Thermo Second Assesment
-
Upload
tom-whittingham -
Category
Documents
-
view
223 -
download
0
Transcript of Thermo Second Assesment
-
8/3/2019 Thermo Second Assesment
1/20
-
8/3/2019 Thermo Second Assesment
2/20
-
8/3/2019 Thermo Second Assesment
3/20
Thomas Whittingham 20036515
INTRODUCTIONto save on the energy required to heat the eco house all most all draughts will be sealed resulting
in no mass transfer of heat energy throught the hot fluid escaping and cold fluid entering.The
Building Research Establishment [1] notes that the exfiltration of warm air can account
for as much as 30% of the heat loss through a buildings envelope.
As the required air changers required per hour for hermitically sealed house is 0.6 per hour [2] the
aim of EP consultancy is to design a suitable air replacement system that can extract as mutch
usefull heat from the warmed slate air , Reducing the enviromental impact of the house. Hope
fully gainin the aproval of channel fours Kevin mercloud..
essesially EP will mathmaticaially desighn and anlise sevral different types of heat exchanger
namely, cross flow, shell and tube and plate type.all spefications will be suitable for the eco
house. design, sizing and rating analysis shall be conducted through several different methods;
Log Mean Temperature Difference.
Epsilon-Number of thermal units.
Overall Heat Transfer co-efficient.
Full descriptions of each method shall be given in the body of the report and all geometric
configurations shall be evaluated and compared from the perspective of price ,effectiveness
,practabilty, size, and expected enegy saving.
heat exchangeres are a device for efectivially tranfering heat between two mediums.the gouvning
principal that drives this heat transfer is the "Zeroth law" stating that if a fluid or medium is in
contact with another fluid whoes tempreature is lower than its own, given enough time the two
fluids will reach thermal equalibrium.the second law of thermodynamics is also of critical
importance to heat exchangers "energy cannot be created or destroyed only converted from one
from to another. meaning that the total energy lost from the warm stale air must be gained by the
cold fresh air. Of course assuming no external heat loss is present and the operation is steady
state.
-
8/3/2019 Thermo Second Assesment
4/20
Thomas Whittingham 20036515
Predicted by theory and proven by practise E.P Consultants will supply a correctly specified and
practically sized heat exchanging air replacement system. Witch shall pay back its initial capital
out lay in ((((years))
ANALYSIS OF HEAT EXCHANGER TYPESTo correctly design a heat exchanger to the highest effencie, whitind a size sutable for unobtruise
operation, Basic fluid stream configurations must be evaluated.
THE 3 MAIN CLASSIFICATIONS FOR FLUID STREAM CONFIGURATION ARE ;
Parallel flow
When the hot fluid and the cold fluid
stream are flowing in the same
direction.as both fluids enter the
exchanger a large temperature
difference will be present. Along the
Couse of the exchanger the fluids
exchange heat, from the hotter to the
cooler, as the length of the exchanger
is increased so the fluids approach
thermal equibrilium.
as can be seen in FIG[3] the highest
possible temperatures the cold fluid
can obtain is the mean of the hot and
-
8/3/2019 Thermo Second Assesment
5/20
Thomas Whittingham 20036515
cross flow
The schematic for the house is shown below.
-
8/3/2019 Thermo Second Assesment
6/20
Thomas Whittingham 20036515
ASSUMPTIONSAssumptions and their respective error are to be quantified later in the report
As the house is a terrace and no data was available for the heating of the
two adjoining properties, so the difference in temperature is assumed to be
zero and so there is no driving force for heat transfer enabling the two
adjoin wall to be eliminated from the model.
all walls are assumed to be isothermal flat plates
-
8/3/2019 Thermo Second Assesment
7/20
-
8/3/2019 Thermo Second Assesment
8/20
Thomas Whittingham 20036515
A simplified (user friendly) model is available to the client upon request.
The following has been included in the model
conduction
1. through walls and plaster
2. through the roof
3. through the basement
4. through the front door
5. through windows
convection
1. forced convection (external)
2. natural convection (internal)
weather data
1. 5 minuet data averaged over 2 years. Gaining temperatures and
wind speeds at applicable times in the day.
