Precision Air-Conditioners is an air-conditioning system which can monitor and control the temperature, humidity within narrow tolerance. It normally has high sensible heat factor exceeding 90%.

Precision air conditioners are different from comfort air conditioners in the following ways:

• High cfm per ton (550 to 600 cfm/Tr) vs comfort air con which is 350 ~ 400 cfm per ton.
• High sensible heat ratio ( > 0.90).
• Suitable for continuous 24 hour and 365 day operation.
• Inbuilt heater and dehumidification strategy to control temperature and humidity within narrow tolerance simultaneously.
• Use of TXV valve in lieu of capillary tube for better refrigerant regulation.
• Microprocessor controls for close monitoring and regulation of temperature and humidity with a user friendly interface which allows high level interfacing.
• Higher efficiency air filtration.

The model name gives the approximate total cooling kW at AHSRAE recommended data centre design condition which is 22°C and 50% RH at standard outdoor air of 35°C, i.e. Unit model VCM07 means the cooling capacity is 7kW.

Air-cooled DX system means the conditioned air shall be cooled directly using refrigerant gas and heat rejection is achieved by air removing heat via air-cooled condensers. Water-cooled DX system means the conditioned air shall be cooled directly using refrigerant gas and heat rejection is achieved by water circulating through water-cooled condensers. Chilled Water PAC means the conditioned air shall be cooled indirectly using chilled water circulating through chilled water cooling coil. The chilled water is normally obtained from building chiller or it can be from standalone mini-chiller.

The suction pressure is in the range of 60-70 psig and the discharge is in the range of 270 to 350 psig.

The typical superheat and sub-cooling is 5°F (-15°C).

It depends on the distance between indoor and outdoor unit and the vertical height different. Please refer to VISIONTEC product OMM.

The low pressure switch is set at 20 psig. The high-pressure switch is set at 400 psig.

The basis of design for estimating a heat load is similar for precision air conditioning and comfort air conditioning. However the following points must be carefully considered and taken into account while making the equipment selection:



a) Inside conditions of temperature and humidity: The recommended inside condition by ASHRAE is 22°C±1°C db and 50% ±5%RH.
b) Outside conditions: Suitable derating of equipment capacity must be considered based on a condensing temperature which is normally 20°F higher than ambient temperature and varies with the ambient temperature.
c) Equipment selection must satisfy the heat load and air quantity requirements. Precision air conditioners have airflow in excess of 550 cfm per ton.
d) Standby requirement: having calculated the heat load, the next step is to decide the capacity and number of air conditioning units to be selected from the standard models available from most manufacturers and accordingly determine the capacity of the standby unit. As an example, if the heat load works out to 75 Kw, one can select three units of 25 Kw as operational plus one unit to work as a standby (commonly referred to as n = 3 + 1) or select two units of 37.5 Kw as operational plus one unit as a standby (referred to as n = 2 + 1). The criticalness of the air-cond area, total price of each selection and the physical constraints at the installation site will finally determine the best choice.

Heat loads should account for the following internal loads in the Server room:

•  equipment loads are normally assumed on the basis of density per sq.ft and it is all sensible heat. It can varies from 15 Watt per sq.ft. for scarcely populated server rack with low heat generating server to 200 Watt Btu/hr per sq.ft. for densely populated server room with high heat generating servers. The most accurate heat estimation is to get the heat rejection load of the IT equipment to be installed. Diversity factor for equipment load is approximately 60-80%.
•  occupancy levels are minimum, normally 2 or 3 persons.
•  lighting loads are normally considered at 1.5 watts per sq.ft.

For greater design flexibility precision units are available either with a top discharge (upflow) for connection to duct distribution system and a blower with external static pressure of 20 to 25 mm or a bottom discharge, (downflow) for underfloor air distribution using a raised floor. The later type is more commonly used in Server rooms and has several advantages such as:

a) No ducting required: The gap between the raised or access floor and the true floor acts as a supply air plenum eliminating ducts and shrinking the project completion time. Incidentally the access/raised floor is installed in any case to simplify electric wiring which is simply laid on cable tray in the raised floor.
b) Natural convection: As the conditioned air is supplied through floor grilles with volume control, the cold air cools the electronic equipment much faster and more efficiently as it moves up, after extracting heat from the equipment. The follows the natural convection path of the air. The warm air is then sucked back from the top of the precision unit, cooled and fed back to the room from the bottom.
c) Even Air Distribution: Air distribution is even and uniform through floor grilles with VCDs (Volume Control Dampers) installed on the false floor tiles.
d) Greater Flexibility: If equipment layout is changed in the future, air distribution can be changed very easily by relocating the floor grilles wherever required.
e) Eliminates Pests: Since the air temperature below the raised floor is between 11-16°C, rats and cockroaches don’t feel welcome and tend to keep away.
f) Lower Fan Static Pressure: Underfloor air distribution requires only 7-10mm E.S.P. against 20-25 in a ducted system, which keeps power consumption down and reduces noise level.