A. Introduction
Variable Refrigerant Flow Air Conditioning Systems are relatively modern compared to other systems (all-air, all-water, etc.) that have been used for many years. Currently there are several manufacturers, mainly Japanese and Korean, that offer this type of systems to the air conditioning market. The patents for the Variable Refrigerant Flow (CVR) or VRF (Variable Refrigerant Flow), VRV (Variable Refrigerant Volume) systems belong to these multinational companies that have incorporated their advances in electronics and control to this type of air conditioning systems. These systems perfectly meet what is demanded today for an air conditioning system: ease of design, flexibility, energy efficiency, reliability, ease of installation, reduced maintenance, silence, ease of use and environmentally friendly. In recent years there are high growths of this type of air conditioning, as for the use of refrigerant R22 other ecological refrigerants such as R-407 or R 410 have been incorporated.
The parameter or variable that is modified in these systems is the flow or flow of the refrigerant, which is regulated thanks to various technologies in the compressors (Inverter) and the electronic expansion valves (EXV -Electronic Expansion Valve) or pulse modulation valves (PMV- Pulse Motor Valve), incorporated in indoor and outdoor units, in order to adjust the capacity to demand. The idea is none other than to deliver to each indoor unit the refrigerant (cooling power) demanded by the area that airs. In this way it is achieved that the consumption is not the total of the system, but that it is a function of the power that is delivered. Because numerous indoor units can be connected in the same copper pipe circuit through which the refrigerant circulates, they allow to have an important climatic independence, to obtain a wide zoning of the air conditioning system.
These systems are ideal for variable load applications, as the Inverter compressor speed adapts to the variability of the building's thermal load. The main commercial and residential applications are in offices, hotels, commerce, single-family homes, building remodeling, etc.
B. VRF Variable Refrigerant Flow SystemsThe 3 types of systems within the Variable Refrigerant Flow exist on the market:
1. Only cold. The power it delivers is exclusively refrigeration. 2. Heat Pump. All indoor units operate in cold mode or heat mode.3. Heat recovery. It provides cooling and heating simultaneously adapting to the needs of each area. Some indoor units may be providing cold and other heat at the same time, mainly in intermediate times. These systems reduce operating costs by transferring energy through the building.
VRF systems can also be differentiated as follows, according to the refrigerant distribution system:
•Two-pipe system: liquid pipe, cold gas suction pipe and heat discharge pipe. •Three-pipe system: liquid pipe and cold gas suction pipe and heat discharge.
Although there are no very exact data from the market on the percentage of installation of the different types of CVR systems, the most used system is the Heat Pump, approximately between 70% and 80% of the total. It is followed by the Heat Recovery system, perhaps because of its higher initial cost and the possibility of finding a viable technical solution with a Heat Pump system. The Cold Only system is mounted at a percentage of approximately 5%.
C. Classification of Systems
VRF according to a didactic point of view. They could be defined as follows:
Direct expansion systems. This type of system is characterized because energy exchanges are carried out between the external environment and the refrigerant, and between it and the premises to be air-conditioned, without resorting to other fluids for the transport of energy. Direct Expansion (DX) systems have one or more condensing units with finned tube batteries.
•All refrigerant systems.- (Depending on the transport or the secondary circuit, according to ASHRAE classifications) For heat pump and heat recovery systems, it can be said that they are air-to-air systems, when the fluid they use for condensation is the outside air. There are also VRF systems condensed by water (water-air systems), although it is true that the percentage of installation is minimal compared to air-to-air heat pump systems.
•Multisplit Inverter Systems. Several evaporators or indoor units connected refrigerated to a single condenser or outdoor unit.
To achieve adequate ventilation and improve indoor air quality it is necessary to introduce fresh air from outside, also called primary or ventilation air to ventilate the different areas of the building to be air-conditioned, depending on the use of the building, its area and number of people. To do this, MANUFACTURERS OF VRF systems complete the air conditioning system with air-to-air heat exchangers, usually with heat recovery. In this way, 2 primary objectives are met, to ventilate the internal areas and also recover an important part of the energy that is expelled through the exhaust air stream.
