Building service energy modelling

P

where heat losses of the "wet" parts of the boiler, present when the boiler

ms

operates (i.e. heat carrier fluid goes through) → present both when the burner is on or

P

off. losses from "dry" parts of the boiler (i.e. no contact with heat carrier fluid)

mb

→ present only when the burner is in operation. The reduction of this losses is pursued

by: adequate insulation of the boiler casing with insulating material, correct sizing of

the generator. An oversized boiler is not economically efficient both for: higher capex,

lower ηs,g, reduction of the average temperature of the heat carrier fluid ( bearing in

mind possible limitations) and a boiler installation in a “protected” environment. Note:

For some installation sites, thermal losses through the casing can be differently

considered. If the boiler is installed within heated rooms → then the heat losses are

largely recovered. In particular it is considered: recovery of 90% type C, and a

recovery of 80%case of boilers of type B. Outdoor installations, even if in the technical

room, no recovery. Finally, for generators installed in technical heating rooms a

recovery of 30%in the form of preheating of combustion air is considered.

For safety reasons, before each burner ignition, the combustion chamber should be

“washed” in order to evacuate any explosive mixtures that can be present. Washing:

operating the burner fan for a few seconds before ignition of the flame. The washing

time is fixed by specific regulations, as function of the fuel and the burner output. The

P

thermal losses of washing are not recoverable to avoid very frequent ignitions

lav

of the burners → when low thermal inertia of the plant or boiler is oversized.  Solution:

install a timer that prevents the re-ignition of the burner before a time lag, such as 10

or 15 minutes. When the burner is off, the draft of the chimney creates a volume flow

of cold air through the CC, cooling it. This stack loss PCBs can be reduced by: use of

burners with air damper to be closed during stop operation, avoid all possible air

infiltration and Limit the chimney draft → reducing the temperature of the flue gas or

using a draft regulation device.

Boiler losses P

Components of boiler losses are Stack gas ,

c

P

incomplete combustion , Transmitted through

inc

P P

case , “washing” CC , Stack with burner

m lav

P

off . The losses listed above can make the

cbs

global seasonal efficiency pretty different from ηg

calculated in nominal conditions. Following we will no

longer consider the losses due to “unburned” fuel → since we consider it as

malfunctioning of the generator. Note: that the electricity consumed by the boiler

should always be considered in a rigorous calculation of the efficiency. The energy

used by the circulation pumps should not be counted, apart from cases where the

pumps are part of appliance (boiler) itself (for example in the case of wall-mounted

boilers).

Classification

Boiler are Classified according : fuel used (gaseous, liquid or solid), typology of

installation (wall, floor standing), the type of burner (atmospheric, pressurised) and

mode of discharge of flue gas (natural convection, pressurized). Two large groups

according to installation: the wall and floor standing → water content (the first small

even less than 10 liters), the latter, without exception, have a high water content

(hundreds of liters). Most used fuel in European buildings → natural gas. The boiler

consists of: a burner, a control system, the «real» burner (heat exchanger).

Hot water boiler

A "real boiler” is a hot water boiler: a water-flue gas heat exchanger where the heat

transfer from the gases to the water circuit takes place due to radiation and

convection. The latter on the walls of the combustion chamber and which are wet on

the opposite side from water. A hot water boiler for space heating is an enclosed

pressure vessel in which water is heated to a required temperature and pressure

without evaporation. Hot water boilers are manufactured according to boiler and

pressure vessel codes (PED, EN, ASME).Boilers are usually rated according to their

gross output heat capacity, i.e., the rate of heat delivered at the hot water outlet of

the boiler (kW). A correct design of the HWB takes into account the chimeney design,

thorugh which the flue gases are released in the atmosphere. Set of burner, boiler and

control system that are identified with the term "thermal unit". In the case of boilers,

the manufacturer shall also indicate the type of chimney (certified). If the flue gas

exhaust duct is not sized correctly according to the boiler capacity it will result in

pressure inbalances in the combustion chamber → boiler efficiency, damages, safety

problems.

Types of Hot Water Boiler

According to their working temperature and pressure, hot water boilers can be

classified as follows: Low-pressure boilers: These hot water boilers are limited to a

working pressure of 1103 kPa and a working temperature of 120°C; Medium- and

high-pressure boilers: These boilers are designed to operate at a working pressure

above 1103 kPa and a temperature above 120°C.

