| Can the existing duct
and SCHW systems be reused in a retrofit project with Dadanco
units replacing the original induction units?
Yes. The primary air flow will be the
basis of the new selection for the Dadanco units. This means the
duct velocities will remain the same, and hence the duct static
pressure losses. The overall system static will reduce when the
changeover is complete and no old units are operational and require
the original higher pressure at their locations.
The secondary water flow will remain the same, although there
have been proposals to increase it by 10% to cater for increased
room loads. This decision is for the designer when deciding if
the increase in pump power is acceptable in the overall energy
equation. With the range of coils available in the Dadanco units
and the secondary air flows achieved with the new nozzles, this
usually means a significant improvement in total unit capacity,
with the original fluid flows and infrastructure.
Can the one primary air AHU feed all of
the perimeter induction terminal units for a floor?
Yes. The correct selection of the primary
air flow and temperature makes this possible. The disadvantages
to the scheme are physical ones relating duct size, ceiling space,
riser locations and the internal zone ducts and boxes. However,
if the building has large areas of glazing on the North face (southern
hemisphere applications), which results in the peak cooling load
for this face occurring in mid-winter, it may be better to have
a separate AHU for the northern zone.
What is the effect of the fan motor heat
on the primary air in a draw-through AHU?
The effect is to raise the temperature
of the air leaving the air handler. The change can be represented
on a psychrometric chart as a sensible heat increase, with the
air leaving condition shifted to the right by whatever is the
calculated rise in dry bulb temperature. This rise needs to be
factored into the system design, as the primary air condition
at the primary air plenum of the units must still be that used
in the selection process.
How is condensation controlled in high
humidity environments?
Outdoor air is pre-conditioned and dehumidified
in the primary air AHU, along with any return air needed to make
up the primary air total. The building is maintained at a slight
positive pressure with respect to the outside to control infiltration
of humid air. Once the dehumidified air is supplied to the space,
the air dew point is monitored as well as the secondary water
temperature, the control system using this data in the control
strategy to provide conditions where condensation will occur.
There are a range of strategies that can be employed to suit any
particular project. These include re-setting the SCHW water temperature
up; switching the SCHW pump off for a period; overriding the primary
air temperature control to increase the rate of dehumidification
at the primary air cooling coil; at start-up after a weekend,
operation with primary air only until the space humidity is reduced
to unacceptable level, and more. Refer to the Controls section
of the Dadanco System handbook.
What about the system shutting down at
night? Won’t the humid air infiltrate and cause a condensation
problem at start up?
The system can and should be shut down at
night to save energy. This will probably lead to some infiltration
of humid air, and the humidity in the space will increase. In
Singapore for example, our experience indicates that the space
humidity increase by as much as 10 – 15% over a weekend
shut down. However, when the system is started up on Monday morning,
the controls always calculate the dew point and maintain the secondary
water temperature slightly above the dew point. During normal
operations, this differential is always maintained with good controls.
This control of the secondary water can be incorporated with the
optimum start program, to achieve savings over a conventional
system. During start up with a conventional system, the air handlers
are typically started and the chilled water valve goes wide open
to provide maximum flow to the cooling coil.
With optimum start the first operation of the cooled and dehumidified
air is to flush the moisture out of the space. This is referred
to in 4.4 above.
Do you need to use high velocity duct in
an induction sytem?
No. With the pressure in the plenum of
an induction terminal unit being 150 – 250Pa, and no longer
450Pa and much higher, the total pressure in the supply system
– the primary air duct system – allows the use of
low pressure and low velocity duct sizing and construction.
How do you maintain the secondary water
temperature?
There are three methods that can be considered. Options 1 and
2 below are for systems where all of the cooling for the installation
originates at one or more chilled water units, or chillers. The
third envisages a separate chiller for the SCHW circuit and will
require the use of an energy program for verification. Brief details
are:
1.
By circulating primary chilled water through
a plate heat exchanger with the secondary water passing through
the other side. The secondary water pump circulates the full secondary
water quantity through one side while a modulating valve controls
the primary water flow to achieve the design secondary water temperature.