Air-to-air ventilation units can be integrated into the air conditioning system, even from a control point of view. When these units are integrated into a heating and cooling system, it decreases the overall size of the HVAC system needed and regenerates some of the energy used to heat the building. For ventilation indoor units can be used in their percentage of allowed air.
D. Description:
Features, advantages, generalities and components.
VRF Variable Refrigerant Flow Systems allow you to connect several indoor units to a single outdoor unit or set of outdoor units by means of a main refrigeration circuit of 2 properly insulated copper pipes. In the case of heat recovery systems, 3 tubes are normally used from the outdoor unit to the distribution boxes or flow selectors and then 2 tubes from these to the indoor units. It can be said that VRF systems are derived from so-called Multi-split systems using Inverter technology and electronic expansion valves to achieve continuous control of the refrigerant flow depending on the demand of the installation. The VRF system individually controls each indoor unit by minimizing operating costs thanks to the Inverter compressor and the electronic control system. Compared to conventional systems that operate on alternating current and regulate the temperature by connecting or turning off the compressor, Inverter technology systems, such as VRFs, are able to vary the current in the compressor from alternating to direct to adjust its capacity to energy demand. With Inverter technology, great energy savings are achieved, thanks to the regulation of the compressor, reduced sound levels, high reliability (the march/stop cycles are reduced), the desired temperature is reached earlier and temperature fluctuations are reduced (greater comfort).
The fundamental elements of the Inverter system are 3: the converter, the inverter and the compressor.
•Converter: component of the Inverter whose specific function is to transform alternating current (AC) into direct current (DC). •Inverter: the fundamental element of the system is an electronic control device located in the outdoor unit that allows to modify the frequency of the current, and in this way, the speed of the compressor. •Compressor: special variable speed compressor. Modulates the ability to adjust to demand.
In the current market you can find systems that combine Inverter compressors with traditional fixed speed compressors and systems that only use Inverter compressors. Likewise, the compressors normally used by manufacturers are Scroll compressors and/or Rotary compressors.
Listed below are a number of features and advantages of VRF systems:
•Maximum zoning. Each user or space has its own control.•Easy design. •Low sound levels. •Energy efficiency and energy saving. High performance and Inverter technology (compressor + electronic valves = capacity adjustment to demand). •Reduced installation space for compact outdoor units. •High flexibility in terms of layouts, system lengths, number of indoor units per system. •Smaller pipe passage spaces. •Reduced operating costs. •Multiple types of indoor units. •Operation in heat mode at lowoutdoor temperatures (te = -15°C). •Versatile control systems (local, central or networked).•Overload factor (simultaneity: possibility of installing outdoor units of lower power than the sum of indoor units (current systems normally allow between 130 to 135% of the capacity of outdoor units in Heat Pump systems).
Among the main disadvantages that are mentioned to VRF systems are:
•High initial cost.•Distribution of refrigerant through a network of copper pipes susceptible to leaks. •In themselves they do not allow humidity control or the option of free-cooling. • Care in the design of the oil recovery, which when diluted with the hot gas is transported through the refrigeration circuit, and solutions must be sought to ensure its return to the compressors.
Due to the wide variety of types of indoor units, there are multiple options for selecting the evaporator unit that can be found in the market:
•Cassette: 1,2 and 4 ways. •Duct: low silhouette, standard and high pressure. •Floor-sky. •Presentation wall. •Floor: with and without furniture. •Vertical floor.
The range of powers of indoor units, allows to cover all needs from 2.2 KW to 14 KW. Some units incorporate condensate pumps as standard. Some manufacturers have high-efficiency filters to improve filtration by standard filters. As for outdoor units, modular or single-block systems can be found In modular systems, by combining different modules or outdoor units, the necessary power is achieved for the projected installation. The different modules are connected to each other by the refrigeration circuit and the oil balancing pipe.
A very important fact that characterizes the outdoor units of VRF systems is the number of pluggable indoor units and that in applications in which there is a varied zoning with very small loads in each space (hotels, hospitals, etc.) this limitation is critical (up to 30 indoor units can be connected). Although condensing units are for the outdoors, they can be installed in internal areas of buildings thanks to the static pressure available from the fans offered by manufacturers. In addition, as an important feature of these systems is that these units can be installed almost glued, leaving a minimum distance of 2cm between them.
