UPS Questions

Precision Cooling Questions

UPS Answers

What is a UPS?

UPS stands for uninterruptible power supply. A UPS is a back-up power system used to prevent power loss or damage to a computer or other critical piece of electrical equipment.

Why do I need a UPS?

There are many answers to that question. A good UPS system can help prevent down time, by providing clean power to critical systems when there is a power anomaly. A UPS can also protect equipment and help it last longer by acting as a power conditioner, filtering out surges, sages, spikes and outages.

A UPS increases the productivity of computer users by keeping their computers functioning more reliably and preventing work loss. A Liebert UPS can provide back up power in a remote or powerless situation with the dark start capability, utilizing the batteries as a power source.

What kind of UPS do I need?

The type of UPS needed depends on how critical the system is which needs protection. There are three main types of UPS: off-line or standby, line interactive, and on-line (double conversion).

1. An off-line or standby UPS is used as an alternative source of power when there has been a sudden loss of utility power. An off-line UPS generally provides a few minutes of back-up power to ride through the outage or in the event of an extended outage, allow a gradual shutdown of the connected equipment. An off-line UPS has few power-conditioning features, and will rely on battery power whenever utility power is unacceptable for the critical load. An off-line UPS is best for desktop computers, point-of-sale terminals or other applications that need some power protection but are not mission critical. An off-line UPS uses a voltage-sensing switch to activate the DC rectifier and draw power from the batteries when incoming power is not ideal. A standby UPS does little or no power conditioning and primarily acts as a switch to draw power from the batteries. Some surge in output voltage occurs when it switches from utility power to battery power. An off-line UPS has no user replaceable batteries and lasts 3 years, on average.

2. A line-interactive UPS is useful for large desktop systems or a set of rack-mounted computers with up to 2200VA of power. While similar in functional layout to the off-line UPS, line-interactive technology also includes a buck and boost capability. This feature compensates for power surges and sags of +/- 25% of the normal incoming voltage without using batteries to regulate the voltage. A greatly reduced battery duty cycle means that the batteries in the UPS will last longer than in the off-line style and that the connected equipment has a greater degree of protection because the back-up power will be more reliable. A line-interactive UPS has about 5 minutes of back-up time when fully loaded, which is enough to ride out 90% of power anomalies and for extended outages, can shut down connected equipment gradually to prevent equipment damage. A line interactive UPS uses a voltage sensing switch to draw power from the batteries when incoming power is outside of the buck and boost input voltage range. A minimal power loss and a surge in the outgoing power occurs at the time of transfer to the batteries. The PowerSure Interactive series of line-interactive UPS have user replaceable batteries and last 3 years, on average.

3. An on-line UPS is the third and best type of UPS. True on-line UPS systems are only those that employ double conversion topology. On-line delta conversion or ferroressonant systems are not on-line UPS systems. Those types of UPS are line-interactive. Liebert on-line UPS systems range from 700 VA to 1,100 KVA in size. An on-line UPS is designed for use with mission critical systems that cannot go down without causing significant work and or financial loss. The typical applications for an on-line UPS are manufacturing equipment, telecommunications systems, Internet service providers, financial networks, data centers and other critical networks or systems. An on-line UPS is constantly active, hence the name. On-line UPS systems convert all incoming power to DC, allows that DC power to pass across the battery circuit, called the DC bus, and then converts the power back into AC power and out to the protected equipment. The critical load is electrically isolated from utility power and receives continued highly regulated power. There is no output power disturbance or gap when there is a power loss or anomaly in the incoming power since the batteries are always connected to the DC bus from which the inverter draws its current. An on-line UPS can also accept as low as 50% of the normal incoming voltage without drawing power from the batteries. This allows the battery systems to have a longer run time and require less maintenance. In single phase UPS, on-line UPS batteries are all hot swappable and will last five years, on average.

What is a VA?

VA is a unit of measurement for power, similar to a watt or horsepower. VA stands for volt-amps. VA is used to signify the total power requirements of a piece of equipment. However, it is generally preferred to use Watts to measure power, since VA can be relative to the type of equipment and Watts is not. VA is the apparent power that is being used, whereas Watts is the actual power being used. Without getting too complicated, the difference between Watts and VA is the power factor. VA = Volts * Amps and Watts = Volts * Amps * Power Factor.

What is Power Factor?

