18 - thermal equipment

8/9/2019 18 - Thermal Equipment

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Thermal central – Equipments forheat and cooling generation

Heaters, boilers, heat pumps.Chillers (Compression and absorption)

Storage tanks and thermal storage systems

....

Heaters

• Classified by the heating system: – Electrical heaters. – Boilers to generate steam or hot water (water

heaters)

– Heat pumps (using electricity or an engine). – Solar panels (usually coupled with another heater).

• Classified b the fuel used: – Natural gas, biomass, Diesel,

• Classified by the fluid generated: Air , Thermal fluid (oil), steam, hot water, mixture.


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Classification of boilers• Fire or water tube – Depending on the fluid circulating in the

tubes.

• Hot water or steam generators (low, medium, high pressure).• Only for DHW or also for ambient heating.• Conventional or condensing.

Fire tubular Water tubular

Condensing boiler

Flue gas exhaust atlow temperature


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Wood burning equipment• Wood burning equipment is divided in:

– Water heating or air heating equipment

10 kWwater heater

Pellets under grate firedgas tubular water heater

Airdistributionis limited

Use of wood logs or pellets


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Safety issues• Boiler (circuit pump, relief valve, water feed).

– Lower and upper limit of water level and pressure – . – Control of water temperature according to use.

• Fuel circuits (Cut off) – Control pressure limits of gaseous or liquid fuels

• Solid fuels – Fuel storage – Loading system (not automatic with logs) – Humidity level of wood.

Boiler efficiency• The main energetic parameter of the boiler/water heater is

its efficiency that is the ratio between the usefull heat(gained by water) and the heat supplied by the fuel (basedon e ower ea ng va ue or ese equ pmen s .

• The boiler efficiency can be calculated by methods:

• The main losses in the indirect method are due to: –

LHV mQ

fuel

Useful Directmethod

Indirectmethod LHV m

Losses fuel

1

. – Incomplete combustion (CO, Carbon In Ash, fuel leak) – Heat loss to the environment.

The heat recovered in the condensation of vapour in the flue gases isan extra gain compared to LHV so the efficiency may be larger than 1but never higher than the ratio between the HHV and LHV.

to fuel flow.


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Influence of parameters• Excess air is one of the main

parameters to regulate the boilerand combustion efficiency

• Flue gas temperature has a directimpact on boiler efficiency and is usedin simple graphs valid for specificfuels

L o s s e s i n f l u e g a s e s

Area of maximumefficiency

b l e

h e a t

l o s s

i n f r a c

t i o n

( % H

H V )

Flue gas temperature ( oC)

Losses in sensible heat

Losses inunburned

Excess air coefficient

Carbon dioxide (%)

O x y g e n

( % )

S e n s i

The boiler efficiency in general islarger with lower water temperature.

Steam generators• Steam generators is the designation of very large

boilers in industry (including electricity production)

generators used for humidification of air streamsby steam.

Small steam generators includethose used in coffee machines,ironing systems, vacuums, etc.

In small steam generators the waterquantity is small and not circulating


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Heat pumps• Most often based on compression cycles, use the heat

rejected in the condenser at temperatures up to ~ 60ºC.• The higher temperatures use two cascade cycles.• The compressor of the system can be supplied by

electricity or an engine.• Heat pumps can also work as refrigerators if reversible.

Dual purpouse heat pump(Alltherma from Daikin)

• Daikin uses for domestic applications two heat pumps

– HP to heat water to ~35ºC for space heating in houses – HP using part of this heat to heat DHW to 65-80ºCThe two heat pump in cascade have a combined COP

(Coeficient of Performance) of 3 – 2,5respectively for the 65 - 80ºC.


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Heated medium• Heat pumps may heat air directly, in direct refrigerant

systems or water for direct use or to distribute heat to air.• A cold medium is needed to supply heat to the

.• Ocasionally the refrigeration may be used in other

applications (as in the unit shown from Trane)

RTWD (Trane) 30RQ (Carrier)

Colling equipment• As for heaters it can be used to generate cold air,

water or a mixture to remove heat from applicatons.•

named chillers and may work in one of the principles: – Compression (reverse of heat pump) with an

efficiency given by Q cold /Work compression (EER/COP) – Absorption (to produce cold water) with an

efficiency given by Q cold /Q heat source . .

The absorption chiller uses heat at a higher temperature(generator) to pump heat from a lower temperature to anupper temperature. This heat and the heat required in theabsorver has to be rejected to the environment.


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Absorption Chiller Theory

Compression chillers cooledby water or air (in condenser)


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Cooling of water fromcondensers

• Water cooled chillers require the cooling of the waterin refrigeration towers.

• The refrigeration cooling towers use forcedconvection and the evaporative cooling effect totransfer the heat to the air. The working temperatureof water may be a problem for Legionella formation.

Absorption Chiller using LiBr-Water In this cycle, water is the working fluid which changes itsvaporisation temperature/pressure relation with differentfractions of LiBr salt dissolved.

better efficiency but requiring heat at a higher temperature.Mechanical work required by pumps is low (


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Single stage absorption chiller

Schematic representation Trane Horizon

A single stage absorption chiller isused at CGD building usingheat from solar panels.

