Ventilation









Overview

The design of the hotel ventilation system is vital to the overall comfort of the occupants who will inhabit the building. The system will bring fresh air with more oxygen and less carbon dioxide at the same time as remove the hazardous chemicals that would be present from the chemicals used in the swimming pool or the dangerous volatile organic compounds which are present in any dwelling to improve the air quality of the indoor spaces.

 Ventilation is the process of continual change of air to a room and there are two means by which to move this air. The first is by Mechanical Ventilation which draws air from the exterior of the building through ducting and fans to the space, the ‘old’ air is extracted from another area within the space to be released to the atmosphere. The other option is natural ventilation, which uses passive processes and makes use of the local micro climate to facilitate the air movement within the building. Both systems will be considered for use within the hotel.

 The balance between low energy use and the comfort levels created is a conflict which will be encountered within the ventilation design strategies. Whilst a low energy design is advantageous, within a hotel environment, occupants expect very high levels of comfort and are paying to be within this atmosphere. The comfort therefore will take greater priority, as the hotel will only be economically viable if it can attract visitors and keep them satisfied.

 Bearing both comfort and energy consumption in mind the two main ways of ventilating the hotel are outlined below with the advantages of integrating the systems.

 Mechanical Ventilation

Advantages

  • Fresh air can be supplied with ease deeper within the building.
  • Not dependant on outdoor weather conditions
  • Air flow rate is easily controllable
  • Air can be directed to allow the output to be passed through a passive heat exchanger

Natural Ventilation

Advantages

  • No noise produced in the operation of the system
  • Completely passive so no energy required.
  • Minimal maintenance required
  • Decreased capital costs

 

Due to the advantages of both systems being highly desirable a ‘hybrid’ ventilation system for the hotel will be used, where as much of the system as possible will be made up from Natural ventilation to save in energy consumption and where there is a shortfall in air change rates, Mechanical Ventilation will be implemented.

 

As much information as possible was gained about the required levels of ventilation for the various areas of the hotel from the CIBSE guide B in order to design a system to meet the hotels requirements.

VENTILATION REQUIREMENTS FOR HOTEL

Room type

As recommended by CIBSE B

Ventilation

Recommended
 fresh air supply

Air changes
per hour

Velocity of air (m/s)

comments

Leisure

 

Changing Rooms

as by occupants

6 to 8

-

supply at low level to aid floor drying

Gym

as by occupants

4

-

 

Swimming Pool (recreational) 15x9x1.5

1350l/s-2025l/s

No recirculation

adequate to remove
chemicals from water surface

should have lower pressure than outside,
inhibits mitigation of moisture and odour

Accommodation

 

Guest Bedroom

opening 1/20 GS

greater than 1

-

-

Staff Bedroom

opening 1/20 GS

greater than 1

-

-

Bathrooms

6l/s per pan

3, (5to10)*

-

Only need run for 10-20mins at a time

Catering

 

Kitchens

>17.5 l/s GS(m2)

20 to 30

0.35m/s extractor

-

Restaurant Area

as by occupants

10 to 15

-

-

Other

 

Lobby

as by occupants

4 to 6

-

-

Chemicals storage cupboard

-

 

0.3-0.6 m/s

Should have sash openings

Laundry

as for machines

10 to 15

-

-

* recommended but not compulsory

Source: CIBSE guide B

Natural Ventilation theory

 The first part of the design process was to determine a strategy for natural ventilation for as much of the hotel as possible. There were some areas highlighted initially from the table of demands that would be harder to naturally ventilate due to the high air flow rates such as the swimming pool and kitchen areas.

There are two main types of natural ventilation

 Pressure driven

Where the wind forces air in one side of the building and out through the roof, or the flow of air through the building draws air in at a lower level known as the venturi effect.

Source: http://nett21.gec.jp

Stack effect

The stack effect occurs where there is a temperature gradient with height. As air lower down is warmed, its density reduces in comparison to the cooler air around it. This causes it to rise due to buoyant forces and fresh air is drawn in as the hot air rises and leaves through a duct in the roof.

 

Source  http://www.cibse.org

 

Heat Recuperators

The downside to the natural ventilation systems is that the out-going air does not pass the in-going air so heat recuperators generally cannot be used. This is not a problem in warmer climates but due to the location of our hotel the predominant load is heating, hence any heat recaptured from the air leaving will be of great benefit in reducing the energy demand.

 

Heat recuperators are passive devices which recover heat from the air leaving the building and pass this to the fresh air entering the building. With an efficiency of about 60% it can be understood that this will reduce the heating demand for the building by approximately the same value.

 

Final Natural Ventilation Design With Heat recovery

The location of this hotel lends itself well to the pressure driven ventilation as the air will be very good quality wherever the intake is located and there are no obstructions upwind of the hotel. The low humidity levels of the air also means that by taking in fresh air the need for dehumidification is nullified.

 

Using a combination of stack effects and pressure driven ventilation, with concepts taken from studies at Cambridge University and the functional sustainable housing development BedZED a design for the natural ventilation was conceived with maximum heat recovery in place.

 

Winter Strategy

The winter strategy is shown above.