A sensor in the outlet of the secondary water line controls the
modulating valve.
2.
The use of a mixing valve that allows an amount
of primary water into the suction side of the secondary water
circulating pump. There needs to be a connection back into the
primary water loop to return a quantity of water equal to that
introduced to maintain the secondary water temperature. A sensor
on the leaving side of the secondary water pump controls the mixing
valve.
3.
The use of a separate chiller to provide SCHW
at the design temperature for this water circuit. The design of
such a plant could have a connection to the primary water circuit,
which would include a mixing valve (as 2 above), with the necessary
isolating valves, to act as a back-up to the SCHW chiller. Remember
the limits on suction gas temperatures if the chiller includes
semi-hermetic compressors for such a higher temperature water
application.
Is the connection to the unit plenum always
on the end?
No. The air connection into the plenum
can be made into the centre of the side of the plenum. This is
recommended where there is limited space between the units. It
often eliminates a 90o bend from the duct connection to the plenum,
which will reduce the duct static pressure losses.
This is applicable to CM perimeter units, Active Chilled Beams
and induction diffusers.
Can an induction system comply with the
building regulations for smoke control with central plant installations?
Yes. AS1668.1: 1998 nominates two methods
for smoke control in buildings. These are purging and zone pressurisation,
commonly referred to as sandwich pressurisation. Induction systems
can perform in either design.
All central plant systems have fire dampers in the supply air
ducts where they leave the riser and enter the floor, as well
as a fire damper in the return air opening into the riser system.
If they are not the motorised type capable of remote control,
a separate motorised shut-off damper will be adjacent and in positions
that do not compromise the operation of the fire dampers.
AS 1668.1 does not give a specific value for the supply or exhaust
rates for either system Rather it describes the required method
of operation to achieve the intended result of excluding smoke
from all non-fire-affected compartments. It also deals with other
possible building air handling systems such as car park exhaust,
but this does not concern us here.
For the zone pressurisation option in a building with an induction
system, the operation of the plant and dampers in the air systems
is as the code. The fact that the total air moved is lower than
for an all-air system is of no consequence. The induction system
is capable of maintaining the minimum pressure difference of 20Pa
between the fire and non-fire compartments. This pressure difference
is tested with all doors closed to required exits. The maintenance
of the pressure differential is a function of the exhaust rate
from the fire floor and the pressurising air supply to the non-fire
floors.
For the purging system, the requirement is for a balance between
the supply of uncontaminated outdoor air and the smoke spill discharge
rates.
For both smoke control methods the current design procedures will
provide an installation complying with the regulations.
It is of interest to note that additional control strategies apply
to VAV systems in AS1668.1. In the commentary in Appendix B for
Section 5, Central Plant Systems, it is required that all boxes
must fail open or be driven open in a fire emergency, to allow
outdoor air to flow into the non-fire floors. This extends to
the wiring of these controls in a fire rated power supply. There
is an alternative, which is to provide a dedicated motorised bypass
damper to allow the air into the non-fire floors. In a number
of layouts this would place this inlet close to the return or
exhaust point.
The major energy saving with induction
systems is in fan power. What is the situation with pumping energy?
There is an increase in total pump energy
as a result of the secondary water system. The primary water flow
to air handlers is reduced, and the primary water pump energy
is reduced accordingly. However, while the total pump energy for
induction systems, primary and secondary, is higher than an all-air
system, it does not significantly reduce the energy savings achieved
by the reduction in the fan power.
As with any exercise in comparing energy usage between competing
systems, each installation must be looked at separately, and the
savings of each assessed and considered against the other factors
of cost, ongoing maintenance and ease of installation.
Can the fan power saving resulting from
retrofitting with Dadanco products be calculated simply?
