Extracting heat from server rooms/warehouses has only grown in importance with today’s high-performing, temperature-sensitive equipment and energy.
Yet, building owners, operators, and engineers must cope with lower or flat maintenance budgets, so keeping equipment energy efficient, sustainable, and operating at original design levels can be a challenge. Because cooling systems represent the most expensive parts of a data center facility to both construct and to operate, even the smallest improvement in energy efficiency can translate to sizeable savings.
It is these very incremental efficiencies that offer an untapped savings opportunity for managers and operators of server rooms and warehouses.
Given the well-documented and often growing cooling demands by data centers, HVAC equipment must operate at its original design capacity. However, as air-conditioning / refrigeration equipment ages, its ability to maintain temperatures and humidity levels decline. Most often, the culprit is reduced coil heat-transfer effectiveness, or the ability of air-handling unit’s (AHU) cooling coils to remove heat from the air.
RESTORING COOLING CAPACITY
These inefficient heat-transfer rates derive primarily by the buildup of organic contaminants on, and through, the coil’s fin areas. Such buildups are eliminated through the use of ultraviolet germicidal irradiation or light in the UV-C wavelength (254 nm). UV-C works by disassociating molecular bonds, which in turn disinfects and disintegrates organic materials.
UV-C lighting systems are not an exotic, new technology. It has been used extensively since the mid-1990s to significantly improve HVAC airflow and heat-exchange efficiency, which can reduce energy use by up to 35%. UV-C by itself doesn’t save energy; rather, it restores cooling capacity and airflow to increase the potential for energy savings.
In new/OEM equipment, UV-C keeps cooling coil surfaces, drain pans, air filters, and ducts free from organic buildup for the purpose of maintaining “as-built” cooling capacity, airflow conditions, and IAQ. In retrofit applications, UV-C eradicates organic matter that has accumulated and grown over time, and then prevents it from returning.
This trifecta of boosting capacity, saving energy, and lowering maintenance is driving more than nine-of-every-ten UV-C installations today.
All of these system-enhancing efficiencies of UV-C technology are discussed in greater depth in the American Society of Heating, Refrigerating and Air Conditioning Engineers’ (ASHRAE) 2011 Handbook of HVAC Applications, Chapter 60.8, which states: “UV-C can increase airflow and heat-transfer coefficient and reduce both fan and refrigeration system energy use.” In other words, UV-C helps to restore, and thereafter maintain, original cooling capacity.
SERVERS’ NEED FOR CONSTANT COOLING
Servers generally require an airflow volume of about 160 cubic feet per minute (CFM), while blade servers consume about 120 cfm of air between 66° and 77°F per kilowatt (kW).
In 2004, ASHRAE recommended a temperature upper limit of 77°F for data centers. In 2008, the association raised the upper limit to 80.6°F and this limit remained in the 2011 recommendations, which may provide some operators a hedge against lost cooling capacity.
Despite, this industry benchmark though, most data center operators believe that higher temperatures lead to equipment failures. Therefore, half of all data center managers strive toward the temperature goal of 71° to 75°F; with more conservative colleagues (37%) aiming for the 65° to 70°F range.
Modifying return air ductwork to capture hot aisle air separately allows return air temperatures to increase, which provides for greater temperature differentials at the cooling coil. So long as coil heat exchange efficiency is maintained to original design specifications, this would allow the cooling coil to operate more efficiently. Liebert Corporation states that capturing hot aisle air in this manner can provide up to a 25% increase in equipment capacity and an increase of 30% in cooling system efficiency.
ROAD TO SUSTAINABLE COOLING
However; in regards to sustainability, these performance-based cooling systems should be of concern for the operator. The main reason relates to the preservation of system cooling coil heat exchange efficiency, or more specifically, capacity. Data center cooling designs vary, but the common reality is increasing heat loads, decreasing capacity from fouled coils and therefore, challenging cooling demands overall.
In some of the designs, variable speed chillers, pumps, and fans along with EC motors, etc., have allowed facilities to initially meet cooling loads in the most cost-effective way. Aiding this equipment are digital controls aimed towards saving energy in, often over-designed, cooling equipment.
Energy efficiency goals among some users is to achieve a rating as close to 1 as possible. This is thought to be accomplished by reducing the energy consumption of cooling equipment. This may be possible in that new facility, but very difficult to obtain from older equipment. Also, intelligent building equipment mentioned above could be masking ongoing losses in heat exchange efficiency (capacity) such, that the +15% surplus of original system capacity may have slowly eroded through lost coil heat exchange efficiency and airflow from coil fouling alone, as seen below.
Energy savings from cooling equipment might not be possible as minor increases in air eaving wetbulb temperatures from a fouled coil can have dramatic effects on system capacity and thus energy use. For operators who have the instrumentation, or access to outside test and balance services, air conditioning unit capacity can be demonstrated through simple measurements using the ASHRAE equation:
BTU = CFM x 4.5 x (h1 - h 2)
h1 - h 2 are wetbulb temperatures in “BTU per degree”
A velocity traverse at the coil is used to establish CFM, and the coil upstream and downstream wetbulb temperatures (h1 - h 2) are used to populate the above algorithm.
Another important measurement is the pressure drop across the coil. Even a small amount of coil fouling will cause the coil’s pressure drop to increase, which will cause the system airflow to decrease, or be digitally compensated for in fan speed (see below). In the equation above we calculate system airflow in CFM, which plays a major role in determining system capacity. In other words, coil fouling will reduce both the system’s heat removal capability and airflow.
