exit from a building. Roy: The primary design consideration is to ensure the refrigerant charge of the system as a function of the floor area served is within ASHRAE 15 limits. Where the limit becomes an issue, ducting the cooling to a number of common heating/cooling profile spaces can improve the charge-to-sq-ft ratio. Agulles: Certainly, the refrigerant charge for each circuit of the system is something that needs to be looked at on every VR project, but ASHRAE Standard 15 compliance is the same as it would be for any DX system. From a design standpoint, you need to make sure that if there was a refrigerant leak, the concentration of refrigerant in the smallest occupied space, by volume, will not harm anyone. All of the major VR equipment manufacturers use refrigerant R410-A in their equipment, which is an A1 safety group refrigerant, the safest category of refrigerant per ASHRAE Standard 34-2007. CSE : In what applications or scenarios are VR systems probably not a good choice? Brantley: Properly designed VR systems work in most applications. Laboratories and hospital areas with high filtration efficiency requirements and increased ventilation rates, however, are not the best application of this type of system. VR indoor units are designed to utilize minimal energy at a lower external static pressure than typically associated with those types of filters. Roy: Spaces that require extensive outside air requirements are always problematic as the VR systems have limited outside air capability, and having to provide a secondary direct outside air (DOA) system can become prohibitively expensive. Agulles: VR systems start to lose some of their efficiency in designs where long refrigerant piping runs are required. In applications where the condensers cannot be located in relatively close proximity to the indoor units, VR would not be a good fit. Also, like any air-cooled heat pump, the heating capacity and energy efficiency of the system drops off significantly as the outdoor temperature decreases below 40 to 50 F. In heat dominant climates, air-cooled VR systems may not make as much sense. Water-cooled VR systems, however, can be set up the same way as a typical heat pump project, with a boiler to inject heat, as needed, into the condenser water loop to maintain a minimum condenser water temperature. Projects with high ventilation loads and a high percentage of outside air will often require a separate HVAC system to address the ventilation loads. Depending on the project, it may not make sense from a first cost or operational cost standpoint to install the central ventilation system plus a VR system. Smith: VRV/VRF systems have many positive attributes that make them a good solution for many applications; however, there are applications where they might not be the most cost-effective solution. These types of applications include those where zoning is not important, applications where there are large open areas where the part load characteristics of VRV/ VRF are not an advantage, or applications that are not specifically comfort cooling or heating applications and the VRV/VRF system is utilized as the primary solution such as data rooms, etc. (VRV/VRF systems are designed for comfort cooling.) CSE : What about ventilation? Do VR systems require separate ventilation systems to meet ASHRAE 62.1? How are ventilation and VR systems psyc homet rically inte grated? Brantley: VR systems do not automatically require a dedicated outside air system to properly ventilate the zone. VR systems serving low-occupant-density spaces can directly introduce ventilation air to the zone via a unit-mounted intake. VR systems with larger ventilation rates can be interlocked with a total energy (sensible and latent) recovery ventilator (ERV) to provide the needed ventilation air at inlet conditions that a VR system can accommodate. Agulles: For ductless systems, if operable windows cannot be used to meet the space ventilation requirements, then some type of mechanical ventilation system will be required. For ducted systems, ventilation requirements can be met in the same way you might address them with ducted conventional heat pumps or chilled water fan coil units. Outside air can be ducted into a mixed air plenum on the back of each horizontal indoor unit. Check with each individual manufacturer for specific requirements, but a good rule of thumb is to limit the mixed air temperature entering the indoor evaporator units to below 90 F dry bulb. 14 Consulting-Specifying Engineer • JANUARY 2009
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