Power factor is a measurement of how the incoming power is being used by a piece of equipment. Most computer equipment has a power factor of 0.7. This basically means that the equipment is using 70% of the incoming VA. For example, a computer designed to use 100 VA at a 0.7 PF is using 70 Watts. Liebert UPS systems are designed to output power with a 0.7 power factor so that the connected equipment does not waste power by not using it. Some equipment is power factor corrected, meaning its power factor is very close to 1.0. This type of equipment is more efficient because it is utilizing more of the incoming power and for all practical purposes the VA rating is the same as the Watts. All of Liebert’s on-line UPS systems are power factor corrected to work with any type of equipment and to be as efficient as possible.

What is the difference between a surge suppressor and a UPS?

The biggest difference is that a UPS has battery backup power, and a surge suppressor or TVSS device does not. However a UPS is much more than a surge protector with batteries. A surge protector is designed to protect a sensitive electrical appliance from being destroyed by power surges and spikes. A UPS has this capability, but also conditions power so that the connected equipment always receives an acceptable voltage. A surge protector only protects the equipment from extreme voltage spikes and surges. A UPS can also protect equipment from brown outs, frequency variation, waveform distortion, outages, as well as voltage spikes and surges, and can shut down a computer safely in an extended blackout.

What size of UPS do I need?

The best way to determine what size of UPS is needed is to measure the amount of amps that are being used by the equipment to be protected. Look on the back of the equipment near the input power cord for a power rating plate. On this plate or label will typically be a maximum amp draw for one or more voltages. For example: a computer may be rated at 8/4 Amps and 110/220 Volts. This computer uses either 4 amps at 220 volts or 8 amps at 110 volts. 4 * 220 = 880 VA, and 8 * 110 = 880 VA, so this computer uses a maximum of 880 VA.

Another consideration is the potential for future expansion.

How much back-up time do I need?

Back-up time is entirely dependant upon the application. If there is an alternative power source, such as a generator, to supply back-up power, a few minutes is probably fine, since a well maintained generator will come on in 30 seconds or less. More critical systems may need protection, to protect against the possibility that the alternative power source fails or isn’t immediately available. If no alternative power source is available, back up times can be extended with additional batteries. However, batteries are expensive, heavy, and large, so be reasonable with expectations, and know that most power anomalies are over in five minutes are less.

What is the difference between three phase and single phase power?

Single-phase power is what most computers and telecommunication systems run on, as well as most things in your home or office. Single phase refers to the fact that one sine wave of voltage and current is being supplied and used. Three-phase power is used when large amounts of power are required for industrial systems, large electric motors and industrial equipment or a facility wide UPS. Three-phase refers to the fact that three offset synchronous sine waves of current and voltage are being used to obtain approximately 1.7 times as much power each of the individual phases.

What is better, a centralize UPS or multiple rack mount UPS’s?

As a network administrator, do you need individual printers for each workstation on the network? Of course not. But this is exactly what some network administrators do when they configure power protection. They buy one UPS for each piece of equipment to be protected.

In many applications, particularly in medium to large sized networks, “clustering” critical network equipment offers your client numerous advantages in terms of greater security, lower costs, and increased performance and quality. This equipment or network cluster should be protected by a single UPS (not multiple), in order to achieve greater security, reduced costs, increased network performance, and increased network service quality.

Precision Cooling Answers

What is a BTU?

The term BTU (British Thermal Unit) is a measurement of a quantity of heat. Specifically it is the amount of heat required to raise the temperature of 1 lb. of water 1 °F.

What are some of the most common conversion factors used in cooling and heating engineering?

°F = (°C x 9/5) + 32

°C = (°F – 32)(5/9)

1 ft³ = 1728 in³

1 U.S. gal = 231 in³ = 0.1337 ft³

1 psi = 2.309 ft of water (pressure)

1 BTU = 778.17 ft lb

1 therm = 100,000 BTU

1 kw = 738 ft lb/sec = 1.341 hp = 3412.14 BTUH = 0.284 ton (refrigeration)

1 hp = 33,000 ft lb/min = 0.746 kw = 2545.1 BTUH

1 ton (refrigeration) = 12,000 BTUH = 3.517 kw

What is the difference between creature comfort (people) cooling and cooling critical electronics?

People produce both heat and moisture (humidity). Electronics produce heat and no moisture. People have a broad temperature and humidity tolerance range. Electronics require tight temperature and humidity tolerances to control static electricity and moisture condensation.

The duty cycle for cooling people is typically only a few hours of the day during the hottest months of the year. Electronics must usually be cooled 7 x 24 x 365 (even when outside air temperatures may be subzero). Filtration requirements for electronics is much more stringent than required for people. Greater airflow is used in an electronic facility to minimize hot spots; 1 or more air changes per minute is typical for electronics and 3 to 4 air changes per hour for comfort cooling.