The effects can be seen in this example: a new 20,000 cfm system with an air entering wetbulb temperature of 64 and an air leaving temperature of 53, would be: 20,000 x 4.5 x (h1 = 64 or 29.31 BTU – h2 = 53 or 22.02 BTU) = 656,100 BTU of cooling. If the current air leaving wetbulb temperature is “only” one degree higher, it would look like this: 20,000 x 4.5 x (29.31 – 22.62) = 602,100, or a drop of (65,6100 – 60,2100) = 54,000 BTU in lost capacity or (54/12) = 4.5 tons of lost cooling capacity.
When a slight reduction in airflow is added in, it would look like this: 19,000 x 4.5 x (29.31 – 22.62) = 571,995, or a drop of (65,6100 – 57,1995) = 84,105 BTU in lost capacity or (84/12) = 7.0 tons of lost cooling capacity or a total reduction of 13% in capacity from some seemingly minor changes in performance. It’s not uncommon to find airflows reduced by 25% and air leaving temperatures increased by 3 degrees. Again, eroded surplus capacity might not be apparent, which warrants the taking of measurements to be sure. Potential energy savings may already be gone.
When capacity is lost (temperatures not being made), fan speed is often increased digitally, manually, or mechanically to help compensate for the loss. However, fan horsepower (energy use) increases to the “cube” of rpm:
This demonstrates that the simple fix of increasing the fan’s speed consumes more energy than most of us realize. It may temporarily satisfy capacity losses; however, when that isn’t the case, further modifications are usually performed.
In chilled water systems, the water’s temperature, which may also be automatically controlled, is lowered, increasing the temperature differential between the air and the coil surfaces, thereby increasing the heat transfer rate. Often, this is enough to return system capacity to when the coil was clean, but at a noteworthy cost. The lowering of the water temperature requires a significant increase in energy use, and it is often accompanied by pumping additional water volume. Increasing pump rpm has the same consequences as increasing fan rpm, a boost in horsepower to the cube of the increase. All of the above makes a case for obtaining and keeping a coil perfectly clean — so that the original design advantages can be captured and maintained for the life of the system.
For direct-expansion (DX) systems, typically, run times are increased, and those DX machines equipped with variable drives could consume additional energy as fans speed up to compensate for the increased pressure drop across the coil. As temperature differentials across the evaporator coil lower, temperature differentials across the condenser coil will decrease as well, yielding an overall loss in cooling capacity. There may be other issues as well, such as head temperatures and pressures at the compressor. The key again is to obtain and keep the evaporator coil as clean as possible, which will restore the unit to near as-built conditions, or more important, capacity and therefore, potential energy savings.
In the future, both energy and water are going to cost more, and the 2012 International Green Construction Code (IGcc) focuses heavily on measures that reduce water consumption, too. Condensate from cooling equipment will be required to be collected and reused. Of interest, when UV-C is used, drain pan water is disinfected and free of agglomerated organic material meaning that it can be easily reused, often without further treatment. When municipalities adopt green building codes, the focus will be on business’ consumption of water, and will most likely include data centers.
Again, UV-C serves to restore the coil’s performance to regain system capacity. And, as system capacity increases, the energy that had been wasted to compensate for lost capacity is returned in the form of lower power consumption.
Equipment manufacturers usually recommend coil cleaning twice a year, and no less than annually to not only prevent mold growth and capacity loss, but to keep contaminants from compacting deep within the coil. With staffs and budgets shrinking, however, time and money for in-house or contracted coil cleaning is becoming scarce. In fact, some building operators report they have not cleaned their AHU’s coils in three or four years. If coil cleaning is not performed regularly, contaminant buildup deep inside internal surfaces can become so difficult to remove that expensive coil replacement becomes the only option. UV-C has been shown to clean even compacted contaminant from a coil.
COSTS AND PAYBACK
Users report that UV-C installations are very cost effective. Most see paybacks in less than six months on energy use alone.
Many users report that their cost for an installed system featuring high output lamps was about $0.10 per cfm (U.S.). For a 10,000 cfm system, this amounts to an investment of about $1,000.
Field reports indicate that the first-cost of a UV-C system (initial investment) is approximately the same (or less) as a single, properly performed coil-cleaning procedure, especially when system shutdowns, off-hours work, associated overtime, and/or contractor labor costs are considered.
The operating cost for a system that is on year-round (24/7/365) is far less than 1% of the power to operate that air conditioning system. This amount is a noteworthy bargain in those systems that return 5% or more of their capacity.
UV-C light is an amazingly effective and affordable technology for keeping critical components of commercial HVAC systems clean and operating to as-built specifications. The benefits of UV-C energy can often sound a little too good to be true. But, with tens of thousands of installs and backing by ASHRAE, it becomes a benefit too good to miss out on. n
Forrest B. Fencl, CEO and co-founder of UV Resources, passed away on Aug. 1, 2015. A lifelong inventor and respected industry leader, Fencl pioneered the modern application of ultraviolet germicidal irradiation (UV-C) in HVACR equipment, writing or co-writing 17 patents and several ASHRAE Handbook chapters related to ultraviolet air and surface treatment. Please direct questions to UV Resources president Dan Jones, firstname.lastname@example.org.
1. 2011 ASHRAE Handbook–HVAC Applications. ASHRAE. Chapter 60: “Ultraviolet Air and Surface Treatment.” https://www.ashrae.org/resources--publications/Table-of-Contents-2011-ASHRAE-Handbook-HVAC-Applications
2. Uptime Institute’s 2013 Data Center Industry Survey http://c.ymcdn.com/sites/www.data-central.org/resource/collection/BC649AE0-4223-4EDE-92C7-29A659EF0900/uptime-institute-2013-data-center-survey.pdf