Relevant Systems

Below is a list of HVAC systems that could possibly be used to service a computer service farm's cooling needs. Each system is evaluated based on why it is appropriate for a computer server farm, what its strengths and limitations are, and relevant numerical parameters.

Constant Air Volume - CAV

What is it?

Constant Air Volume (CAV) is a simple system in which the flow rate of supply air is constant, but with varying temperature. This is done to meet the thermal loads of the occupied space. This system is normally small and serves single thermal zones, but with the right variations they can be applied to larger buildings.

Types:

Single Duct
Mixed Air

Limitations:

Inefficient Use of Energy

Affects the efficiency of the system when it supplies the same air flow rate when a slower flow rate would be sufficient.

Oversized Ducts
- Causes increase in design cost as well as higher chances of air leakage during distribution.

Limited Temperature Control for Single Ducts

Need for Additional System

Relevant Parameters

Parameters of this system type rely on size, capacity, target temperature, and overall function of a space. Occupancy, amount of equipment, and room function contribute to the overall cooling load needed for the space. Outdoor conditions can have an effect on the overall conditions inside the space if calculated correctly. The number of air changes per hour that this system is required to perform is determined by the formula:

N = (60*Q)/Volume

Here Q = Flow rate of air, N = Number of times all air in a space is cycled out within an hours time. Another relevant equation to consider is that caused by occupancy and room function and is as follows:

Q = qs*n*CLF

Where Q = Heating Load, qs = Heat gains per person, n = Occupants, CLF = Cooling loads per person. Common loads can be found in the following table.

Furthermore, the following systems must be sized accordingly so that the system can provide an effective function to the space.

Fan Sizing
Duct Sizing
Chiller Selection
Boiler Selection

A more in depth explanation of these parameters can be found here: http://tcastonguay24.wix.com/ae390assignment5#!untitled/c15b1

Appropriate for a Computer Server Farm?

A CAV system would be appropriate for a computer server farm due to it's ability to provide a constant air supply to a space. Furthermore, its ability to vary the temperature being blown into the space allows for constant temperature control of the space. Server farms are typically housed in large rooms, so it would make sense to place a few of these per large space and regulate them to the same temperature in order to cool the space.

Variable Air Volume - VAV

What is it?

Variable Air Volume or VAV is a system that conditions a space by varying the amount of air flow introducted to a space at a constant temperature. A constant temperature of about 55 degrees Fahrenheit is blown through the system at various rates to condition the space in such a way that gains and losses of the space are kept under control.

Components:

Central Cooling Tower
Boiler System
Fan Network
Duct System
Thermostats
Diffusers

Advantages

Precise temperature control.

Maintains constant supply air temperature.
- Reduces energy consumed by fans.
- Lowers building cooling load.
- Increases life span of system.

Humidity control.

Pressure control.

Limitations

Maintenance of comfortable temperatures in spaces that require different heating/cooling loads is limited.

Relevant Parameters

VAV control boxes provide the main size-dependent components of a VAV system. They are based on the area of the specific space in question. Normally located out of sight and hidden above a suspended ceiling. Unit length depends on its overall capacity. Typically units reach about 5ft in length. The table below describes the relationship of maximum airflow versus inlet/outlet sizes in VAV boxes.

Appropriate for a Computer Server Farm?

A VAV system works in the opposite fashion as a CAV system does. This type of system is still relevant for a computer server farm due to it ability to keep the space at a constant temperature. While this system type may not be one hundred percent effective, based on size of the farm, this system may be a viable option depending on the location.

Rooftop Air Handling Unit

What is it?

A rooftop air-handling package unit (AHU) is designed for larger commercial needs and is normally placed on the roof of a structure. This unit regulates and circulates air in order to provide heating, cooling, and ventilation. This system is considered a Primary system, but also has secondary system functionality.

Limitations

Initial Costs can be high due to multiple system parts and repairs.

Operating costs and maintenance can become expensive.

Life span is not as long due to exposure to the elements.
- Life span will vary based on climate of outdoor conditions.

Can be difficult to install in a pre-existing building versus a newly built one.

Large Equipment Loads.
- Supports must be designed to carry large loads.

Units are only capable of servicing 5-stories at a time.
- Multiple units may be necessary based on structure height.

Lack control of temperature to individual spaces.

Relevant Parameters

Rooftop Handling Units are available in numerous different sizes which allow there to be various numeric parameters associated with each individual system. Ranges are used for a majority of parameters where they are deemed appropriate. Of all the parameters the most consistent one is of the fan motor speed which is run at 1800 rpm, typically. The following components are considered for numeric parameters of a Rooftop Package Unit System:

Energy Efficiency
- Seasonal Energy Efficiency Ratio (SEER)
Fan Efficiency
Heat Recovery Efficiency
Dimensions
Weight
Temperature Control
Outdoor Air Temperature Suitability
Heating/Cooling Maximums

A more in depth explanation of these parameters can be found here:

http://kmoyer225.wix.com/ae-390-hvac-variety#!about1/ckqh

Appropriate for a Computer Server Farm?

