In data centres with a 1-3kW/rack, the most popular IT racks have been 600 mm (24 inches) wide, 1070 mm (42 inches) deep, and 42U tall.  However, most data centres today support a wide variety of IT equipment densities and form factors that require appropriate racks and accessories. For example, in racks housing 5 kW and above, the most popular rack size is no longer optimal because deeper equipment, higher density rack mounted power distribution units (rack PDUs), and increased cable loads crowd the inside of the IT rack.

This is according to a white paper, entitled How to Choose an IT Rack, by Schneider Electric, the specialist the global specialist in energy management and automation.  Another issue that the research highlights, is that information technology (IT) racks are available in three forms; two-post rack, four-post rack, or a cabinet or enclosure.  Two-post racks typically support telecommunications equipment, four-post racks can be used for networking equipment, and cabinets or enclosures support compute and storage equipment.  IT racks not only save floor space by allowing the stacking of equipment, but they also play a role in mounting heavy hardware, providing an organised environment for power distribution, air flow distribution for better cooling performance, network cable management, rack environmental monitoring, security, and so on.  To avoid confusion, we will use the term “IT rack” in this article to refer to the IT rack frame and enclosure.

So what are the key size and feature options for IT racks, and what criteria for selection should you use? According to the research there are several matters to be considered:

Components

IT equipment and accessories are mounted upon the vertical mounting rails, while the frame also provides space to mount some non-IT equipment and accessories to provide power, cooling, and cable management.  The perforated front and rear doors provide access to mount the equipment in a lockable secure environment without limiting the airflow though the IT equipment.  Side panels can optimise the rack airflow and eliminate mixing of air between racks.  The rack roof provides two key functions; one is to protect the IT equipment from falling debris and the other is to provide an entry point for power and network cabling.  Rack roofs should provide wide cable penetrations that allow full cable bundles to pass through.  Also, roofs should have brush strip around cable penetrations to prevent air leakage.  Some rack roofs also serve as a mounting point for overhead cable troughs.  The weight rating of the castors and levelling feet is verified to ensure they can support the specified loading of the rack.   

Decision criteria

Before a rack is selected, some decision criteria should be considered, such as dimensions, operational design, structural design, material, and colour.  Racks are closely tied to the operation of a data centre and as such have a significant effect on how long it takes complete rack-based work orders.  In general, the lowest-cost racks require more time for cable management and mounting which, in large quantities, can have a material impact on operational costs.   

Dimensions

The vast majority of IT equipment has a standard width of 482.6mm (19 inches), including the edges or ears, which allow for mounting in 19-inch racks. The usable vertical mounting grid is often specified in “U”.  1U is equal to 44.45mm (1.75 inches).  If a rack is described to be 42U, it means that there is a physical interior vertical space of 1.87m (73.5 inches) available for equipment mounting. As mentioned prior, the most prevalent IT rack dimensions have been 600 mm (24 inches) wide, 1070 mm (42 inches) deep, and 42U tall, but deeper IT equipment, higher cable densities, and higher power densities are driving the need for deeper, wider and taller IT racks along with a wider array of rack accessories.

In general, networking racks should be 750 mm (29.5 in) wide by 1070 mm (42 in) deep to accommodate the network cabling trunks.  Also, server racks should be 600mm (24 in) wide by 1200 mm (47.2 in) deep to accommodate deeper servers and provide room for cable management at the back of the rack.  When choosing a wide rack, ensure that the rack vendor provides wide vertical rails that prevent cold air leakage and hot air recirculation (see side bar).  Racks should be no taller than the lowest door dimension to simplify transportation and installation.  42U racks should be specified.  Finally, if racks pre-configured with IT equipment are being moved, ensure that the vendor offers shock packaging to avoid damage to the IT equipment and rack during transportation.   

Operational design

In addition to the attributes discussed above, some features increase data centre operations efficiency, such as speed of the deployment and ease of maintenance, amongst others.  These include, but are not limited to, the following:     

  • Tool-less mounting, which decreases time required to assemble doors, side panels, roof, and accessories during installation and maintenance;
  • Time-saving adjustable vertical, tool-less mounting rails;
  • Numbered U positions to assist in mounting IT equipment and also in identifying server locations when creating work orders or an asset database;
  • Half-height removable side panels to make handling easier and safer
  • Pre-installed levelling feet that allow fast adjustment with a cordless screw gun on unlevelled surfaces such as a concrete slab; and
  • Pre-installed castors to help move IT racks without the need for a pallet jack.  

Structural design

Static and dynamic load capacities are critical parameters for the rack frame.  Different manufacturing techniques, such as the formed or folded sheet, tubular sheet with continuous seam welding, or stitch welding yield different capacities.  Each technique has its own strengths and weaknesses.  For example, the folded sheet technique is used in the majority of industrial racks with a relatively low cost and high static load capacity.  However, this technique is not recommended for IT applications because of its weak dynamic load capacity which limits the movement of IT racks around the data centre.  Another example is in the type of welding technique used on seams.  Robot welding is preferred over human welding due to reduced human error and more consistent quality.  Ultimately, the static and dynamic load capacity, the higher the capacity the stronger the rack.  

All rack components (such as the door, frame, mounting rails) should be bonded to ground for human safety.  The simplest way is to connect the components to electrical wires, usually by screws.  Some racks can attain continuous grounding without screws through their structural design, which increases speed of deployment and lowers human-error during installation and maintenance.   

Material

Considering the increase in average rack power density over the years, more IT equipment is mounted in a given rack.  Therefore, a rack’s load bearing capacity should be evaluated and confirmed in advance.  The required load bearing capacity of a rack can be from several tens to thousands of kilograms.  IT racks are commonly made of metallic material, like steel and aluminium depending on cost, strength, and corrosion resistance requirements.  Steel construction represents the majority of racks due to its high strength and low cost.  

Colour

The surface of cabinets or accessories can be finished to suit an end user’s colour requirements such as black, white, grey, or even matching the company’s colours.  Typical colours are black and white.  In general, dirt and touch-up repairs are less noticeable on black racks than white racks, lowering the cleaning lifecycle cost.  White racks tend to fade in colour after about two years with high-temperature airflow.  Differences in white shading are also easier to notice compared to black racks.  However, white racks reflect more light than black racks and provide more contrast to IT equipment and cabling, making it easier to see when working in the rack.  Because of the increased reflection, lighting may be operated at a slightly lower power level.  For lighting that is always on, this may result in a measurable lighting power savings, but is less so when lighting management is used.   

“An IT rack is the foundation of the data centre and affects the availability, serviceability, flexibility, and manageability of the data centre for years. That’s a lot to think about,” concludes Schneider Electric southern Africa IT business unit Vice President, Bruce Grobler.