2. extensive use of VBA to extract above values
interpolation of air properties linked to VBA coding
iterative calculations to accurately calculate values for h
CALCULATIONSMost of the formulas used in the report are taken and adapted from "introduction
to THERMAL and FLUIDS ENGINEERING" {3}
Giving conductive heat transfer as
-
8/3/2019 Thermo Second Assesment
9/20
Thomas Whittingham 20036515
Q=k*A*T1-T2L
{3}
And conductive heat transfer as
Q=h*a*(Ts-Tf)
{3}
Combing the two expressions for the wall facing SSW gives (noting that the arrears are exactly
the same for each surface)
And then for the windows facing SSW
With the h values being worked out with a series of calculation's
Analysed first is forced convection (h outer)
(considering fluid flow over an isothermal flat plate.)
Firstly calculated is the Reynolds number given by (where x is the length of plate under
consideration)
Re=*v*x {3}
And Pr is given by
Pr=kcp {3}
Then imputing the two values gained in to below formula
Nu=Hlk=0.664Re12*Pr13
{3}
Re arranging to give the value for the external h value
-
8/3/2019 Thermo Second Assesment
10/20
Thomas Whittingham 20036515
Now for the internal value of h (natural conduction )
First needed is the volume expansivity of air:
=1k For an ideal gas {3}
Next the Grashof number:
Gr=g**Ts-Tf*L32
{3}
Multiplication of the Grashof number by the prandalt number gives rise to Rayleigh
number: and if the geometry of the plate is known a Nusselt number can be
gained. From:
Nu=0.1*Ra13
{3}
or more aptly
Nu=0.825+0.387*Ra161+0.429Pr9168272
As the Ts is not known a guess has to made initially iterations are then carried out
through the rearrangement of the above formulas. a detailed run through one
iteration will be included in the appendix.
All k values were taken from {4}
For the iteration of h values (internal) the following sequence was followed
A guess was made for the surface temperature. giving Ts as (Tf -2)
Allowing a Q value to be calculated. and thus a value for h
Re arranging the formula to give a nusselt number.
Nu=hlk
Taking this back to the Rayleigh number
-
8/3/2019 Thermo Second Assesment
11/20
Thomas Whittingham 20036515
And then back to the surface temperature (in this case for the roof)
Re inserting this value of Ts in to the original formula (remembering to enableiterative calculation's)
Giving accurate h values for all walls and roofs.
INTERPERELATION of air properties the below table was graphed and interpolated
for all the relevant air properties.
C kg/m3 kg/m.s kj/kg.K W/mK Temp(c) Cp K Pr
-20 1.3958 1.6222000E-05 1005.4 0.022507 0.72467
-15 1.3687 1.6478000E-05 1005.4 0.022903 0.72337-10 1.3426 1.6731100E-05 1005.5 0.023296 0.72212-5 1.3175 1.6982100E-05 1005.5 0.023686 0.720920 1.2933 1.7231000E-05 1005.6 0.024073 0.719775 1.2699 1.7478100E-05 1005.7 0.024548 0.71866
10 1.2474 1.7722000E-05 1005.8 0.02484 0.7175915 1.2257 1.7965100E-05 1005.9 0.025219 0.7165720 1.2047 1.8205100E-05 1006.1 0.025596 0.7155925 1.1845 1.8444100E-05 1006.3 0.025969 0.7146530 1.1649 1.8681000E-05 1006.5 0.026341 0.7137535 1.1459 1.8915100E-05 1006.7 0.02671 0.71289
40 1.1275 1.9148100E-05 1006.9 0.027076 0.7120745 1.1098 1.9379100E-05 1007.2 0.02744 0.71128
QUANTIFICATION OF ASSUMPTIONS"Radiation heat loss will be near zero"
-
8/3/2019 Thermo Second Assesment
12/20
Thomas Whittingham 20036515
As shown above the total radiative heat loss for all the windows is a pitifully small value not worth
addition to the total heat loss.
Radiation only becomes an apparent heat transfer mechanism at larger temperatures due to theforth power. Exhibiting exponential transformation.
"all walls are assumed to be isothermal flat plates"
If actual surface roughness was taken in to account the solution would be far
beyond the scope of our model and having a small effect on the major causes of
heat loss.
"In the double glazing no convection of heat takes place only conduction
mechanisms are present."
Again too complex for the model to include. As the air is in an enclosed surface
heat rising on the inner surface will then fall down the outside surface as it cools
transferring heat along its journey. a reasonable assumption therefore is
conduction of air is the only medtoh of transport.
