The Executive Summary
The purpose of this blog post is to compare and contrast two types of critical facilities as they pertain to the HVAC heat transfer design strategy. The Hospital Operating Room (OR) and the Data Center Equipment Floor Space. This post is supplemented with consolidated data and information from ASHRAE (Journal and Standards) as well as my own experiential data.
Who is the Client?
Aka, "the V.I.P.", aka "the focus", aka "the revenue stream", aka "the reason you exist".
Depending on who you ask, the revenue from a data center could range from many thousands to tens of thousands of dollars per square foot per year. Not having proper heat transfer to cool the equipment can be a risky proposition. In a hospital Operating Room (OR) where we consider the possible loss of life, I'm fairly confident that the risk is far greater.
What are the Requirements?
As a seasoned Project Engineer will tell you, often times the client doesn't know what he or she wants. However, it is the Engineer's job to provide options, to communicate those options, and to provide possible solutions in a manner that the client can understand.
While there is an almost infinite number of variables involved; the ME is skilled at narrowing down the requirements to basic principles (e.g. Temperature, Pressure, and Flow). More specifically, here we discuss Heat Transfer, Humidity Ratio, Laminar air flow, and Air Quality.
Heat Transfer
In general, the amount of space heat (BTU/Hr) to be removed originates from people, equipment, or the heat absorption from external sources. Although the hospital Operating Room will have heat generated from the surgeon and his or her staff, the equipment, and the lighting; by far the most extensive amount of air conditioning energy will be from the conditioning of 100% outside air required to exhaust all odors & contamination. The heat needs to be removed (or added) such that the OR is always kept at the optimum Temperature and Humidity levels as mandated by the client and codes.
As mentioned below in the airflow discussion, the Heat Transfer or Cooling Load profile in a data center (DC) is very different from a hospital OR but the underlying principles remain the same. On average, the DC equipment waste heat is on the order of 200 Watts per Square Foot with very little cooling load from technical staff (or even lighting). Taking into account experiential data on stand-by engine size and design power usage, this represents an estimate of 10,000 Sq Ft. of equipment space (or rack foot print). Of course, the required heat transfer in a typical hospital OR is minuscule compared to a data center.
Relative Humidity (RH)
Driven by Temperature, the target RH level in a Data Center may be difficult to maintain tightly but can vary from 45% to 55%
according to the expanded ASHRAE range for Data Centers. While the hospital Operating Room (OR) requires a lower RH from 20% to 35%. OR's have typically mandated the 35% minimum but newer (ASHRAE 170) standards recognizing the removal of flammable materials within the OR require a 20% minimum which may save money depending on psychometric design.
Laminar vs. Turbulent Air Flow in the OR
It may be obvious that the goal here is to minimize contaminated airflow over the body of an OR patient. The non-trivial issue is how. For example, how does the Mechanical Engineer design the HVAC system airflow to ensure that particulate matter is kept away from the patient? If the airflow enters the room from the ceiling and falls to the floor, there is a chance that contaminated particles from the floor are kicked up and re-circulated to the patient. To minimize contamination, the engineer must ensure that the air flow passes over the patient once and is NOT re-circulated toward the operation area but returned to the system for filtration and/or exhaustion.
The Multiple Panel System:
At much lower airflow rates than the typical system, these perforated "laminar airflow panels" are designed with the assumption of straight air movement toward the floor from the face of the outlet. The laminar airflow concept developed for industrial clean room use has attracted interest from some medical authorities. There are advocates of both vertical and horizontal laminar airflow systems, with and without fixed or movable walls around the surgical team. Some medical authorities do not advocate laminar airflow for surgeries.
The Air Curtain:
Air curtains are jets of air projected across envelope openings with the intention of reducing air exchange and the entrance of dust and insects, for example. The performance of air curtains is highly dependent on factors such as jet characteristics, wind, and building pressurization. In the OR, they are generally created using a couple of linear slot diffusers with supply plenums - designed to deliver air evenly over the length of the air slot.
Now, due to the effects of buoyancy, air entering a room at a relatively cold temperature to its surroundings start off at a low velocity but will increase its speed toward the floor. This happens much the same way as an iron ball falls to the bottom of a deep pool of water. And, if this happens, there is a much higher chance of the flow becoming more turbulent and kicking up contaminated particulate matter back to the patient.
Immunosuppressed patients (including bone marrow or organ transplant, leukemia, burn, and AIDS patients) are highly susceptible to diseases. Some physicians prefer an isolated laminar flow unit to protect the patient; others are of the opinion that the conditions of the laminar cell have a psychologically harmful effect on the patient and prefer flushing out the room and reducing spores in the air. An air distribution of 15 air changes per hour supplied through a nonaspirating diffuser (unidirectional downward airflow from the ceiling with minimum entrainment of room air) is often recommended. With this arrangement, the sterile air is drawn across the patient and returned near the floor, at or near the door to the room.
Airflow in the Data Center
In contrast to the OR patient who is always located in the same place, the "hot spots" on the floor of a Data Center may not only be in different areas but the floor configurations of the equipment may change. Where it is critical to keep a patient clean of any contaminated particulate matter in the OR, it is critical to efficiently cool Data Center equipment by properly aligning cooling capacity to the heat sources.
Floor Space Configuration:
While the precursors to the modern data center had equipment mounted on the concrete slab with flexible duct drops hanging from the iron supports above; more modern data centers (outside of the newest modular construction) are typically designed with Hot/Cold aisle separation between inlet/outlet of the equipment cooling fan flow. As many know, the reason for this separation is to minimize the mixing of cold & hot air streams leading to inefficient cooling. As eluded to above, many variables may reduce the efficiency of heat rejection on the floor including variable equipment manufacturer / type / heat output; inconsistent and/or non-homogeneous equipment floor space planning; and surrounding structure impeding the proper flow of air.
