Air Conditioner Sizes

Critical cooling load refers to the load in a server room or datacentre environment of the usable cooling capacity. The critical cooling load does not include equipment in support areas outside the datacentre or server room floor.

HVAC is an industry acronym that stands for ‘Heating, Ventilation and Air Conditioning’. These three systems are often combined into one overall system within a datacentre building with specialists known as HVAC engineers. Air conditioning specialist suppliers may also be known as HVAC companies. The aim for an HVAC system is to provide a thermal comfort and acceptable air quality in an economically viable and energy efficient manner.

The equipment within a server cabinet can include servers, switches, routers and rack mount UPS systems. These devices tend to draw air ‘front-to-rear’ as a result of their design and fan operation. A hot-aisle/cold-aisle containment is an arrangement of server cabinets so that the front of the cabinets face each other (cold-aisle). Air conditioned (cool) air is drawn in from the front of the cabinets and exits the server cabinets into a hot-aisle. From the ‘hot-aisle’ the warm air is drawn back into the cooling system for cooling and re-circulation back through the cold-aisle.

The term ‘hydrocarbon refrigerant’ is also known as HC and refers to a family of chemicals that contain only carbon and hydrogen, that are suitable for use as a refrigerant in cooling systems. Typical examples would include:

- Propane (R290)
- Isobutane (R600a)
- The CARE range with zero Open Drip Proof (ODP) and a very low Global Warming Potential (GWP)

Latent Cooling Capacity (LCC) is the amount of energy added or removed from the air in order to increase or reduce the moister content (humidity) during the air conditioning process. Latent Cooling Capacity is measured in Kg/Kg of dry air.

A Precision Air Conditioning (PAC) system is one designed for cooling datacentre and server room environments rather than one designed for general building (homes, commercial offices and retail). Precision Air Conditioning systems typically offer superior design and reliability and have a high ratio of sensible-to-total cooling capacity (COP).

A refrigerant is a fluid used within a refrigeration or cooling system to transfer heat. The refrigerant is circulated and absorbs heat at low temperatures and pressures and transfers this heat at high temperatures and pressures. Refrigerants are typically man-made fluorocarbon compounds but there are natural refrigerants including Ammonia, CO2, hydrocarbons, water and air. Within an air conditioning unit, the term ‘refrigerant charge’ is also used. This is the amount of refrigerant, measured in Kg within the cooling system.

Relative Humidity is the ration of partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a particular temperature. Relative Humidity is shown as a percentage and measured in: rh. In a high temperature area the difference can be as high as 10 or greater leading to potential damage and corrosion of metals, electrical and electronic devices. A typical example would be Singapore and other tropical-type environments where the average relative humidity can be 80% or more. In a datacentre or server room environment it is important to maintain low humidity levels.

R22 is a Hydrochlorofluorocarbon (HCFC) refrigerant used in air conditioners, process chillers and industrial plant cooling systems. R22 is an ozone depleting gas and EU legislation banned its stockholding and system topping up from 1st January 2016. As a result air conditioners using this refrigerant cannot be maintained and must be replaced.

Duty of Standby is a configuration scenario where one cooling system is referred to as the ‘Duty’ system and the other as the ‘Standby’. The choice between which device acts as the Duty is known as the service. In normal operation, the Duty air conditioner is running and the Standby is does not run. This type of configuration can suit server room environments using either single or three phase mains power supplies and where there is a need for redundancy (N+X) in the cooling operation. Server rooms with three phase mains power supplies can install either a three phase air conditioning system or single phase systems with one cooling unit per phase.

SEER stands for Seasonal Energy Efficiency Ratio and is a term used in air conditioning to compare units for their energy efficiency when heating or cooling over an entire year. The measure is driven by the EU’s Energy Related Products (ErP) Directive (the Eco-design Directive) which specifies the minimum Eco-design requirements that manufacturers must integrate into their energy-using products. The new measures reflect a devices true energy consumption as they measure energy efficiency over a complete year including temperature fluctuations and standby periods.

A British Thermal Unit (BTU) is used to measure thermal or heat energy and is the amount of energy needed to raise 1 pound of water 1°F at sea level. BTU/hr is measure of the thermal or heat energy generated per hour and is a measured to used specify air conditioners. The formula for calculating BTU/hr is: Heat Dissipation (Watts) x 3.4192 = BTU/Hr. So if a server has a heat dissipation (output) of 500 Watts the thermal or heat energy generated is 1709.6 BTU/hr.

The terms air conditioner, power inverter and heat pump are sometimes referred to by different air conditioner manufacturers because air conditioners as well as having a cooling function can also provide a heating one. In the cooling industry, a power inverter normally has a higher COP (Coefficient of Performance) and can cope with a longer pipe run as it will generally have a different condenser, more efficient compressor and a bigger coil to dissipate more heat. The term heat pump may also be used as a an air conditioner or power inverter has the ability to cool as well as provide heating. This is a different use of the team ‘heat pump’ than that associated with air-source or ground-source heat pumps.

COP stands for Coefficient of Performance and is sometimes referred to as the CP or CoP and can be used to refer to the ratio of useful heating or cooling provided compared to the output required. The higher the COP, the higher the efficiency and the lower the operating costs. COP is a measure used for heat pumps, refrigerators and air conditioners.