E. Compressors
There are manufacturers that offer their condensing units with modulation compressors other than the Inverter, such as Super PC Compressor (Sanyo) or Digital Scroll (Midea), which is the most cutting-edge technology in the refrigeration industry. The following advantages of the MDV-D Digital Scroll System (developed by Copeland) over the Inverter system are indicated below:
•Power regulation range between 10% and 100%. In the inverter system the minimum capacity is 30% (when the interior capacity required in the Inverter system is less than 30%, the system needs several bypasses to modulate the total capacity, which makes it less efficient). •Effective continuous regulation of energy. •Very high service life (few compressor starts and stops). • High efficiency and stability of the refrigerant with the use of high resistance seals. •The compressor operates at constant speed (it does not require a converter and inverter with its controls, which are very expensive) regulating its capacity by periodically opening and closing a bypass valve to control the flow of refrigerant. This control cycle typically lasts 10 to 20 seconds between two valve openings. By changing the ratio between how long the valve remains open and how long it remains closed, you get precise control of the amount of compressed refrigerant. •Continuous modulation of demand and not by steps such as the inverter system (savings of 15% of energy during capacity modulation). • It does not generate electromagnetic interference, unlike the inverter system, which makes it suitable for use in telecommunications, laboratories, etc. •No need for an oil separation and recirculation system. The gas velocity is sufficient to drag the oil along the system when the compressor is in a state of charge, while when the compressor works in vacuum (10%) the oil does not leave the compressor and remains static in the pipes. It has an electronic and mechanical control system to maintain a correct level of oil in the crankcase of the compressor. In this way, the problems of oil return and its complicated network of pipes in the condensing unit of the Inverter system are avoided. •Better dehumidification capacity. •High EER (Eficciency Energy Ratio) in total load (11.5) and partial load.
F. Analysis of Energy Consumption in Buildings (according to Daikin's background)
In a building, energy consumption is distributed approximately in the following percentages:
Air conditioning = 47 % Lighting = 32 % Other services = 21%
•The consumption of centralized systems with chillers is:
Equipment = 42% Pumps = 15% Fans = 43%
•Influence of the size of the installation on the transport of energy•Analysis of energy consumption in buildings. Energy losses from centralized systems•The capacity delivered and the consumption of the installation are not proportional. •Consumption due to heat transport is constant. •The consumption of the equipment is variable and proportional to the capacity delivered.•Energy losses in centralized systems. •Question: How to reduce energy consumption in heat transport? •A more effective heat fluid. •An installation with less inertia. •More personalized, more precise and efficient control.•Comparison of available systems (energy transport)•VAV System (Variable Air Volume)
on Sensitive heat air with differential of
0°C = 2.4 kcal/kgoC x 10°C = 2.4 kcal/kg.
•VWV System (Variable Water Volume) with Differential Ice Cream Water 5°C 1 kcal/ltoC x 5°C = 5 kcal/lt•VRF System (Variable Cooling Volume) with latent heat of evaporation and condensation for R22 = 49 kcal/kg.
It can be observed that the transport capacity of the VRF direct expansion system is about 10 times greater than that of water and 20 times greater than air. In addition, it does not require additional equipment for the transport of energy, such as pumps in the case of water or fans in the case of air, so it does not have additional consumption. On the other hand, being a fluid under pressure, the section of the pipe necessary for the transport of energy will be smaller compared to water and air.
Characteristic values of energy consumption for systems of 200 Tons of cooling are:
•Ice water chiller 1.2 kW/ton x 200 tons = 240 kW•VRF system 0.9 kW/ton x 200 tons = 180 kW
Inverter System Efficiency at partial loads
•HRV (Heat Reclaim Ventilation) heat recovery units
With these units you get:
•Reduction of the load due to the outside air•Reduction of the capacity in the cooling unit (size and consumption)•Heat recovery system (Inverter System 3 pipes – Simultaneous cold & heat)
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