Heat densities are much higher in an electronic facility (1 ton of cooling for every 10 – 60 ft² of space) than for a space occupied by people (1 ton of cooling for every 200 – 400 ft² of space).

How are these considerations manifested in the design of precision (electronic) cooling systems?

Precision cooling systems tend to be highly integrated, self-contained, modularized units for cooling one room or a small portion of a larger facility. They are relatively easy to install. The maximum possible amount of work content is performed at the factory to assure the highest possible quality of installation. Given that electronic generated heat is dry (all sensible heat), these cooling systems were designed to have very high sensible heat ratios (sensible cooling capacity/total cooling capacity). The result is a highly efficient system for this application. Since people emit moisture, comfort-cooling systems are designed to provide both sensible and latent cooling and are efficient for that requirement. Precision cooling systems typically include a humidifier whereas comfort-cooling systems do not. Because of the much more stringent duty cycle imposed on them and the criticality of their mission, precision cooling systems are designed to be far more robust and reliable. Many such units incorporate dual refrigeration systems and might make use of dual cooling sources. To provide tight tolerance control over temperature and humidity, precision systems commonly use advanced state of the art microprocessor based controls which have the ability to interface with Network Manage Systems and/or Building Management Systems to allow remote alarming, monitoring and control. Typically a simple thermostat controls comfort-cooling systems. Larger fans are used in the precision models to obtain the desired airflow. Further, filter efficiencies of 30 to 60% are common.

What problems would likely be encountered if a comfort-cooling unit was used for electronic cooling?

Since a comfort-cooling system has a low sensible heat ratio it would be necessary to sufficiently oversize the unit to provide the required sensible capacity. In addition to higher initial cost and the waste of energy, this would likely lead to over-dehumidification of the space. Temperature would be hard, if not impossible, to control within the desired range and there would be no control of humidity unless a separate, stand-alone humidifier was installed. Filtering would probably be inadequate and the ability to monitor and control the system remotely is doubtful. It is unlikely that system reliability and life would be acceptable. Smaller evaporator fans would produce fewer air changes and hot spots in the critical space could be expected.

What are some of the configurations of precision cooling systems?

A complete description for each of the systems is available in the Precision Cooling pages by drilling down into the Guide Specifications, Technical Manuals, Installation Manuals and Operation and Maintenance Manuals. However, as an overview systems are classified by size (cooling capacity), method of heat rejection (air cooled, water cooled, glycol cooled, “Glycool” cooled or chilled water) and mounting location (floor, wall or ceiling).

In an air-cooled system the refrigerant is directed through a condenser (normally outdoors) where it transfers heat to the environment. In a water-cooled system the heat is removed from the refrigerant in a condenser (heat exchanger normally within the indoor unit) by water. Typically the water carries the heat to a cooling tower (outside) where it is rejected to the atmosphere. However, in a few applications water passes through the condenser once and is directed down a drain. A glycol-cooled system is similar to the water-cooled system except that a water/glycol solution carries the heat from the indoor condenser to a drycooler (closed system cooling coil) outside where the heat is rejected.

Some systems have the ability to use two different sources for cooling (commonly air-cooled refrigeration system for primary cooling and chilled water for backup cooling). Many options are available for each model to meet the specific needs of the client.

How do I determine what type of system is best for my application?

Many factors enter into this decision. Some relate to the configuration of the facility. Others depend on such things as the characteristics of the heat load, local code requirements, operating costs, the type of environment the system will be operating in and, certainly, installation costs. Following are some examples of these considerations:

  • How much space is available for the cooling system? If floor space is limited but there is ceiling space, as much as 10 tons of cooling in a single system can be installed above a dropped ceiling. Small systems (up to 3 tons) that can be wall mounted are available. External wall mounted systems to 5 tons are available for structures in which there is little or no floor space, such as telecommunications shelters. Larger systems (up to 60 tons) will be floor mounted.
  • What is the characteristic of the load? Are there hot spots (areas of high heat density) that need to be cooled? Many of the systems allow ducting of the supply air directly to the exact point of need. Downflow systems deliver the supply air under a raised floor where it is distributed to the heat source through perforated tile. Otherwise, supply air is commonly discharged through plenum grilles.
  • What type of heat rejection option should be used? Generally, an air-cooled system has the least up-front cost. However, installation cost will be more than that for a water-cooled or glycol-cooled system since the refrigeration lines that are run between the indoor unit and outside condenser must include proper slopes and traps. These constraints do not apply to water or glycol lines. Standard outside condensers used in air-cooled systems can operate at ambient temperatures of -20°F. The optional Lee-Temp configuration increases this range to -30°F. Be careful to check with local codes. To minimize energy consumption some jurisdictions require the use of cooling systems that incorporate air or water economizers. Introducing large amounts of outside air prohibits the control of humidity in the critical space, which is unacceptable. A good solution is to use a system that includes a “free” cooling coil (“Glycool”).
  • If the building has an existing cooling tower with enough capacity to allow the addition of the precision cooling system on the loop, then a water-cooled unit may be a good alternative. Similarly, if the building has an existing chiller with the capacity to support the precision cooling load, a chilled water unit would be an option. Core must be taken to assure redundancy in the chilled water system. Losing a chiller pump can cause the computer cooling system to go down.
  • A glycol-cooled system provides ease of installation (no special routing considerations for the glycol piping other than not placing it over electronic equipment) and good performance in cold climates. If a glycol cooled system is chosen, upgrading to a “Glycool” configuration should be considered. It is not uncommon for the added capital cost to be recovered, by energy savings alone, in 6 to 18 months, depending on climatic conditions.

