BACKGROUND [PDF]

Energy Efficiency Forum '94, Varna, Bulgaria - 17 June 1994

CONDENSING GAS BOILERS FOR ENKRGY EFFICIENCY AND REDUCTION OF СО г AND NOx

E. Melville Stewardson MCIM Commercial Director - Beeston Heating Group Ltd., Great Britain

BACKGROUND ll is my belief that the murker for gas fired hoi water boilers in individual outputs ranging from 50 to 500kW will represent a substantial portion of the total heating market

in Bulgaria as gasification progresses from industry to urban communities,

where it will become a clean, controllable and economic fuel for heating apartment blocks, public buildings and any other multiple occupied establishment. Britain

is only

surpassed by the smaller

Netherlands

in having over 85% of

all

buildings in the country at the end of a natural gas supply line and has the longest and most experienced knowledge base on gas and gas distribution, as well as the highest of safely standards evidenced by its published results. In (he European countries their

Union,

and France,

traditional

the most dominate

advanced natural gas niaikels, the

condensing

product base being sectional

boiler

cast

market,

Mriiaiu, Benelux primarily

iron atmospheric

This type of product presents great output and performance variety

due to

gas boilers. from two or

three cast iron patterns, and is highly cost effective in terms of volume production. All

boilers were designed to specifically

avoid condensation which could corrode

the raw material and cause premature failure. In the lute 70's, with the risk of fuel crisis always present, work commenced to design to

boilers

recover

the

which

were

latent

more

heat

efficient,

contained

in

which water

meant vapour

promoting condensation which

results

from the

combustion process and represents around 10% of the total heat content of a Group 20 natural

gas. The

first

since when experience,

boilers were commercially

redesign and volumes

marketed in the early 80's,

have steadily grown, particularly

in

countries where legislation and some subsidies have driven the market to its current levels. In the European Union (CE) there are around 6,000 commercially sized condensing boilers

sold

annually,

accounting

for

2.5%

background, the motivation and the status quo.

of

the

market.

So

much

for

the

-

2

PRODUCT TYPES A l l condensing boilers have a heat exchanger which is made from a corrosion proof material, since wet

flue gas products are acidic.

There are many configurations

between the flame, heat exchanger and flue, and the following illustrates the main types: f sheet attached behind this page J The most common type in Europe is 4.2 and is the cast iron seiHional boiler type which we manufacture, as do other market

leaders in Holland and France.

The

condensing heat exchanger is built in 10 or 12 heat recovery sizes and added, along with more sophisticated controls and a fan, to the standard product. MA ILK IA I.S (a)

Stainless some

steel

ferritic

304 or 316 are highly

resistant

to corrosive attack,

as art;

stainless steels, but welding must be carefully controlled, and

I he material is particularly expensive. (b)

Aluminium

alloys

show

good

corrosion

resistance,

all hough

some

surface

corrosion may occur, proper design and use of thick wall tul)es should give good life expectancy. use this materiai.

15 to 20 years are attainable and most manufacturers

Welding, like stainless steel, should be carefully controlled.

At Beeston we do not put any weld joint in the flue gas path, and so avoid the possibility of this type of corrosion. (c)

Other materials like high purity and thick cast iron are used. does

corrode

over

time

and

is

environmentally

leaching of iron oxides into the drainage system.

unacceptable

However, this due

to

the

Copper has the same effect

and is also excluded for this reason. (d)

Coatings on materials

have not

proved successful

due to the difficulty

achieving total coverage on irregular or extended surfaces.

of

Surface treatment

techniques may not suffer this way, but are as yet unproven. (e)

External effects can give rise to corrosion, for example the air which helps gas combustion

must

be free

from chlorides or

fluorides which will,

the combustion process, damage the condensing heat exchanger. fresh air from outside may be ducted into the boiler.

after

If necessary,

-Flu* Burner

Л

-Induced drought fan -Dilution airinlet

y heotexchanger

""Secondaryheat exchanger Air vent

Drain Figure 4.7

Burner

Integral finned tube boiler - downward Tiring (concentric Figure 4.2

Слыгоп 3ccuon.1l boiler with add-on secondary heat exchanger (inverted U-iype) Pnmary heui exchanger

1_

X-

I /•

Burner

M

rWner *

k

— Secondary heat exchanger

у

Drain Figure 4.8

- 'Optional third heatexchonger

Drain

Shell boiler with add-on secondary heat exchanger Figure 4.5

Steel/copper tinned lube boiler with integral jad-.) tindary (N-iype)

flue

Hue

л

Induced draught fan

Dilution air

Secondary heal exchanger

Secondary heat exchanger

heol exchanger

Pnmary

Dilution air

—

:

Drain

4

Aif vent '/ Avvenl /,

Burner 4.i

Burner

Cj&t-iron »cctional boiler with add-on secondary heat exchanger (otful lypt) 4.4 ...