"The house will be steady state"
For the purposes of the model, major sources of heat loss have to be identified not
a complete analogy of the house. So for the purposes of the model constructed atransient model would require more time and experience resulting in a higher
price for the customer.
-
8/3/2019 Thermo Second Assesment
13/20
Thomas Whittingham 20036515
RESULTS
Price including vat = 660.764
Co2 emission from {6}
Price for kWh of gas {5}
Shown below is the heat loss for one day in May at time's 16:00-22:00
-
8/3/2019 Thermo Second Assesment
14/20
Thomas Whittingham 20036515
SUGGESTED IMPROVEMENTSAs observed from the above table a huge relative heat loss is present in the ground floor windows
facing NNW.
Calculations were then carried out by our team inserting a double glazed window in to the
mathematical model where there once was a single glazed window.
With the double glazing added:
Q calculation'sfor windowNNW GFtemp inner 20
temp outer11.057
34total area ofwindow 5.68length of glass 0.004length of air 0.08
-
8/3/2019 Thermo Second Assesment
15/20
Thomas Whittingham 20036515
gapk for glass 0.96
k for air0.0252
19
h inner
10.210
97
h outer25.360
88
Q=15.309
13
Giving rise to a calculated saving of 43.25 per year
With the new windows and fitting costing around 700 pay back can be expected within 16 years.
More expensive and invasive heat retention technology is available on the market. Theseinnovations can save the client much more on their energy bills a discussion of the clients wished
capital expenditure is advised and a suitable energy saving strategy can be implemented.
Some examples to consider are:
Floor insulation,Solid wall insulation,a chimney balloon, planting trees on the front
and back of the properties to block the flow of wind. Less obvious methods include
having 2 Mediterranean neighbours.
CONCLUSION
The total calculated heat loss for the house under investigation is Kwh/Y
With a carbon cost of KG/co2
And a capital cost of 660.7
-
8/3/2019 Thermo Second Assesment
16/20
Thomas Whittingham 20036515
With the installation of double glazing on the ground floor NNW an expected saving of 43.25/y is
possible
Further consultation between client and engineering team will lead to capital identified asavailable for improvements to be properly managed. Resulting in greater carbon and capital
savings for you the costumer.
REFERENCES{1}
T = Tmean - Tamp * e[ -Depth * (/365/)0.5] * COS {2/365 * [tnow - tshift - Depth/2 * (365//)
0.5]}
-
8/3/2019 Thermo Second Assesment
17/20
Thomas Whittingham 20036515
From
Kasuda, T., and Archenbach, P.R. "Earth Temperature amd Thermal Diffusivity at
Selected Stations in the United States", ASHRAE Transactions, Vol. 71, Part 1,
1965
{2}
http://www.sheffieldweather.co.uk/Weather%20Data.htm
Taken on 20/10/2011
{3}
"introduction to THERMAL and FLUIDS ENGINEERING " by Deborah A. Kaminski. Michael K Jensenpublished by WILEY
{4}
http://www.engineeringtoolbox.com/ on the 20/10/2011
{5}
http://www.biomassenergycentre.org.uk/portal/page?
_pageid=75,59188&_dad=por tal
{6}
http://www.engineeringtoolbox.com/co2-emission-fuels-d_1085.html
examples of excell sheet
http://www.sheffieldweather.co.uk/Weather%20Data.htmhttp://www.engineeringtoolbox.com/http://www.biomassenergycentre.org.uk/portal/page?_pageid=75,59188&_dad=por%20talhttp://www.biomassenergycentre.org.uk/portal/page?_pageid=75,59188&_dad=por%20talhttp://www.engineeringtoolbox.com/co2-emission-fuels-d_1085.htmlhttp://www.engineeringtoolbox.com/http://www.biomassenergycentre.org.uk/portal/page?_pageid=75,59188&_dad=por%20talhttp://www.biomassenergycentre.org.uk/portal/page?_pageid=75,59188&_dad=por%20talhttp://www.engineeringtoolbox.com/co2-emission-fuels-d_1085.htmlhttp://www.sheffieldweather.co.uk/Weather%20Data.htm -
8/3/2019 Thermo Second Assesment
18/20
Thomas Whittingham 20036515
-
8/3/2019 Thermo Second Assesment
19/20
Thomas Whittingham 20036515
-
8/3/2019 Thermo Second Assesment
20/20
Thomas Whittingham 20036515