VRF AIR CONDITIONING - NEW SECTION   vrf-air-conditioning-img

NEW MASTERSPEC SECTION

Published 1 June 2016

Introducing a new Masterspec section 7628 VRF AIR-CONDITIONING SYSTEM in Services which relates to the design, supply, installation and testing of Variable Refrigerant Flow (VRF) air conditioning.  It includes:

  • 2-pipe and 3-pipe VRF inverter heat pump multi-system air-conditioning plant with nominal cooling capacities greater than 9 kW
  • air cooled and water cooled types including cooling only and heat pump recovery systems

Although most commonly used in commercial situations, VRF can also be used in domestic buildings.

WHAT IS VRF?

Variable Refrigerant Flow (VRF), sometimes known as VRV, is a step up in complexity from standard split unit heat pumps and comes in two systems:

2 Pipe System

Has heating and cooling mode just like a standard heat pump, but generally with one outdoor condensing unit and multiple indoor units (IDU's).  Although heating or cooling is possible it means that all IDU's have to do the same thing at the same time, although some manufacturers have the facility to vary this.

3 Pipe System    3-pipe-vrf-diag

The three pipe system (sometimes referred to as Heat Recovery HR) has a special box that controls the flow of suction gas and hot gas through the evaporator, indoor, coils. By switching from suction gas in the coil to hot gas, the individual IDU's can be switched from cool to heat while other IDU’s remain in cooling mode.  So individual IDU's can be heating, cooling or off, regardless of what the others are doing.

VRF zoning systems are sustainable and cost-effective HVAC solutions. Additional benefits include energy savings, increased comfort, design and installation flexibility, lower maintenance costs, and quiet operation.

DESIGN CONSIDERATIONS

Deciding what HVAC system best suits your application will depend on several variables such as building characteristics; cooling and heating load requirements; peak occurrence; simultaneous heating and cooling requirements; fresh air needs; accessibility requirements; minimum and maximum outdoor temperatures; sustainability; and acoustic characteristics.

VRF ADVANTAGES

Comfort - responds individually to fluctuations in space load conditions, simultaneous cooling and heating, flexibility for zoning, (in particular 3 pipe)

Design Flexibility - single condensing unit can be connected to wide range (types) of IDU's,

Flexible Installation - modular and self contained, enable floor by floor installation and commissioning,

Energy Efficiency - Inverter compressor technology is highly responsive and efficient. The modular arrangement permits staged operation, cooling and heating in a single system which avoids duplicating systems, (in particular 3 pipe).

Energy Savings - Heat recovery, where waste heat removed from cooled rooms is effectively used as a heat source for heating other rooms, (in particular 3 pipe).

Reliability - Continuous operation is possible even if trouble occurs at an individual IDU, similarly for multiple outdoor units.

Retrofit - Particularly good for retrofit situations with restrictive space and different environments, (in particular 3 pipe).

VRF CHALLENGES & LIMITATIONS

Refrigerant Piping - The maximum lengths of refrigerant pipe work for a VRF or any other split system is related to the compressors ability to overcome the pressure drop and for the system to maintain proper oil return. All 'split' systems therefore have a maximum allowable vertical and total refrigeration pipe work length.

Compliance with ANSI/ASHRAE Standards - VRF systems sometimes find it difficult to comply with American standards on refrigeration; especially where a long length of refrigerant piping is involved.  This may be a reflection of the focus of the American standards which tends to be other types of systems not VRF.

Oil Management - As the system has a larger spread, the refrigerant pipes traverse long lengths; hence their pressure testing and protection becomes critical. Long refrigerant piping loops also raise concerns about oil return.

Fresh Air Requirements - Like all split systems, VRF systems do not provide ventilation of their own, so a separate ventilation system is necessary.

Particulate Matter Removal - VRF filter types may not achieve some higher requirements

Environmental Refrigerant Concerns - Not so much of a concern as the new Masterspec section specifies refrigerants R 410A or R407C.  Both these refrigerants are rated very low (0) for ozone depletion, and are relatively low for global warming potential (R407C very slightly lower).  (Source: DuPont Fluorochemicals).  Note; R410A operates at higher pressures than R407C.

 

LINKS AND SOURCES

ASHRAE Journal - Variable refrigerant flow systems

https://www.ashrae.org/File%20Library/docLib/Journal%20Documents/April%202007'/20070327_goetzler.pdf

The Construction Specifier - VRF Zoning Energy-efficiency Data: Five questions specifiers should ask

http://www.constructionspecifier.com/vrf-zoning-energy-efficiency-data-five-questions-specifiers-should-ask/

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