Stay Cool Round-the-Clock: 7x24 Cooling in Health-Care Facilities
By J. Patrick Banse, P.E., Smith Seckman Reid, Houston
Creating and maintaining comfort conditioning in hospitals has long been the design goal and an operational requirement of HVAC systems, not only for patients and health-care providers, but also for equipment such as telephone rooms and computer rooms.
Lately, it is becoming even more of a challenge. Most central air systems operate 24 hours per day, seven days per week to provide comfort conditioning and minimum air change rates as required by code. But the increasing use and need for digital communications, computerized patient records, digital radiology imaging and storage systems are creating an increasing need for year-round cooling in hospitals and related health-care facilities.
Many new types of equipment are creating larger cooling loads: digital radiographic equipment such as computed tomography (CT) scan equipment and heat exchanger; cardiac cath lab and EP lab computer equipment, magnetic resonance imaging equipment, computer rooms themselves; and uninterruptible power supplies. Adding information technology (IT) closets and server rooms for security, nurse call and telephone systems can create a 75- to 150-ton continuous equipment cooling load in the average 500,000-sq.-ft. hospital. Server densities are also becoming greater, creating a higher need for heat rejection (cooling load).
These are some of the causes of the problem. But one also needs a fuller explanation of why. Computer imaging and storage equipment must remain “on” and ready in order to be functional when needed. The loss of cooling to a space with this equipment can cause equipment overheating and malfunction, rendering its useless as a diagnostic tool. Loss of access to patient medical records or a patient’s imaging records is also a possibility. It could slow down the treatment or admitting and transferring of a patient and lost productive time for staff and caregivers—not to mention lost revenue. Loss of power or internal high temperature shut down of imaging equipment may require recalibration when the equipment is re-energized.
Where and how
These are some of the “whys.” Now for some of the “wheres” and “hows.” A central building distribution frame (BDF) room is usually located at grade or point of entry for the outside communication system such as telephone service. IT closets can be scattered throughout a large facility or neatly stacked from floor to floor. IT closets at a nominal 10'x10' can house racks of servers, routing and termination points for CATV, MATV, telephone, data, security, telemetry and nurse call cabinets as well as code blue communications to the appropriate response team. Each of these components is vital to the operation of the hospital and care of its primary occupants, and creates a heat load in this room.
Most IT rooms have a cooling load of 9,000 to 12,000 BTUh. Diagnostic/treatment equipment, such as CT scanners or cardiac cath labs, has computerized control rooms, signal generators and imaging equipment. They can also have water-cooled heat exchangers, provided by the equipment vendor, for heat rejection for the imaging equipment. The heat from these heat exchangers is also rejected to the room depending upon the medical equipment manufacturer. Moreover, new technology such as magnetic navigation cath labs have the added requirements of enclosed equipment rooms and separate magnet cooling units.
Sometimes a majority of these loads can be served by all air systems from central station air handlers, provided the air delivery temperature is low enough and the system operates continuously. During a normal cooling season, a facility's central cooling plant can provide the necessary chilled water at 42-44oF to the central air system.
But what happens when the building cooling load is not at peak condition during the fall and spring, or completely goes away in the winter, as is the case in some climates? The air system can go into an economizer mode using all-outdoor air. Computer room A/C units, however, do not have this capability. Central station unit discharge air temperatures can also rise and still meet the load and space conditions as the cooling and dehumidification requirements are lessened. However, the equipment cooling loads go on unchanged. Temperature- sensitive computer processors and water-cooled heat generators care little about the rest of the building operation or its human occupants. So, how can this equipment and its cooling loads be kept happy and operational? Let’s explore some ways.
A satisfied system
The simplest way is to have the air-handling system and computer equipment connected to the central building chilled-water system and operate the chillers, pumps and cooling towers year-round, regardless of the remainder of the building load. This arrangement could lead to inefficient central equipment operation and would certainly use more energy than desired.
A second method is to use a separate smaller chiller, either air-cooled or water-cooled, with a winterized cooling tower of sufficient capacity to handle the telecom and equipment loads. A second or redundant chiller and pumps should be provided in the event of a primary cooling equipment failure. The building cooling system may also be used as the redundant or backup cooling source. This source, however, may be more difficult to start and operate in the winter due to the cold entering condenser water temperatures.
Data center computer rooms have long used separate cooling systems for equipment cooling and heat rejection. Direct expansion (DX) cooling and chilled-water systems with glycol to inhibit water freezing have both been used successfully. These systems can also serve the computer and communications rooms. Some specialty process chiller manufacturers match chiller load exactly to the medical equipment heat rejection. For single pieces of equipment, this may be appropriate. Redundant cooling system sources would need to be provided so as not to compromise the operational integrity of either system.
In appropriate climates, air-side economizers are used for free cooling using outdoor air. This allows building chiller plants to shut down. In this case, a third cooling source is a plate frame heat exchanger for tower free cooling. This source, which uses cold condenser water to reject heat and cool the building chilled water loop, allows for cooling of areas of constant heat load. (For example, the process cooling and sensible cooling requirements of the equipment room and communication room loads.) Problems can exist though, as the outdoor air increases to the point where the chilled water temperature approaches the upper limits of what the equipment can stand, usually 55-60oF maximum.
This is where the supplemental mechanical cooling is required. A small chiller can then provide reliable cooling or the building chilled water source is required to be activated. An economic evaluation should be performed to determine a cost-effective means to provide equipment coating. Emergency or standby power for cooling systems needs to be considered as well, and it needs to match the equipment operational scenario under loss of normal power.
New facilities should consider separate cooling sources for communication closets and sensitive electronic equipment rooms in the design of their HVAC systems. These can be through separate fan coil units or separate air-handling units serving only those rooms which can then use cooling sources separate from the building sources. Some hospitals or facilities include certain areas or occupancies such as offices or clinics, which can go into an unoccupied mode, thereby reducing the cooling requirement and resulting capability or in some cases shutting the air unit off altogether. This energy saving scenario, while benefiting the building, can be very detrimental to those areas requiring 24/7 cooling.
Many older health-care facilities have been retrofitting their existing buildings by adding IT rooms and closets to provide space for the electronic data racks and communication equipment. This process requires re-evaluation of the building cooling requirements and cooling capabilities as the building cooling loads increase when this much technology is added. A means for providing the cooling requirements for the year around load can be a challenge. Studies should be performed and planning identified to make this happen. Ways to provide reliable, efficient, cost effective year-round cooling are available and need to be incorporated into both new and renovation projects involving HVAC and technology design.