Bedrooms:

The air for the windward bedrooms is drawn in through ducts located at the bottom of the large lower windows, as the air is forced in due to the wind pressure. The air then passes through the gap between glazing as is shown in the diagram. This captures the heat otherwise lost through conduction and due to the positioning of a low emmisivity coating on inside of the outer glass pane the longwave radiation for areas such as the swimming pool and the restaurant. The warmed air will then enter the room through a controllable vent at ground level, where it will pass through the room and leave through the en suite bathroom. The air then travels up a duct past a heat exchanger where it exits through a cowl on the roof. The exiting air is forced out by both thermal buoyancy effects and due to the negative pressure created behind the cowl.

 

The cowl design from the BedZed development will be used to ensure the rear bedrooms are fully ventilated. The cowl rotates to point into the wind and uses wind pressure to force the air into the cowl and over a heat exchanger, where it gains heat from the air leaving both windward and rear bedrooms. The air then passes through ducting to enter the bedroom at floor level, and exiting again through the en-suite. 

Source http://www.bedzed.org.uk

Both bedrooms will have extractor fans mounted in the en suites to be used at the occupants discretion.

Restaurant/office:

The air will be taken in through large vents located on the windward side of the atrium, where the air can pass through a heat exchanger and recover energy from the leaving air. The air will be pressure driven, but with the option of a fan to supplement the air intake to allow the air to reach the far reaches of the building. The air will be ducted down through the floor, passing closely by the under floor heating to allow the air to be gently warmed before it is vented into the restaurant through strategically located ducts. The hot air will then rise and collect in the channels of the ceiling, which have a gradient to allow the air to flow with buoyant forces back to the atrium where it will rise, pass the heat exchanger and leave the building.

This system allows heat to be recovered whilst using mainly natural ventilation, with the addition of mechanical ventilation where required.

Summer strategy

The winter ventilation system will easily be changed into a summer ventilation system by the closing and opening of vents, making the transition very straight forward for the occupants or the staff of the hotel to control.

 Bedrooms,

For the summer ventilation of the bedrooms, the lower vents are closed and the windows opened to introduce fresh cool air into the rooms, the front room will operate on wind driven ventilation while the rear bedroom will make use of the venturi effect caused by the flow from windward bedroom to the cowl exit which will have a reduced pressure.

 

Restaurant,

The vents for the upper bedroom are redirected into the restaurant and the under floor air supply shut off to allow fresh air into the building. The ‘old’ air is again channelled towards the atrium where it is expelled.

Overview

Where the natural ventilation is installed this will offset almost all of the mechanical ventilation required, with the fan ventilation only expected to run for very short periods of time. By being able to incorporate the heat recuperators it is also possible to reduce the heating demand by up to 60%.

Mechanical Ventilation

High ventilation rates are required in the kitchen and in the swimming pool and due to the locations within the building heat recovery would be difficult with natural ventilation. In the kitchen a permanent air velocity is required and hence extractor fans will be used due to the variability of natural ventilation.

 

Kitchen Ventilation

High efficiency fans such as Aerofoil Bladed centrifugal fans will be used with efficiencies of approximately 80-85%. There  is a greater noise related with these types of fans but noise reducing filters will be positioned in front of the fan to not only to reduce the noise but also the grease and dirt build up in the fan and heat exchanger. These filters will be easily accessible for cleaning purposes. Although this may reduce the fans efficiency slightly the reduced maintenance costs on the fan and heat exchanger will be appreciable.

Source http://www.ebigchina.com.htm

Using the CIBSE ventilation guide and a lower end efficiency for the fan, to take into account the use of filters, the power used by the fans for the kitchen will be 300 watts (calculated from the minimum air velocity and air exchange rates).  These however will only be used during food preparation hours and hence will run for approximately 10 hours a day, giving a total energy consumption of 1.095MWh per year, which is considerably less than using other types of fans.

 

Swimming Pool Ventilation

The greatest concern within the swimming pool environment is the humidity of the air. Greater moisture contents are not desirable as they increase the chance of condensation and hence mould growth, the relative humidity should therefore be kept at about 60% or 16.3g/Kg for 30°C as is shown in diagram.

 

 Source http://www.recotherm.co.uk/how.htm

As is the case with the kitchen,  the swimming pool having heat recovery and natural ventilation is not feasible. Due to the higher temperature required within the pool it was calculated that throughout the year mechanical ventilation uses considerably less energy than that which would be lost due to the lack of heat recovery.

 

Use of a fresh air ventilation system can reduce the moisture content of the air within the swimming pool to adequate levels and using the CIBSE Ventilation Guide H, high efficiency fans would required  1700 watts to ventilate the swimming pool space.

 

The extractor fans located in the bedrooms, restaurant, and offices will have to be less noisy making them generally less efficient. Quiet fans such as a multi-vane centrifugal fan would be used with an efficiency of 60%. The rating of the fans in the bedrooms would be approx 50 watts, but would run intermittently at the occupants discretion.

References:

Environmental Science in Building, Randall McMullan. Published by Palgrave, 2002

Bed Zed development, http://www.bedzed.org.uk/

U.S Department of energy efficiency and renewable energy, http://www.eere.energy.gov

CIBSE, http://www.cibse.org

ASHRAE Ventilation Standards,http://www.ashrae.org

Swimming pool ventilation, http://www.recotherm.co.uk/how.htm

Low noise fans  http://www.ebigchina.com.htm

Global environmantal centre foundation http://nett21.gec.jp






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