SUPPLY AIR
ΤΟ
RM.
MIXING CABINET
WARM AIR COOL AIR
DUCT
DUCT
temperature is achieved by a ther- mostatic ог manual device which operates the control gear within the cabinet. This gear regulates the air discharged from the warm duct and cool duct to suit the temperature re- quirements of the space.
a
Fan coil units, Fig. 3., comprise a supply and recirculation grille, heater/cooling battery, fans and an air filter.
Operation of the fan draws air from the room and discharges it through the heater/cooling battery into the space being conditioned.
or
Control of temperature is obtained by either regulating the warm chilled water serving the battery or by operating the fan at varying speeds.
With this system it is necessary to introduce fresh air and this is done by means of a fresh air ducting sys- tem or air can be drawn by the unit from outside the building.
The induction unit, Fig. 4, is similar in appearance to the fan-coil unit but instead of using fans to draw the air from the room. high velocity induction nozzles are used.
The re-
quired room temperature is achieved by thermostatically controlling the warm or chilled water passing through the heat exchanger of the unit.
Window units, Fig. 5, are common- ly used. They are relatively inexpen- sive in capital cost and simple to operate but are rather expensive with
High velocity air-conditioning systems
It has always been desirable to keep ducts and equipment sizes to a
SUPPLY AIR
TO RM.
FROM RM.
122
72
RETURN AIR:
CONTROL DAMPER GEAR||
Fig. 2.
regard to electrical consumption. minimum on the grounds of cost. Space too is costly and if a building owner decides to go to the expense of air-conditioning his building he naturally expects as much space as possible to be available for useful occupation. Thus in modern building operations there is a great pressure on the engineers to reduce the space occupied by the
the services to absolute minimum.
an
Such considerations have led to the use of higher velocities in the order of 4,000 to 8,000 ft./min. in the ducts. Such systems, which are common in certain other parts of the world are now coming into use in Hong Kong.
To propel air at such speeds re- quires several times the fan horse- power that would be absorbed by the orthodox system operating at about 1,000 ft./min. in the ducts.
It is necessary with high velocity systems have perfectly air-tight ducts and this necessitates special fabrica- tion techniques. In addition, air moving at this speed gives rise to noise and sound absorbers are an es- sential part of the system. These consist of specially constructed sound absorber boxes which are lined with acoustic material either in the run of the duct or at the outlet or discharge unit.
a
Any consideration of the use of high velocity as compared with conventional system of ductwork must take into full account the econo- mics, which will include the space saving of high velocity ducts as com- pared with the higher operating costs that will be experienced due to greater
CHILLED & HEATING WATER PIPES
DRAIN
IFT TO
COOLER & HEATER BATTERY
SUPPLY AIR
GRILLE
APPROX.
4 FT. OIN.
FAN
RETURN AIR GRILLE AND FILTER
APPROX.
2 FT.OIN
Fig. 3.
consumption of electricity, plus the extra capital cost due to the special fabrication of ducts and the sound at- tenuators.
Ventilation
A mechanical ventilation system installed for purposes of thermal comfort' does not include facilities for cooling the air although a method of heating may be provided.
The application of mechanical ven- tilation systems is therefore limited to buildings where heating is required. rather than cooling and consequently in Hong Kong there is little call for the use of mechanical ventilation for comfort reasons.
In Fig. 6 is shown a simplified ver- sion of a psychrometric chart which is graphical representation of certain thermal properties of air.
By way of example as to the dif- ferent conditions that would be ex- perienced in a building with either an air-conditioning or ventilation system, condition points have been plotted. State B at approximately 94°F. dry bulb and 56% relative humidity re- presents the condition that would be obtained with a mechanical ventila- tion system providing 12 changes of air per hour, and this occurs when the outside air is at a condition of 85°F. dry bulb and a relative humidi- ty of 70%.
The air-conditioning system can however reduce the temperature and moisture content of the air below that of outside of the building and on the chart the conditions have been plotted at 75°F dry bulb with a relative humidity of 50%.
In the case of the air-conditioning plant the conditions to be obtained in the building are dependent on the heat gains to the spaces and also the thermal condition of the air supplied to the rooms and this latter supply condition is shown on the chart at state C. The mechanical ventilation system however supplies air to the various rooms without any cooling and the air so supplied is at the con- dition as existing outside of the build- ing.
Therefore in the case of a me- chanical ventilation system the tem- perature and relative humidity ex- perienced within the rooms will be a function of the air quantity provided. the heat gains of the spaces and the characteristics of the building struc-
ture.
Curves 1 to 5 are of particular in- terest as they represent five different buildings each equipped with an iden- tical ventilation plant which provides the same quantities of fresh air to each building. Each building also experiences identical heat gains due to occupants, electric lighting, solar heat etc. but the slope of the curves differ due to the varying proportion of heat transfer by conduction as compared to the heat of occupants, electric lights, etc.
In Table 1 is given the results of calculations applied to the five build- ings and it is seen in column 4 that
Far East Architect & Builder October, 1965