The IT servers used within server rooms, datacentres and IT networks generate heat. The amount of heat an IT server generates depends upon the level of utilisation (loading) and the energy efficiency of the server design. Within a confined space, such as a server rack cabinet, heat levels can rise to damaging levels and present a fire risk. Typical server racks can see power demands of up to 30kW and the need to cool or dissipate up to 5kW or more of excess heat. Air conditioners and cooling systems are used to reduce the heat within server racks and also the room in which the rack cabinets are installed.

Split air conditioner systems consist of two parts: the indoor unit includes the cool air handling assemblies and supplies temperature controlled air into the room, whilst the outdoor unit includes the hot coils and condenser assemblies. This type of ‘split’ arrangement reduces the size of the indoor unit, lowers the audible noise within the room and can reduce pipe installation costs where the outdoor unit is installed close or on the outside wall to the indoor cooling unit.

The Energy Performance of Buildings Directive(EPBD) is the EU’s (European Union) main legislative instrument which aims to promote the improvement of the energy performance of buildings within the EC.  The Energy Performance of Buildings Directive is designed to help reduce carbon emissions from buildings. One of its articles states all air conditioning systems over 12kW should be inspected regularly (at least every five years). These inspections will highlight ways to reduce carbon  emissions and may also reduce running costs.

The 2014 EU Regulation for fluorinated greenhouse gas (F-Gas) usage, is intended to reduce emissions of F-Gasses which are used as refrigerants in some air conditioning units. The gasses in question include R134a, R407C and R410A which all come under the regulation. The objective is to ensure their containment by raising professionalism during installation, operation and maintenance and good record keeping. The regulation also covers the phasing out of HFCs and product bans.

The optimum temperature for a server room is 20-21°C (68-71°F). This provides a comfortable environment for technicians and engineers to work in and one that can help to maximise the electronic and electrical equipment within the room including the servers, UPS systems and air conditioners.

Server room air conditioners are typically sized in kW (kilo-watts) or BTU (British Thermal Units). To quickly calculate the BTU for a server room, multiply the room dimensions in feet by 5. So for a small server room that is (30×20×16) x 5 = 48,000 BTU. To convert BTU to kWh (kilo-watt hours), multiply the BTU figure by 0.000293. So for this server room the kWh = 48,000 x 0.000293 = 14.06kWh. This calculation does not take into account the room orientation (north or south) and whether there are any windows.

EER stands for Energy Efficiency Ratio and measures the energy efficiency of an air conditioning unit. An air conditioner’s EER is calculated by dividing the cooling capacity of the air conditioning unit (measured in BTU, British Thermal Units) by the power input required (in Watts). The higher the EER ratio, the more energy efficient the air conditioning unit.

Sensible heat is the heat that causes an object to change temperature. When an object is heated the increase in heat is called the ‘sensible heat’. When the temperature of an object falls, the heat removed is called ‘sensible heat’. Latent heat is the heat the heat added to an object in order for it to change state. All natural substances can change state; solids become liquids (ice turns into water) and liquids can turn in gasses (water turns into vapour) when heat is added removed from them. However, latent heat does not affect the temperature of a substance or object. Water for example boils at 100°C and the latent heat keeps the water boiling. Total capacity in an air conditioner is the sum of the sensible and latent heat values. The term ‘sensible capacity’ defines the cooling capacity of an air conditioner, whilst the term ‘latent capacity’ defines the capacity of the cooling unit to remove the moisture from the air.

GWP in air conditioning units stands for Global Warming Potential. GWP is a relative index to quantify the amount of heat trapped in the atmosphere by a greenhouse gas. By definition the GWP of carbon dioxide (CO2) is equal to 1.

DX stands for direct expansion and a DX air conditioning unit uses a condensed refrigerant liquid in an expansion and compression cycle to cool air. In a server room or datacentre the DX unit cools the air coming in through a supply plenum and returns cool air to the area where it is required.

Active cooling is the practice of applying cool air to an area or system or device via an air conditioning or cooling unit. Active cooling can also be referred to as forced cooling or forced convection. The alternative is natural convection or passive cooling.

Adiabatic cooling in a datacentre environment is referred to as ‘free air cooling’. The cooling process is a naturally occurring phenomena for regulating temperature and relies on a change in air pressure and volume expansion to reduce heat. When the pressure on a volume of air is reduced, the air expands and the volume increases. As it does so, energy (heat) within the air volume reduces and so does the heat.

Liquid immersion cooling is a cooling solution for high-density racks in server rooms (40-50kW) and datacentres that relies on heat reduction through the immersion of the server hardware in a dielectric liquid that can conduct the heat. A dielectric liquid for this type of cooling application should have good thermo-physical properties (high conductivity and specific heat), low freezing and burst points, a high atmospheric boiling point, good chemical and thermal stability, high flash and auto-ignition points, be non-corrosive to materials including metals and plastics, have no or minimal regulatory constraints and be economically viable as an alternative to air-based and gas refrigerants.

In an air conditioner, an inverter is used to control compressor motor speed to allow continuous temperature regulation. Without an inverter the air conditioner could only control temperature by either working at full speed or being powered off. An air conditioning unit with an inverter is more energy efficient and can have a longer working life  than a standard AC unit. The outdoor compressor also tends to run quieter.

R32 refrigerant is a Hydrochlorofluorocarbon (HFC) refrigerant and is used as a replacement for R410A in low temperature refrigeration and air conditioning applications. R32 cannot be used as a retrofit refrigerant.