What are some of the considerations that should be made when designing an electronic facility requiring precision cooling?

Location of the space within the building is an important consideration. Locating it within the core of the building provides isolation from seasonal environmental load influences. The space should not be adjacent to any mechanical room or unconditioned area to prevent thermal impact on the space.

When calculating the cooling load it is important to consider all of the load factors, not just the electronic equipment heat rejection. These factors include heat from the adjacent areas, including from above and below; heat load from windows if on an outside wall (considering direction of exposure); heat from people regularly in the room and, importantly, heat from lighting (usually in the range of 3 watts/ft²).

To maintain the desired humidity in the controlled space and avoid costly humidifier run times and dehumidification cycles it is imperative to minimize (if not eliminate) the incursion of outside air. One of the key factors in doing this is to seal the room with a vapor barrier. For example, plastic sheets placed between sheetrock in the walls in new construction provides an excellent barrier. A rubber- or plastic-based paint can be used on concrete walls and floors. Doors should not be undercut or have grilles. A proper vapor barrier can reduce moisture migration by as much as 80%.

Electronics are commonly installed in rooms with raised floors. What issues need to be addressed when installing precision air conditioning in this type of facility?

Raised floors provide a great and flexible alternative for routing cables and piping as well as distributing cooling air. For this type of application a downflow cooling system delivers the cold air to the space under the floor where it is directed to the desired location either through vents or, more commonly, perforated tiles. The space under the floor is, in essence, a supply air plenum. Raised floor heights are commonly in the 12 to 18 inch range. However, they may be as low as 6 inches or as high as 24 inches.

Cooling systems are heavy. Therefore, floorstands fabricated to the height of the raised floor are normally, but not always, used to provide structural support. Obviously, the strength of floor must be evaluated when making this decision. Using a floorstand also allows the cooling unit to be installed, piped, wired and inspected prior to the installation of the raised floor to allow easier access. A floorstand also provides vibration isolation while eliminating the need for cutting special floor panel openings under the unit. Floorstands can be manufactured to meet local seismic requirements. It is important when installing the system that the floorstand be bolted to the subfloor and the cooling unit bolted to the floor stand. Otherwise there would be no restraint in a seismic event. If the height of the raised floor is less than 12 inches a turning vane should be ordered with the floorstand and installed to assure proper air distribution.

For underfloor air distribution, the units (if more than one) should not be placed too close together or in a long, narrow space or the effectiveness of the air distribution will be reduced. Air supply grilles or perforated panels should be selected to minimize circuit pressure loss. Air volume dampers on grilles are usually detrimental to airflow.

Care should be taken when laying out the piping, wiring, etc. under the floor to avoid blocking the free flow of cooling air. Wherever possible all piping should be run parallel to the airflow.

What is the status of R-22 phase-out?

R-22 has been the refrigerant of choice used by most cooling system manufacturers for decades. Because it is mildly toxic to the atmosphere it was included in the provisions of the Clean Air Act Amendments of 1990. This Act stipulated phase-out dates for various refrigerants, including HCFC-22 (a Class II substance). Essentially it says that no new products will be built containing R-22 after January 1, 2010 and no R-22 will be produced after January 1, 2020. Systems operating with R-22 will be able to continue using that refrigerant after the 2020 date. However, with the cessation of R-22 production replacement refrigerant will become more difficult to obtain. Equipment manufacturers will undoubtedly develop products using new, acceptable refrigerants prior to the cutoff date in 2010. In fact, Liebert is beginning to sell products using R-407C refrigerant (although those same products can be ordered with R-22 until the phase-out date). R-407C was designed to have operating characteristics similar to R-22.

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