Download (buoyancy-driven) stack ventilation

Ventilation for Low
Energy Buildings
Dr Nick Kelly
Energy Systems Research Unit (ESRU)
Mechanical Engineering
University of Strathclyde
Overview
• Ventilation and Infiltration
• Natural Ventilation
– wind-driven pressure
– buoyancy-driven flow
• Mechanical Ventilation
– fans
– system resistance
– operating conditions
– heat recovery (heat exchangers)
Ventilation and Infiltration
• all buildings require some form of
ventilation
– supply of fresh air (comfort)
–for removal or dilution of contaminants
(health)
• ventilation is the deliberate supply of air
to a building
– natural ventilation
– using a fan and ductwork
• infiltration is the unwanted leakage of
air onto a building through cracks and
apertures in the building fabric
Measuring Infiltration
• measurement of air leakage [infiltration]
in buildings is done using a blower door
test
• the building is pressurised to 50Pa and
the decay rate of pressure measured to
get a leakage rate
• … this can be divided by 20 to get the
typical infiltration under normal
conditions
• some typical infiltration levels are:
> 1.5 (leaky [older house])
< 1.5 – 0.5 (average)
< 0.5 (low leakage)
0.03 (PassivHaus)
Natural Ventilation
• is the ventilation type in most
smaller UK buildings
• driven by wind pressure and
density variations (buoyancy)
– single sided ventilation
(buoyancy-driven)
– stack ventilation (buoyancydriven)
– cross flow ventilation (wind
driven)
Natural Ventilation
• driving force will often be a combination of
wind + density (buoyancy) forces
• influenced by:
– wind direction
– wind speed
– ventilation opening location
– interior/exterior temp. difference
– internal gains
– building geometry
• no energy required for ventilation
• but results in highly variable flow (magnitude
and direction)
Buoyancy Driven Flow
• from the Gas Laws
(PV=mRT); air density 
1/T
• The weight of two vertical
columns of air at different
temperatures separated
by a vertical surface will
differ and a pressure
difference, Ps, will be
created across the
intervening surface. If
openings exist in this
surface, Ps will cause a
flow of air to occur.
Pa
P  gz
Ra
 1 1
  
 T1 T2 
 1 1
P  3462 z  
 T1 T2 
Buoyancy Driven Flow
• The pressure gradients in the two columns will differ, the
greater of the gradients occurring where the temperature
has the lower value.
Tint
Text
Ph2
Height
h2
neutral
plane
Ph1
h1
external
pressure
internal pressure
gradient
Pressure
Buoyancy Driven Flow
• The total induced stack effect pressure is:
Ps
=
Ph1 + Ph2
 1
 1
1
1
Ps  3462h2 


  3462h1

 Text Tint 
 Text Tint 
 1
1
Ps  3462h2  h1 


 Text Tint 
Wind Induced Pressure
• when wind blows over a building it creates regions of
either positive or negative (below atmospheric pressure)
• these can be used to promote flow
Wind Induced Pressure
• the generated pressure on a surface is can be
approximated using:
1
2
Pi ,d  Cid U d
2
• Ci,d is a pressure coefficient for a surface at some angle i in
relation to the wind direction d
• the coefficient is generally positive for surfaces facing the
wind and negative for leeward surfaces
• lists of ‘typical’ coefficients are tabulated for different
surface types at 22.5o intervals
• alternatively, Cp values can be taken from C FD models or
wind tunnel tests
Flow Through Fabric
• wind-induced flow can occur deliberately (open window) or
be unintentional – though cracks and other small openings
• typically the flow rate through the opening is expressed as
a function of the pressure difference across it
m  f ( P )
• e.g. power law flow
n

m  a P
Flow Through Fabric
Natural Ventilation
• given the range of driving
forces and general
complexity of natural
ventilation (strongly
coupled with
temperatures) computer
modelling is often used to
assess natural ventilation
schemes
• gives an indication of the
variability of flow and the
influence on internal
temperatures, comfort and
air quality
Natural Ventilation
the reality!
the drawing …
Mechanical Ventilation
• ventilating a building mechanically requires one or more fans and a
distribution system
Fan Performance
• in mechanical ventilation systems
fans can be used to move large
volumes of air from one point to
another
• the pressure and resulting flow
generated by a fan is an order of
magnitude greater than that
achievable from either buoyancy or
wind induced pressure
2
 m 
 m 
 m 
P  ao  a1    a2    a3  



P
• the flow induced by a fan is typically
expressed as a 3rd order function
m/

3
Ducting Pressure Losses
• the job of the distribution system is
merely to convey air from one point to
another
• the fan is required to overcome
various frictional losses in the system;
there are proportional to the air
velocity2
P
V2
pL  K
2
4 fL V 2
pL 

D
2
• these losses occur:
- at the interface of the air and the
ducting
- at fittings (junctions, expansions,
etc)
- due to equipment (heating/cooling
coils, etc)
m/

System Flow Rate
• the flow rate achieved by a particular fan is determined by
identifying the intersect of the fan performance and system
resistance
P
m/