XIX-

*

1 Air damper

Siccl/copper finned lube boiier with add-on secondary heai exchanger {in-line type) Burner

Flue

- Secondary healeachanger

Опи» i Flue

-Pnmory heat exchanger

Burner

I !

Airvenl/ Drain

3

MATERIALS (continued) (f)

System water corrosion may occur if the system has not been flushed and cleaned, or it is taking in air or fresh water clue to leakage elsewhere. Minimum flow rates through boilers must be maintained to avoid localised boiling which could accelerate corrosion. It is normal to add chemical protection to mixed metal hot water systems to prolong the life of boilers, pumps and valves.

FLUE & CONDENSATE Because the latent heat has been extracted into useable but low temperature system water, the flue gases are no longer buoyant enough to leave the boiler and a fan is used to both exhaust and sometimes partially dilute the final products of combustion. These graphs show the effect of, firstly, \l] typical flue gas temperatures relative to the boiler return water temperature, which is in the order of 10-12° before dilution. The second [2] graph shows the theoretical maximum condensate in ml per kWh and a typical range relative to the boiler return water temperature, and the third [3J illustrates an example of the condensing boiler flue and condensate disposal system. Flues must be stainless steel, sealed water and gas tight, drain through traps into the boiler and/or the common drain in the plant mom. A nozzle is used at (he outlet to clear ihe inevitable plume away from the building and the flue must be high enough to ' d unpressurised by surrounding buildings. Condensate is in the order of 3 to 4.5 pH, less than vinegar, and generally mixes with the largely alkali effluent in the public drainage system. KEY MEASURING POINTS In Britain efficiency measurements are quoted as "heat to water" based on the gross calorific value of the fuel, and since the documents I have used as support employ this method, all subsequent reference to efficiency will be on that basis, the following graph [41 illustrates the performance of the current generation of non-condensing but highly efficient boilers against the performance of typical condensing boilers. The difference is obvious immediately, and much more so when the boiler water temperature is allowed to decrease from the normal 70°C. In testing condensing boilers with their huge additional heat exchanger surface area and highly variable operating load conditions, the new European Standards recognise two points on boiler temperature scale as the efficiency measurement points. These are: flow temperature 80°C, return temperature 65°C, and flow temperature 55°C, return temperature 40°C. At these return temperatures, boilers should achieve 86% and 90% gross.

8Qi

1

ISO,

20 lumper л line lo boiler) Q

40

50

70

60

Boiler telurn water temperature { C)

Plume Flut terminal nazile Minimum heiohl ubovti roof level

Sniuraied

Lessnaiura! buoyancy

Fall -5

Lower flue gai temperaiurti

Leiivolumeol fluegaiei Flue IJIUIII

Tundish

100

.

_

lOOj

-:

-2

0

.. .

' Good modem | conventional boilers

75 20

30

40

50

60

Rerurn wafer lemperatuf е (*Q

70

"80

2

4

6

8

10

12

14 Э

External temperature ( С)

16

18

2 0 22

4

-

APPLICATIONS How can the lower water temperatures produced at high efficiency in condensing boilers be used practically? The opportunities are considerable and too detailed for the purpose of this paper, however many condensing boiler installations replace older equipment to great effect. A building which already has conventional radiators and poor control may benefit by the installation of mixed condensing and non-condensing boilers and a compensating valve operated by outside weather sensing. The effect of this means that at the lowest outside temperature the radiators receive water at the highest design temperature; as the outside temperature increases - and your climate in Bulgaria is highly variable day by day during the heating season - then the water temperature reduces to the radiators. Under these conditions the condensing boiler's efficiency, heat to water, rapidly increases as the graph [5j shows. Condensing boilers may show substantial savings in underfloor heating, variable air volume air heating, and high volume domestic hot water production. The key elements of choice relate to the number of hours the system will be operated during a season, the thermal capacity of the building i.e. badly insulated - better payback, system type and opportunity to exercise control over the heat produced. We market small microprocessors at around $1000 which will programme, sequence, optimise start and stop times and compensate for outside temperature changes.

SEASONAL EFFICIENCY Seasonal efficiency, rather than full load efficiency, provides the best measure of boiler performance. It compares the useful heat output with the total fuel consumed over the heating season. In Britain, the Meteorological Office records all relevant weather data in every town and city, as well as rural and tourist locations. This data is compiled into degree/day tables and the information used by Building Services Engineers to formulate their heat loss/gain calculations at the design stage of installations. The basis of information will not identify exactly with Bulgaria, although you do have substantial variations in your country, just as we do in Britain.