Heat Transfer Strategy:
Of course this is an over-simplification but removing heat from data center equipment is sort of like blowing out a candle. If you blow the wrong way or not hard enough, the candle will stay lit. In essence, blowing the candle the "right way" in a data center is aligning the equipment along the floor space such that the air flow will transfer the heat efficiently; making sure that the source of the cooling is as close to the equipment heat sink as possible such as in an in-line cooling configuration; and/or ensuring that the cooling capacity is large enough to take the heat away more efficiently as is the case for liquid cooling inside the server racks. It should be noted that this type of cooling is generally not accepted due to lack of maintenance access and leak risks but is growing in popularity.
Computational Fluid Dynamics (CFD)
CFD models of particle trajectories, transport mechanisms, and contamination propagation are commercially available. Flow patterns and air streamlines are analyzed by computational fluid dynamics for laminar and turbulent flow where incompressibility and uniform thermophysical properties are assumed. Design parameters may be modified to determine the effect of airflow on particle transport and flow streamlines, thus avoiding the cost of mockups.
Some major features and benefits associated with most computer flow models are:
- Two- or three-dimensional modeling of cleanroom configurations, including people and equipment
- Modeling of unidirectional airflows
- Multiple air inlets and outlets of varying sizes and velocities
- Allowances for varying boundary conditions associated with walls, floors, and ceilings
- Graphical representation of flow streamlines and velocity vectors to assist in flow analysis
- Graphical representation of simulated particle trajectories and propagation
Research has shown good correlation between flow modeling by computer and that done in simple mockups. However, computer flow modeling software should not be considered a panacea for design because of the variability of individual project conditions. "It is…more difficult to model the air currents resulting from complex mix of convective, radiative, and conductive heat flows in a typical office with high induction outlets…we found very little data that draws a parallel between CFD and physical measurements in these situations" - see You Have to Prove It by Dan Int-Hout, Fellow ASHRAE (ASHRAE Journal October, 2013)
Air Quality
Hospital operating rooms may be classified as cleanrooms, but their primary function is to limit particular types of contamination rather than the quantity of particles present. Cleanrooms are used in patient isolation and surgery where risks of infection exist.
Systems must also provide air virtually free of dust, dirt, odor, and chemical and radioactive pollutants. In some cases, untreated outdoor air is hazardous to patients suffering from cardiopulmonary, respiratory, or pulmonary conditions. In such instances, treatment of outdoor air as discussed in ASHRAE Standard 62.1 should be considered.
Outdoor Air Intakes. These intakes should be located as far as practical (on directionally different exposures whenever possible), but not less than 25 ft, from combustion equipment stack exhaust outlets, ventilation exhaust outlets from the hospital or adjoining buildings, medical-surgical vacuum systems, cooling towers, plumbing vent stacks, smoke control exhaust outlets, and areas that may collect vehicular exhaust and other noxious fumes. The bottom of outdoor air intakes serving central systems should be located as high as practical (minimum of 12 ft recommended) but not less than 6 ft above ground level or, if installed above the roof, 3 ft above the roof level.
Exhaust Air Outlets. These exhausts should be located a minimum of 10 ft above ground level and away from doors, occupied areas, and operable windows. Preferred location for exhaust outlets is at roof level projecting upward or horizontally away from outside intakes. Care must be taken in locating highly contaminated exhausts (e.g., from engines, fume hoods, biological safety cabinets, kitchen hoods, and paint booths). Prevailing winds, adjacent buildings, and discharge velocities must be taken into account. In critical or complicated applications, wind tunnel studies or computer modeling may be appropriate.
Air Filters. A number of methods are available for determining the efficiency of filters in removing particulates from an airstream. All central ventilation or air-conditioning systems should be equipped with filters having efficiencies recommended by code and/or ASHRAE standard. Appropriate precautions should be observed to prevent wetting the filter media by free moisture from humidifiers. Application of filter beds should follow ASHRAE Standard 170. All filter efficiencies are based on ASHRAE Standard 52.2. For example, a hospital OR filter of MERV 16 rating is designed to remove particulate matter in the size range of 0.3 to 1.0 micrometers. In the surgical environment, this will take care of bacteria, smoke, some viruses, and droplet nuclei.
Optical particle counters (OPCs) are widely used and likely to become more so. They are very convenient and provide real-time, size-selective data. Individual aerosol particles are illuminated with a bright light as they singly pass through the OPC viewing volume. Each particle scatters light, which is collected to produce a voltage pulse in the detector. The pulse size is proportional to the particle size, and the electronics of the OPC assign counts to size ranges based on the pulse size. ASHRAE Standard 52.2 defines a laboratory method for assessing the performance of media filters using an OPC to measure particle counts up- and downstream of the filter in 12 size ranges between 0.3 and 10 μm. Filters are then given a minimum efficiency reporting value (MERV) based on the count data.
Other Factors
The "Black Box" or "Closed System" are analogies that Engineers like to use to define the borders through which the various inputs & outputs of the system cross. Although this discussion has been limited to either the Hospital OR or the Data Center floor space, ANY ingress or egress of air flow must be reviewed and integrated as a complete system to minimize contamination. For example, air ingress from other parts of the hospital from either rooms or from elevators must be taken into account with the use of special sterile procedures.
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