A low-pressure hot water boilers are generally used for a low-temperature water (LTW)

heating system in a single building, regardless of the building’s size. Medium- and

high-pressure boilers are often used in medium-temperature water (MTW) and high-

temperature water (HTW) heating systems for a large number of buildings (quarters)

in which hot water temperature may range from 150 to 205°C.Based on their

construction and materials, hot water boilers can also be classified as fire-tube boilers,

water-tube boilers. Water-tube boilers, mainly used for steam at higher pressure and

temperature, are not discussed here.

Floor standing boilers: Fire-tube boilers

Floor standing boilers (Fire-Tube Boiler) → consistent mass of water that remains inside

the boiler itself and is in contact with the surfaces heated by the combustion. A fire-

tube boiler’s combustion chamber and flue gas passages are in tubes, which are all

enclosed in a shell filled with water. Heat released from the combustion process and

the flue gases is absorbed by the surrounding water, the temperature of which is

increased to a required value. Many kinds of fire-tube boilers have been developed.

One of the more recently developed models is known as the modified Scotch marine

boiler, which is a compact and efficient design originally used on ships. The Scotch

marine boiler is probably the most popular hot water boiler manufactured today. They

has a thermal capacity between 80kW and 14MW and are classified according to

number of fire-tube passes: two or three fire-tube passes.

2 fire-tube passes (with inversion in the combustion chamber): the flue gas

 reverses direction and surrounds the flame → higher average temperatures of

the flame for longer time intervals → favors the formation of NOx (backward

flow of flue gases at high temperature reduces the direct heat transfer from the

flame to the combustion chamber cooled by water)

3 fire-tube passes: flue gases inside the combustion chamber do not reverse,

 but pass through a water cooled room in the second fire-tube pass and then in

the third → the flue gases do not prevent the heat transfer from the flame to the

internal boiler walls → higher heat exchange rate and a lower production of NOX

The component of a Floor standing boilers are:

The burner: atmospheric / pressurized. The most modern are the seconds able

 to guarantee higher efficiency. The burner must be consistent with the capacity

of the boiler to which it must be applied. The burner is composed of a mixing

system and a ventilation system that pressurizes the combustion chamber. (gas

oil burner can also be blown air with exhaust gas recirculation). The dart of

flame must be compatible with the length of the combustion chamber: short

flame → recirculation anticipated of flue gas (reduced heat transfer); flame

excessively developed → overheating and possible damage of the combustion

chamber. Before each ignition must perform a washing cycle of the combustion

chamber to eliminate the possible presence of vapors or any trace of “unburnt”

fuel → external air (to room temperature?). The latter causes cooling CC with an

effect on the overall global efficiency

The combustion chamber (cc): It must be sealed in order to prevent water

 leakages (water as heat carrier goes by the external surface --> dangerous

unwanted pressurre rise. CC such as to ensure the best possible heat exchange

by direct radiation (flame) and convection with the flue gas. Designed so as to

achieve an ideal distribution of the temperature on the surfaces and have a size

and shape to allow optimal path of the exhaust gas (countercurrent exchange

gas-water).

Fire-tube passes: In boilers with 2/3 fire-tube passes, it is the stage

 immediately after the combustion chamber. In traditional boilers at a constant

temperature it is required that the return water to the boiler is kept at

temperature higher than 60° C in order to prevent condensation of the flue gas.

To increase the yield of production → operate with lower temperatures →

technological solutions are implemented to prevent condensation → boilers

sliding temperature or condensation pump.

The boiler: Watertight envelope containing the heat carrier fluid that is crossed

 by fire-tubes and within which develops the entire combustion chamber. It is a

real “tank” crucial for the safety operation of the generator: resistance to

corrosion, watertight and structural soundness. It is the main element of the

boiler itself.

Control system: Component that implements the control algorithms of the

 boiler in order to optimize the performance under variable boundary conditions.

Thermal insulation: important in floor-standing boilers → significant mass of

 water stored and large heat exchange surface. It is in contact to the entire outer

surface of the “tank” and to the supply and return hydraulic connections. Type :

from glass wool to different recyclable and CFC-free foams...

Actual technology: floor standing boilers

The distinction between ηg and ηs,g is crucial for energy efficiency. In the past years

increase of ηg through reduction of Pc and Pms (burner on) Pc → lowering

temperature of flue gas from 180°C to about 100°C (increase heat exchange surface –

3 fire-tube passes, particular geometries to promote HT) and reducing excess air

(bearing in mind limit for “unburnt” production). Reduction of T flue gas was less

sensitive in diesel boilers: sulfur makes more dangerous effects of a possible

condensation → solution adopt corrosion resistant materials (as stainless steel). Effects

compared to pa