5

-

The measurement accuracy of efficiency, as agreed within the European Union for product approvals, is in the order of 2%. Ft is not the exact science claimed by many engineers. Measuring device tolerances, manufacturing tolerances and delivered fuel tolerances are all accounted for within this 2% margin. The key factors influencing seasonal efficiency are (a) heat emitter sizing, (b) condensing boiler sizing, (c) mixed boiler sizing, (d) system and control design and (e) boiler design. As this graph [6] shows, in an existing building with accurately sized radiators for design temperatures, you may expect a seasonal efficiency of 86.5 to 87.5%. In a new build situation it would be possible to use the lower water temperature which a condensing boiler produces so efficiently, to heat oversized radiators - say by 50% - and move the seasonal efficiency to between 88 and 89%. This is where economics enter the calculation - greater heating surface costs more money, which must be economically recovered in fuel savings. More about this later. The next graph [7| shows that it is imperative to size condensing boilers accurately to the load required. Make them work hard on the days where minimum design temperature is not required. A maximum of 20% oversize is desirable. The next graph [8| shows the effect of mixing non-condensing boilers with condensing to reduce the capital outlay ami achieve economic paybacks. It is normal to install 50/50, 60/40 mixed condensing and non-condensing boilers. Systems containing only condensing boilers are generally used for buildings with continuous heating demands, 24 hours daily for the whole 34 weeks' heating season. The next graph [91 again shows the dramatic difference between boilers when run at part load. It is important that systems should be designed or adapted to run at variable temperatures compensated by the outside temperature. Boiler designs do make a difference; however unless the system has some peculiarities, most modern condensing boilers will operate at their measured and approved efficiency points - 86% at 65°C and 90% at 40°C. The next graph f 10] shows a typical demand curve of load against time. All these factors can he fed into a spreadsheet programme, with the meteorological data for particular locations as shown on the next illustration fllj. This represents all the factors I have spoken of and is actually based on the city of BRISTOL in the south west of Britain. After all the calculations, the seasonal efficiency is determined as 86.94%, but please remember my comment on accuracy!!

Return wqrer temperature 4d«ignr (°C) fl

aO

55

50

J5 90

Heoloutput

•a

I

d

39

|

S8

~

871

S

Я6

S

3

Boiler sizing rano =

I

Nannul юно

Howtemueraruie - 83"C Кегигп temperature - 72°C

tu d3L 10

10

15

20

2b

Boiier sizing ratio

3.0

1.5 Emitter sizing rano

A. Umleifloor heulifig ьунет

8

B. Srandard sizing rodtarars C. Standard sized raaiutors

IП0

10 \ Г

eriiuency conventional boiler

100 200 Days per annum f00% Conventional boilers

50/50

Mix of boiler types 1%)

100% Condensing boiler

100

80

60

40

20

Heating season {%)

300

-

6

This next illustration [12) shows the economic evaluation between different types of boiler plant against seasonal efficiencies and any capital cost differences between installations. The payback period is the time it takes to recover the additional capital out of running cost savings. Prices here are based on British equipment prices and the cost of natural gas to commercial consumers. You may view this as a representation of what may occur in Bulgaria with world gas pricing and competitive but high quality manufactured goods. It is obvious that 2 of the 4 options shown will give a reasonable payback in financial terms and go on saving fuel for the 15/20 years' life cycle of the equipment. HMISSIONS Condensing boilers use less fuel for a given output than other boilers, therefore they emit less C0 2 , "greenhouse gas", into the atmosphere - an objective all countries shared at the Rio Earth Summit meeting. European Union legislation is also in process to reduce NOx, the low level pollutant gas, and has already been drafted into the Standards Pr EN 303 and Pr EN 656, which will limit NOx (o below 260 mg/kWh and classify products into A classes, the lowest of these being under 100 mg/kWh. The next illustration [13| shows an example of how my company helps customers to make valued judgements on efficiency, emissions and costs, both financial and environmental. The following illustration [I4J shows a completed example with the options of under 2 years or under 4 years payback, whilst reducing CO a from 103 Tonnes to 89 Tonnes and NOx from 153kg to 72.5kg per year. MARKETING With concepts as complex as condensing boiler systems it is not easy to convince potential customers to spend extra capital at the time of installation. It is important to make the offer of higher efficiency, therefore lower fuel costs, and higher environmental standards at the earliest design stage when capital is being talked about and the financial benefits can be explained. In Britain it is usual practice to view the financial worth of buildings and their mechanical and electrical services on the basis of life cycling. The calculations do not only account for capital expenditure but also running costs, maintenance, durability and replacement costs at the end of their life cycle. Condensing boilers of quality design and construction, installed and operated correctly, should last between 15 and 20 years. Fuel costs will not reduce over that period, therefore they do represent a sound investment both economically and environmentally.

11 Table A l l

м Upper limit of external temperature band ("Q

Bxamplc spreadsheet

(W

Number oi days that external temperature falls m interval