ASHRAE Journal - May 2013 - 79

A

B

Figure 1: Wind-driven natural ventilation in the single corridor type hospital with wind entering the ward first. A) Section plan;
B) floor plan.2
ventilation is used for infection control, the minimum ventilation should be much higher than the existing requirement for
mechanical ventilation to significantly reduce the infection
risk, as the airflow direction may not be fully controlled.
Although the airflow direction cannot be fully controlled,
the strong dilution effect allows the diluted contaminated air
being emitted to present a reduced risk. Still, the choice of
airborne precaution areas and placement of patients within
the areas need to be carefully planned and designedto further
reduce the risk of infection for people in the surrounding
areas.
The panel of WHO2 suggests that the minimum ventilation
rate is to be set as 24 ach for airborne precaution rooms when
natural ventilation is used, doubling the ventilation requirement relative to mechanical airborne negative pressure isolation rooms. When natural ventilation alone cannot satisfy
the recommended ventilation requirements, mechanical or
mechanically assisted natural ventilation modes should be
activated. 12 ach is equivalent to 80 L/s (170 cfm) for an
isolation room of 4 m × 2 m × 3 m (13 ft × 7 ft × 10 ft).
This estimate has been the basis of WHO2 specifying for
natural ventilation a minimum hourly averaged ventilation
rate of 160 L/s (339 cfm) per patient for airborne precaution
rooms (with a minimum of 80 L/s [170 cfm] per patient). The
specification of the minimum hourly averaged ventilation rate
also addresses the issue of airflow fluctuation. WHO2 made
similar arguments and specifications for natural ventilation
requirements in other wards, outpatient areas, and corridors.

Natural Ventilation Strategies

WHO2 classifies suitable natural ventilation design strategies based on the relevant basic architecture design elements
(corridors, courtyards, chimneys, wind catchers, etc.) and
building layout. These design elements define the routes of
airflow, and thereby, the basic natural ventilation strategy.
Five basic natural ventilation strategies are discussed. It
is possible to combine some of these strategies to suit the
local climate and particular needs of a given hospital. WHO2
does include a note with each recommended strategy: “This
May 2013

conceptual drawing should be used with care, and sufficient attention is needed to consider realistic limitations.”
1. Single-side corridor type (Figure 1). A corridor is placed on
either side of the wards. The airflow will maintain a unidirectional
flow either from the ward to the corridor or from the corridor to
the ward depending on the incident wind direction. The former
unidirectional flow is beneficial to prevent cross infection, while
the latter is not. The design of operable openings (e.g., windows
and doors) is crucial for this design. It is best to align operable
openings and passageways within rooms and create a path of
minimum resistance to flow for cross ventilation.4
The 18th century architect Beer is credited with having initiated
the corridor hospital, where all the rooms are arranged alongside
internal walkways. His hospital in Bern, Switzerland, built between
1718 and 1724, was the first of this type.5
2. Central corridor type. A central corridor type may be derived
from the single-side corridor type by adding another series of
wards on the other side of the corridor. The possible airflow path
would be from one ward to the corridor, and then to the ward on
the other side. When the wind is blowing parallel to the windows,
adding a wing wall may help drive the outdoor air to enter the
wards first, then meet and exit from the central corridor.
3. Courtyard type (Figure 2). Courtyards are traditionally
enclosed outdoor zones that can help channel and direct the
airflow that is promoted by large openings (gates, doors, arches,
etc.) and thus modify the microclimate around the buildings.
Based on the relative position between wards and corridor, this
type of natural ventilation design can be divided into subtypes,
i.e., an inner corridor and outer corridor type. In either case,
the courtyard needs to be sufficiently large. The outer corridor
type has the advantage over the inner one as it can avoid cross
infection via the connected corridor by delivering clean outdoor
air into the corridor first.
The first hospital designed according to these geometrical
principles was during the Renaissance. The hospital, the Ospedale
Maggiore, was founded in Milan in 1456 and designed by Antonio
Averulino, better known as Filarete. It has a symmetrical rectangular plan with a large central courtyard; on both sides of it, the
wings of the building delineate four smaller courtyards.5
ASHRAE Journal

79



ASHRAE Journal - May 2013

Table of Contents for the Digital Edition of ASHRAE Journal - May 2013

ASHRAE Journal - May 2013
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
VAV Reheat Versus Active Chilled Beams & DOAS
A Stable Whole Building Performance Method for Standard 90.1
Technology Award Case Studies:
PSU Design Build Project
Passive Cooling for School
Standing Columns
Building Sciences
InfoCenter
Refrigeration Applications
IAQ Applications
Engineer's Notebook
Products
Data Centers
Emerging Technologies
Classified Advertising
Advertisers Index
ASHRAE Journal - May 2013 - ASHRAE Journal - May 2013
ASHRAE Journal - May 2013 - Cover2
ASHRAE Journal - May 2013 - 1
ASHRAE Journal - May 2013 - 2
ASHRAE Journal - May 2013 - Contents
ASHRAE Journal - May 2013 - Commentary
ASHRAE Journal - May 2013 - 5
ASHRAE Journal - May 2013 - Industry News
ASHRAE Journal - May 2013 - 7
ASHRAE Journal - May 2013 - 8
ASHRAE Journal - May 2013 - 9
ASHRAE Journal - May 2013 - 10
ASHRAE Journal - May 2013 - 11
ASHRAE Journal - May 2013 - 12
ASHRAE Journal - May 2013 - 13
ASHRAE Journal - May 2013 - Letters
ASHRAE Journal - May 2013 - 15
ASHRAE Journal - May 2013 - Meetings and Shows
ASHRAE Journal - May 2013 - 17
ASHRAE Journal - May 2013 - VAV Reheat Versus Active Chilled Beams & DOAS
ASHRAE Journal - May 2013 - 19
ASHRAE Journal - May 2013 - 20
ASHRAE Journal - May 2013 - 21
ASHRAE Journal - May 2013 - 22
ASHRAE Journal - May 2013 - 23
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ASHRAE Journal - May 2013 - 31
ASHRAE Journal - May 2013 - 32
ASHRAE Journal - May 2013 - A Stable Whole Building Performance Method for Standard 90.1
ASHRAE Journal - May 2013 - 34
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ASHRAE Journal - May 2013 - PSU Design Build Project
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ASHRAE Journal - May 2013 - Passive Cooling for School
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ASHRAE Journal - May 2013 - Building Sciences
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ASHRAE Journal - May 2013 - InfoCenter
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ASHRAE Journal - May 2013 - 73
ASHRAE Journal - May 2013 - 74
ASHRAE Journal - May 2013 - Refrigeration Applications
ASHRAE Journal - May 2013 - 76
ASHRAE Journal - May 2013 - 77
ASHRAE Journal - May 2013 - IAQ Applications
ASHRAE Journal - May 2013 - 79
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ASHRAE Journal - May 2013 - 81
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ASHRAE Journal - May 2013 - Engineer's Notebook
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ASHRAE Journal - May 2013 - Products
ASHRAE Journal - May 2013 - 87
ASHRAE Journal - May 2013 - Data Centers
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ASHRAE Journal - May 2013 - 90
ASHRAE Journal - May 2013 - 91
ASHRAE Journal - May 2013 - Emerging Technologies
ASHRAE Journal - May 2013 - 93
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