Fan Laws
• the so-called fan laws can also be applied to assess the effect of
changes in fan speed
• volume flow (m3/s) varies proportionally to the fan speed
Q 2 N 2


Q1 N1
• Pressure (Pa) varies as the square of the fan speed
P2  N 2 

 
P1  N1 
2
• power consumed (W) varies as the cube of the fan speed
p2  N 2 

 
p1  N1 
2
MVHR
• modern low energy buildings are typically tightly
sealed and employ mechanical ventilation heat
recovery rather than relying on natural
ventilation
• this gives a consistent supply of fresh air,
without a significant heating energy penalty
• heat recovery in an MVHR system is typically
achieved using a plate heat exchanger
• this takes heat from the warm exhaust stream
and transfers it to the incoming
Exchanger Basics
•
As the name implies a
heat exchanger is a
device that promotes the
transfer of heat between
two or more fluids.
•
Heat exchange can take
place due to:
– mixing of the fluids;
– heat flow between
fluids separated by a
solid surface (no
mixing can take
place).
hot fluid into heat
exchanger at
temperature T1
cold fluid exits heat
exchanger at
temperature T4
heat exchange from
hot to cold.
hot fluid exits
heat exchanger
at temperature
T2
cold fluid enters heat
exchanger at
temperature T3
Consider the simple heat exchanger shown above: as a
"warm" fluid passes over the exchanging surface it losses
heat; this heat is absorbed by the "cold" fluid, which is in
contact with the reverse side of the heat exchanging
surface.
Energy Analysis
•
Heat exchangers can be analysed
using the steady flow energy equation
(SFEE).
heat transfer with surroundings – work = output energy rate – input energy rate

Simplifies to:
Energy Analysis
• For a simple two stream heat exchanger the energy balance
is:

If

Then the equation becomes
heat lost from hot stream = heat gained by cold stream
Basic Flow Types
• There are two basic heat exchanger configurations:
– parallel-flow exchanger;
– counter-flow exchanger.
Parallel Flow
•
•
In a parallel-flow
exchanger, the fluid inlet
ports of the two air
streams are located at
the same end of the
exchanger.
The stream-to-stream
temperature difference is
the greatest at the inlet
and at it’s smallest at the
outlet.
Hot fluid outlet 2
Hot fluid inlet 1
Cold fluid inlet 3
Cold fluid outlet 4
1
Temp. T
Hot fluid temp.
2
4
•
The greatest heat transfer
between the streams
occurs at the inlet.
Cold fluid temp.
3
Length
Counter Flow
•
•
In a counter-flow
exchanger, the fluid inlets
are located at opposite
ends of the exchanger
results in a near constant
temperature difference
throughout the length of
the exchanger
Hot fluid outlet
2
Hot fluid inlet
1
Cold fluid outlet 4
Cold fluid inlet
3
1
Temp. T
Hot fluid temp.
•
near constant heat
exchange per unit length.
2
4
Cold fluid temp.
3
Length
Efficiency
•


In any heat exchange process the efficiency can be defined as:
The primary stream is the stream in the heat exchanger that gets the
“useful” effect of the heat transfer: the stream being heated or cooled.
The secondary stream performs the useful effect, removing heat from
or giving up excess heat to the primary stream.
Efficiency
•
By this definition of efficiency equation (5) would give us an ideal
efficiency of 100%!
Hot fluid outlet
2
Hot fluid inlet
1
Cold fluid outlet 4
Cold fluid inlet
3
Qloss

A more realistic form of equation 5 is therefore:
Effectiveness
•
A useful measure of a heat exchanger’s ability to transfer heat is its
effectiveness.
•
The maximum theoretical energy transfer occurs when the cold fluid
exits at the inlet temperature of the warm fluid.
•
The fluid with the smallest heat capacity, (W/K), will experience the
largest temperature rise and the maximum amount of heat transfer is
dictated by the maximum amount of heat which this fluid can lose or
pick up.
Effectiveness
•

The maximum possible heat transfer is
therefore:
The effectiveness for a cold primary stream (fluid being
heated) is:
Hot fluid outlet
2
Hot fluid inlet
1
Cold fluid outlet 4
Cold fluid inlet
3
Qloss
Effectiveness

Similarly, the effectiveness for a warm primary stream
(fluid being cooled) is:
Hot fluid outlet
2
Hot fluid inlet
1
Cold fluid outlet 4
Cold fluid inlet
3
Qloss

It is important to recognize the correct primary stream to
get a correct value of effectiveness!
Common Heat Exchangers
•
•
•
a common heat
exchanger in
ventilation systems is
the plate heat
exchanger
used for transferring
heat between two
gases or two liquids
used extensively in
buildings for
ventilation heat
recovery
Common Heat Exchangers
•
•
•
•
finned tube heat
exchangers are
used to transfer
heat between a
fluid and a gas
the fluid flows
inside the tubes
the gas flows over
the fins
heat is transferred
much more readily
to and from a fluid
than to and from a
gas
the fins act to
greatly increase
the heat transfer to
the gas