A Rooftop Package Unit is an appropriate system for a computer server farm due to its ablilty to meet large commercial needs. By being placed on the roof of the structure it provides more space for computer equipment inside the space as well as providing effective cooling over a large area.

Chilled Beam

Chilled beams utilize convection to heat or cool a space in a large building. Beams can be suspended from the ceiling of the space or integrated into the spaced internal structure. Cool water is allowed to flow through the beam system and in turn "chills" the air surrounding the beam. This air then sinks to the floor, which allows the warmer air to rise up and repeat the cycle. Chilled beams are considered a secondary system type.

Types:

Active
Passive

Limitations

Cannot remove particulates from the surrounding air.

Condensation is a constant issue that requires a separate system to be controlled.

Small heating capabilities.

New building system.
- Needs more exposure to become a more commonly used system.

Occupy significant ceiling space.
- Coordination with other systems of the space are necessary.

Relevant Parameters

One of the most important parameters is heating and cooling capacity of the system. This capacity is computed using the following equations for primary and secondary flows:

Equation 1: q = S(nozzle) * u(in)* ρ* Cp * (Tair - Tamb)

Equation 2: Q= S(cross) * ((1/Wbat) * ∫u dx) *ρ* Cp * (Tair - Tamb);

Where S(nozzle) is the total inlet surface of fresh air for 1 m of length and S(cross) is the crossing section for fluid flowing through the battery ( Wbat is battery width and Tamb is ambient temperature).

The table above highlights the physical properties of air and solid materials that are used in relevant calculations to figure out the sizing of a chilled beam system.

A more in depth explanation of these parameters can be found here:

https://sites.google.com/site/assignment5hvac/chilled-beam/chilled-beam

Appropriate for a Computer Server Farm?

This secondary system is appropriate for a computer server farm that is on a smaller scale size wise. This method of cooling is effective, but can only be applied to a computer server farm is used on a small scale due to time needed to process the heat out of the space. This should be considered when considering chilled beams as a relevant HVAC system for a computer server farm.

Active Solar HVAC

Active Solar HVAC systems utilize solar energy to heat a fluid. Some systems can use air, but others use liquid as the primary fluid. This system requires a secondary system, on top of the primary fluid system, to distribute or store the heated fluid, usually some type of storage tank that provides circulation. The primary system is a solar collector which is normally found on the top of the structure. The fluid present on the collector absorbs solar energy and transfers it to the secondary system.

Collector Types:

Flat Plate (Most Common)
Batch (aka Breadbox)
Evacuated Tubes

Subsystems:

Circulation
Control
Storage Tank

Limitations

Climate effects amount of solar energy absorbed.

Backup systems are necessary.

Storage tanks must be kept in room termperature locations.

Tank weight, and potential freezing hazards can be dangerous to the structure they are installed in.

Relevant Parameters

Active solar systems use both direct and diffuse solar radiation to provide HVAC to a building space. Parameter wise the tile of the collector panel must be considered as well as the size of the collector in order to provide optimum efficiency from the unit.

A more in depth explanation of these parameters can be found here:

http://whitesellconstruct.wix.com/ae390group13#!numerical-parameters/c1tam

Appropriate for a Computer Server Farm?

This type of system is not necessarity appropriate for a computer server farm. While the solar energy can be converted into a power source to run the server systems, it does not provide adequate cooling capabilities for the space in question.

Radiant Cooling

What is it?

A radiant cooling system refers to a temperature-controlled surface that cools indoor temperatures by removing sensible heat and where more than half of heat transfer occurs through thermal radiation. When the floor is cooled, it is often referred to as radiant floor cooling; cooling the ceiling is usually done in homes with radiant panels.

Advantages

Suitable for arid climates.

Offers lower energy consumption than conventional cooling systems.

Radiant cooling appears to have lower first costs and lifecycle costs compared to conventional systems.

In the US, on average, savings are in the range of 30% compared to conventional systems.

Limitations

Problematic for homes in more humid climates.

Condensation caused by humidity is a limiting factor for the cooling capacity of a radiant cooling system.

Relevant Parameters

Appropriate for a Computer Server Farm?

This type of system, if applied in a ceiling aspect, would be appropriate for a computer server farm's needs. The cold air sink would allow cooled air to filter its way down into the space and cycle the warmer air out of the space.

Assignment A6

HVAC SELECTION

Relevant Systems