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Ventilation in Healthcare
Ventilation is used extensively in all types of healthcare premises to provide a safe and comfortable environment for patients and staff. More specialized ventilation is provided in primary patient treatment areas such as operating departments, critical care areas and isolation units. The sophistication of ventilation in healthcare premises is increasing. Patients and staff have a right to expect that it will be designed, installed, operated and maintained to standards that will enable it to fulfill its desired functions reliably and safely.
Reasons for ventilation
The Building Regulations require that all enclosed workspaces be ventilated by either natural or mechanical means. The following are some of the factors that determine the ventilation requirements of a workspace:
human habitation (minimum fresh-air requirement);
the activities of the department, that is, extraction of odors, aerosols, gases, vapors, fumes and dust- some of which may be toxic, infectious, corrosive, flammable, or otherwise hazardous;
dilution and control of airborne pathogenic material;
the removal of heat generated by equipment (for example catering, wash-up, sterilizing areas, electrical switch rooms, and some laboratory areas);
the reduction of the effects of solar heat gains;
Mechanical ventilation systems are expensive in terms of capital and running costs, and design solutions have been be sought in this project which take advantage of natural ventilation to meet the criteria mentioned above.
Natural Ventilation Constraints in Health Care premises
According to the 'HTM 03' (1), planning constraints caused by a building's shape and/or the functional relationships of specific areas will invariably result in some measure of deep planning as most healthcare buildings are, thus reducing the opportunity for natural ventilation. Also noted is the fact that if natural ventilation is single-sided, it will usually only be effective for a 3m depth within the space. Beyond that it will need to be supplemented by mixed-mode or forced ventilation thus increasing energy cost and increase in CO2. Current guidance restricts the opening of windows for safety reasons; also, as many designs are top hung, their ability to permit natural ventilation is limited. It may therefore be necessary to provide dedicated ventilation openings in the fabric of the building to allow a sufficient natural flow of air into and out of the space. In all cases, excessive heat gain, indoor air-quality requirements or external noise may limit or preclude the use of natural ventilation.
As we know that designing for a comfortable internal condition in healthcare buildings is a necessary goal for occupants' well being, good health and high staff productivity, this project has been aimed to achieve the principles of passive ventilation with the employment of wind catchers for natural ventilation, glazed atrium area to provide ample fresh of air into the internal spaces and all windows designed with horizontal pivoting and thus reducing the energy consumption at no extra cost to the building overall.
Ventilation costs have also been minimized by ensuring that, where practicable, core areas such as the sanitary facilities, dirty utilities and those rooms where clinical or functional requirements have specific environmental needs; have been grouped together to provide with a small amount of mechanical ventilation servicing these spaces.
1. Health Technical Memorandum 03-01: Heating and ventilation systems. Specialised ventilation for healthcare premises. Part A: Design and validation.
Ventilation System Design Calculations
This section describes the step taken to sizing the windcatcher system for the design project. The first step taken in designing the windcatcher ventilation system for the healthcare project is to determine the airflow quantities necessary to satisfy the required IAQ and thermal comfort criteria. These flow rates will vary for each season of the year. There are a number of methods available and each, method has a part to play in the design process. The methods are:
design charts based on parametric analysis
manual methods included in the Volume A of the CIBSE Guide
In this section, I will be explaining further on the simulation software because of its usage in the design project. I have used a software called URBVENT which offers natural ventilation potential in urban context. It helps also to assess the potential for natural ventilation of a given site by comparison with the base rates installed on the software. The program also provides a set of graphs which are shown below.
Natural ventilation potential graphs
Three graphs have been retained for the purpose of this project:
the average squared wind speed;
the average stack temperature difference;
the fraction of time when free (passive) cooling is possible.
The first two are strictly associated with (hygienic) ventilation potential. Indoor temperatures have been determined on the basis of the ventilation strategy to be applied. The third graph provides the fraction of time when free cooling is possible. The functionality is accessed by pushing the graphs button of the URBVENT software after all data have been entered in.
