Basic Mechanical Systems

Ben Gurion Airport
Ben Gurion Airport

All buildings generate unwanted heat, whether it be from solar heat gain, or generated by the occupants and equipment on the inside. This heat needs to be removed in order to keep the environment at a suitable temperature for critical equipment and the occupants inside.

The design cooling load (or heat gain) is the amount of heat energy to be removed from a space by the Heating, Ventilation, and Air Conditioning (HVAC) systems to maintain indoor design temperature when worst case outdoor design temperature is being experienced.

Air conditioners are designed to cool the air, but air temperature is influenced by humidity, and that needs to be part of the evaluation when you are sizing an air-conditioning system. Water vapor holds heat and requires energy to remove it.

Electronic equipment such as server racks cannot tolerance fluctuations in moisture and temperature. Maintaining proper humidity is also important to reduce unwanted static electricity in the air.

Data centers and server rooms should be maintained between 40% and 60% RH. Low humidity tends to result in a higher risk of static electricity buildup. It can lead to static electric discharge in a computer room.

Types of Cooling Load

Sensible cooling load is a measurement of the amount of energy that must be removed from, for example, the air inside a building, in order to maintain a certain temperature, regardless of the temperature outside. Also called the dry bulb temperature.

Latent cooling load is a measure of the amount of energy that is necessary to dehumidify the air in a building, for example, regardless of the outdoor humidity. Also called the wet bulb temperature.

Types of HVAC Systems

There are several different individual components that make up an HVAC system. Each component in the system has its own job, and its design is designed for the particular application and efficiency requirements.

AHU, Air Handling Unit

An Air Handling Unit (AHU) takes in hot return air from a conditioned space and supplies conditioned air via the evaporator and is responsible for cleaning the air and adjusting for the desired humidity. The AHU is part of the chilled water loop on the inside of a building. An air handler will typically include: Supply dampers, filters (panel or bag style), Coils (heating/cooling), humidifiers, supply fan, return fan, and exhaust dampers (controls mix of fresh/recycled air).

CRAC, Computer Room Air Conditioner

The computer room air conditioner (CRAC) is a close-coupled, in-row, chilled water cooling system for efficient cooling in small spaces. These compact units are perfect for small spaces and are able to communicate with Building Management Systems.

CRAC units offer better air filtration, higher airflow, and better humidity control mechanisms than standard air conditioning systems. These units are not designed for comfort, they offer high heat transfer, high velocity air output, and operate 24/7/365.

Air is cooled by blowing over a cooling coil using a direct expansion refrigeration cycle. The cooling coil is filled with refrigerant, which itself is kept cool by compression. The extra heat is ejected using a glycol mix, water or ambient air.

CRAC cooling units can be used to pressurize the spaces below raised floors. They vent out cool air through perforated tiles into the server intakes. After moving through the server, the cool air is pushed out as hot exhaust and returned to the CRAC unit for re-cooling.

Newer models of CRAC units can vary the airflow with the assistance of multistage compressors, but most older models only have on/off capability. This method of cooling works best for small and low-density data centers. CRAC units are ideal for data centers with electrical loads of less than 200 kilowatts (kW) and lower availability requirements.

CRAH, Computer Room Air Handler

The computer room air handler contains cooling coils that are filled with chilled water rather than refrigerant, which typically comes from a separate chiller or chilled water plant. The unit draws in warm air from the computer room, and the air flows over the chilled water coils. Heat transfers from the air to the water, which then returns to the chiller.

Air handlers are generally designed for data centers with electrical loads of 200 kW or more and higher availability requirements. They use a more efficient cooling cycle than CRAC units, and have greater heat removal capabilities, while still having the same energy footprint.

RTU, Rooftop Unit

A rooftop air conditioning unit (RTU) is a type of HVAC system that contains all the components needed to provide conditioned air in one compact unit. These units may be used in human occupied areas and are typically used in light and large commercial applications. They also are very popular with retail and industrial properties.

RTU’s connect directly to a duct-work system that distributes the conditioned air through a space. There is usually a clear, insulated divide between the evaporator coils that cool the return air, and the condenser coils and compressor that eject heat to the atmosphere.

FCU, Fan Coil Unit

Some FCU’s will be heating only, some will be cooling only and others will have both heating and cooling capabilities. The coil heat exchangers will typically utilize a hot and/or chilled water supply which is distributed from the buildings boilers and chillers.

Electrical heaters can be used for heating purposes and some coils use a direct expansion coil fed by a refrigeration system for cooling. The cooling coil will remove the moisture from the air. This condensed liquid will run off the coil and collect in the drip tray at the bottom. A drain line will also need to be connected to drain this water away.

CAV, Constant Air Volume

Constant Air Volume systems a means of delivering conditioned air around a building. These units connect to the branches of duct work, are fairly cheap to install, and are simple to operate. The main disadvantage of the CAV is all rooms in the condition space receive the same temperature air regardless of their heat load.

CAV’s work very well if all the rooms are of a similar condition or require a similar cooling/heating condition, otherwise they can be inefficient. Supply air is often conditioned for the highest heat load, which can result in uneven cooling and the need for localized reheating of air. For this reason, they are not usually found in new construction applications but they may still be present in older buildings.

VAV, Variable Air Volume

Variable air volume systems supply air at a variable temperature and airflow rate from an air handling unit. Air flow sensor measures the change in pressure and changes the flow rate. The unit also connects to a thermostat in the room it is serving.

Because VAV systems can meet varying heating and cooling needs of different building zones, these systems are found in many commercial buildings. Typical components include a controller, actuator, damper which controls the flow. They may also contain a re-heater (electric, boiled water, or heat pump) to compensate for specific zone temperature.

Chiller Systems

In large-scale industrial applications, a centralized cooling system consisting of a chiller and cooling tower is used to improve efficiency. The chiller will usually be located either in the basement or on the roof and this depends on what type of chiller is used. Roof top chillers are usually “Air cooled” whereas basement chillers are usually “Water cooled.”

How a Chiller, Cooling Tower and Air Handling Unit work together by The Engineering Mindset.

Chilled Water Loop

The evaporator of the chiller is where the “chilled water” is generated. The “chilled water” leaves the evaporator and is pushed around the building by the chilled water pump. Glycol is often added into chilled water systems to serve as an antifreeze agent.

The chilled water flows up the height of the building to each floor in pipes known as “risers,” where it branches off into smaller diameter pipes which head to the fan coil units (FCU’s) and Air Handling Units (AHU’s) to provide air conditioning.

The chilled water enters the AHU/FCU and passes through the cooling coil (a series of thin pipes) where it will absorb the heat of the air blowing across. The chilled water heats up and the air blowing across it cools down.

The warm chilled water then heads back to the evaporator a refrigerant will absorb the unwanted heat and move this over to the condenser. The chilled water will then leave cool again, ready to circulate around the building and collect more unwanted heat.

The refrigerant collects the heat from the “chilled water” loop in the evaporator and moves this to the “condenser water” loop at the condenser.

Condenser Water Loop

The condenser of the chiller is where the unwanted heat is collected before being sent to the cooling towers. Once the condenser water has passed through the condenser and picked up the unwanted heat, it will head up to the cooling towers to dump this heat and return cooler ready to collect more heat.

Cooling Tower

The cooling tower is usually located up on the roof and is the final destination for the unwanted heat in the building. The cooling tower contains a large fan which blows air through the unit. The condenser water then collects and heads back to the chillers condenser ready to collect more heat.