Thermo Second Assesment

8/3/2019 Thermo Second Assesment

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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.


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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


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cross flow

The schematic for the house is shown below.


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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


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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


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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


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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


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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"


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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.


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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


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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


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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


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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]}


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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


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Thermo Second Assesment

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