All graphs have been produced using the URBVENT software.
Source: Natural Ventilation Software
Figure 5.1 Natural ventilation potential graphs (situation in Stoke Aldermoor, Coventry)
Graphs of degree hours
These graphs represent the degree hours associated with the heating demand; the cooling saved by ventilation; and the cooling demand.
Source: Natural Ventilation Software
Figure 5.2 Degree-Hours graphs (situation in Stoke Aldermoor, Coventry)
Only airflow due to stack effect is displayed in figure 5.3 below without accounting for the effect of wind pressure. This graph represents airflow in a very simple example, comprising a room with two openings as sketched in the top left corner. This is a monthly averaged value which does not guarantee that the airflow is always sufficient.
Source: Natural ventilation software
Figure 5.3 Degree-Hours graphs in distribution form (situation in Stoke Aldermoor, Coventry)
The data obtained from the software will be used to calculate the required number of wind catchers and size needed in the design of its ventilation system.
Wind Catcher Calculations
I have decided to use the waiting area/reception area as a model for the windcatcher system calculations.
Reception /Waiting Area
Fig 5.4 Room used for calculations
115W per person
Building U Value
0.25 U value
Qty of Equip.
Electrical Lighting level
Ceiling Mounted Exam. Light
Window Air leakage
Door Air leakage
Fresh Air Requirements
The basis for any natural ventilation strategy is to provide a fresh air to the occupants. Guidance is given within BS5925 1. I have recommended also 12l/s per person as a fresh air supply figure in accordance with new CIBSE 2 recommendations.
Minimum air flow (m3/s) =
3 x 12l/s/person
Minimum air flow (ac/hr) =
0.036m3/s x 3600 s/hr
= 1.96 ac/hr
Having established the requirement for fresh air, the next step is to provide sufficient airflow to dissipate the heat gains from solar gain, the structure, people and equipment. Based on the data set out in the table above, the calculations are as follows:
Net ventilation required in relation to net heat gains.
Heat gain from:
6.4 x 0.55 x 0.3 1.1kW
People heat gain
No of people
Equip heat gain
No of equip
Add. Electric equip
Lighting heat gain
Heat gained to be ventilated =
1. BS 5925: Code of practice for ventilation principles and designing for natural ventilation.
2. CIBSE Applications Manual AM10: Natural Ventilation in non domestic buildings
Maximum Ventilation rate required due to the above heat loads
Net Heat gained
SHC of Air
For these calculations, I assumed a temperature difference of 9oC; and using the data above, first will be to calculate in m3/s and then convert to ac/hr using the formulae below.
ρx cpx (Δ Temp)
3600 x Q (m3/s)
Room volume (m3)
where H = net heat gained (W); ρ = Air density (kg/ m3); SHC of Air (kJ/kg K)
Substituting the values,
ρx cpx (Δ Temp)
1.293 x 1005 x 9
3600 x Q (m3/s)
Room volume (m3)
3600 x 0.46
Maximum ventilation rate =
Reduction factors (ac/hr) for windows, doors and the building envelope
building air leakage (Moderately 'tight' building (complies with 2005 CIBSE regulations))
Effective contribution from perimeter windows / doors
Windcatcher residual airflow required
Predicted Ventilation Rates and the recommended systems
Figure 5.1 graphs above shows the average wind speed for the proposed site at Stoke Aldermoor, Coventry ranging from 2.4m/s to 3.9m/s at different months. The MET office states an average wind speed across the UK of approximately 4 to 4 ½ m/s. However, my aim is to select the optimum Windcatcher system that will provide the desired ventilation rate for the proposed development. According to Monodarught's website, the company responsible for supplying and manufacturing the windcatcher system, a size 1200 Square Windcatcher System is predicted to provide the target ventilation rate at an external wind speed of between 2 and 3 m/s.