Yes. As the change is to an existing
system, there is no change to fan type, ducts and other airside
characteristics, for which the formula for fan total efficiency
can be used. This is:
Total Efficiency % = qV x ptF
10 PR
where qV = volume flow of air, m3/s
ptF
= fan total pressure, Pa
PR
= power absorbed by the fan, kW
The 10 is a constant in this formula. If the fan total pressures
are the same, as is the fan efficiency, then the power absorbed
by the fans is in the ratio of the two air flow rates.
The question can be answered best by giving an example from
a project.
Example
The following is an example of how to calculate the reduction
in fan power when Dadanco units replace old units that operated
on higher plenum pressures.
Given: The induction units in an existing system
required 380Pa (1.5”wg) primary air pressure at the nozzles.
The replacement Dadanco units require 200Pa, a reduction of 180Pa.
The original and design total pressure at the primary air fan
was 1500Pa (6.0”wg). With the Dadanco units the total pressure
reduces to 1320Pa. What is the saving in fan power?
Answer: From the fan laws, the relationship of
the new total pressure to the old is given by:
p2 = p1 x (n2 / n1)2 x (d2 / d1)2 x (r1 x r2)1, where
p = pressures, 1 original and 2 the revised values for p, n, d
and r.
n = speed of the fan, r/s
d = fan diameter – which does no change and this element
can be deleted
r = density of the air, which does not change and can also be
deleted
Then
p2 = p1 x (n2 / n1)2, and p2/p1 is 0.88.
As n2 / n1 the square root of p2/p1, the ratio of the speeds is
0.9381.
If we now consider the law relating the change in power, PR, it
is similar to the pressure change equation, and can be reduced
similarly to:
PR2 = PR1 x (n2 / n1)3
PR2 = PR1 x 0.93813
PR2 = 0.8255 PR1
This represents a reduction of 17.45% in fan power.
The above presentation draws from the well presented Fan Laws
page in the Fantech® catalogue.
Please note that it was the total fan pressure that was used.
What proportion of the load is handled by the primary air and
what by the secondary coil?
If you are starting out on a design and need
a feel for the division of the load between the primary air and
the secondary water, allow for the primary air to handle all of
the transmission load, the room latent load and a part of the
sensible loads of the space being considered.
The load to be handled by the secondary water system is the sum
of the people, lights, office equipment and the radiant and transmitted
solar load. The first three you will have based on normal design
standards or the values in a brief, and the others by reference
to solar load tables or your heat load calculation program. The
secondary load will vary from one project to another, but as a
first approximation, allow for the secondary circuit to handle
60% of the load on a room or zone.
Do not include the outdoor air load, as the primary air handler
will cope with this. If you use Camel, the program will include
a part of the outdoor load equal to the bypass factor component.
The “error” resulting from this is small and can be
neglected. A word of caution though, don’t let the bypass
factor get stuck at 0.15. It will be much smaller for a larger
or central plant unit and coil, and possibly lower for a packaged
fan-coil unit. Check it out.
How many units can be controlled from one
control valve?
Depending on the piping design and layout,
it could be one or several. A single temperature sensor can control
a zone, and then control one or more automatic control valves,
each valve controlling the flow to one or more induction terminal
units. The piping and valves after the control valve should be
such that the water flow to each unit is at the required design
flow. It is not out of the question for each unit to have its
own control valve, if the zoning or office requirements want this.
It is not unusual for a pair of units on the perimeter next to
one another to have a single control valve. They may even have
their primary air supply from a single duct take-off.
How low can the secondary chilled water
(SCHW) temperature be without causing condensation?
A basis for deciding on a secondary chilled
water temperature is to relate it to the room dew point temperature.
In theory, a surface at room dew point temperature has the potential
to condense water vapour from the air. In still air conditions
the air film on this surface will act as a layer of insulation,
and allow the temperature of the surface to drop below the room
dew point before condensation commences. The effectiveness of
the air film depends on the velocity of the air over it. In still
air conditions, the air film has the greatest insulating effect
and could “increase” the surface to dew point temperature
gap by around 1.5K. Therefore for a room dew point temperature
of 13oC, a minimum secondary water temperature would be 11.5oC.
|