Integrated units• Absorption chiller usually work at relatively constant

regime and therefore are often combined withcompression chillers, also because the later may producelower temperatures (~4ºC) than the first (~8ºC).

• Absorption chillers may use waste heat from engines.

Refrigeration

Absorption chillerusing engine fluegases waste heat

Gas fuelled engine andcompression chiller.

owers


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Water distribution

Storage tanks and heaters intanks

Support of solar panels heatersuse small tankswith heaters.


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Thermal inertia and storage• As the heating or cooling requirements and its cost of

generation changes considerably during the day, insome cases it is interesting to store thermal energy.

• The use of storage increases the thermal inertia of theinstallation and reduces the peak power capacity of thethermal energy generation system.

• For absorption chillers producing cold water at constantload storage is important.

air and it depends on the storage as well as on thewater quantity in the installation.

• The higher inertia requires also larger start up times.

Thermal Energy Storage• Energy storage is done using sensible heat:

– In water or other thermal fluid used – Using solid materials (e.g. Morse-Trombe wall)

• or p ase c ange ea : – Solidification/fusion of water or aquous solutions – Solid/fusion of mixtures with pre-defined fusion temperature. – Use of solids that change their cristaline structure (solid/solid)

• The theoretical capacity is the amount of energy to performthe chan e of states: Qt = M h – h – M is the mass of substance and h TS and h TI are the

enthalpies at the maximum and minimum temperatures. – For phase change the difference can be h SL or equivalent.

• The storage efficiency is the ratio between the realcapacity compared to the total and is around 90%.


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Acumulação sensívelDomínio de Massa Calor Densidade

u(oC) (kg/m3) (J/kgoC) (kJ/m3 oC)

Aluminio max. 660 2700 920 2484Ferro fundido max. 1150 7200 540 3889Tijolo refractário 2100 - 2600 1000 2100 - 2600Água glicolada(50%C2H4(OH)2 + 50% H2O)

0 - 100 1075 3480 3741

Sal (50% NaNO3 +

a e r a

50% KNO3)-

Granito 2400 790 1896Sódio líquido 100 - 760 750 1260 945Água 0 - 100 1000 4180 4180

Luis Roriz, 2008

Acumulação latenteT fusão Massa esp. Massa esp. Calor fusão

(oC) Liq (kg/m ) Sol (kg/m ) (kJ/kg)Parafinas * 10-71 750-830 785-930 190-255Outros compostos orgânicos C. química (kg/m3) (kJ/kg)

Material

Ácido acético 17 C2H4O2 1266 (sol) 273Ácido mirístico 54 C14H28O2 862 (sol) 199Ácido palmítico 62 C16H32O2 853 (sol) 164Fenol 41 C6H5OH 1070 (sol) 120P-Diclorobenzeno 53 C6H4Cl2 1250 (sol) 121Nitronaftaleno 57 C10H7NO2 1220 (sol) 103Azobenzeno 68 C12H10N2 1090 (sol) 121Compostos inorgânicos C. química (kg/m3) (kJ/kg)Água 0 H2O 1000 (liq) 333Óxido de fósforo 24 P4O6 64Césio 28 Cs 1880 (sol) 15Gálio 303 Ga 5900 (sol) 80Nitrato de sódio 307 NaNO3 2260 (sol) 199

* C n H 2n+2 com n>15 Luis Roriz, 2008


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Use of ice storage and

comparison to water

banco de gelo convencional banco de gelo fundente

• For the same storage volume, ice has 15 times more capacity

• The efficiency of cooling generation is smaller with ice due tothe lower temperature necessary and the formation of icearound tubes that decreases heat transfer. Further there arehigher heat transfer losses from the environment.

Other substances• Parafines: C nH2n+2 with n>15• Fat acids: CH 3(CH 2)2n COOH•

– CaCl 2.6H 2O (30ºC) and Na 2SO 4.10H 2O (32ºC)

• There are several suppliers of phase change materialsto be used at temperatures from negative up to 120ºC.• There is interest in using alternative fluids to water,

namel to avoid freezin in the circuits: – Solar thermal systems at night in winter. – Climatization systems with equipment outside when stopped. – Other systems for refrigeration applications (not climatization).


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Brine (Salts e.g. of NaCl; CaCl 2)

• The phase change temperaturedepends on the salt concentration• e m xture may ave one ormore euctetic points (where it

behaves as a pure substance).

Weight percent NaCl

Glycol – water mixtures• Glycol has two hydroxil groups (CH 2)n (OH) 2 while alcool

CnH2n+1 (OH) has only one.• Ethanediol (CH 2)2(OH) 2 is the main glycol used and is

miscible with water in all proportions.• It’s fusion temperature is -13ºC, but when mixed with

water with mass fraction larger than 25% it has lowerfusion temperature with minimum at 70% of -51ºC.• It is h armful if swallowed and eye irritant so there is a

less harmful and with a mass fraction of 60% leads tofusion temperature of -51ºC.

• Both are combustible but not all properties arestudied for the later.

Ethanediol inflamation temperature of 111ºC.

18 - thermal equipment

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