Heat Recovery from bw-Pressure Steam Boiler~lue Gases .-

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Herbert ~onstodt, PE, Herbert Kunstadt konsulting ~niineen, New York

The heat In the bofler flue gases In an apartment h o u ~ complex Is avaflable for preheatfng feedwater, combustton alr, heavy fuel011 and comestic hot water, provfdlns substantlol savings for the owner.

THIS STUDY shows that heat recovery from low pressure steam boiler flue gases is both feasible and economical for preheating,of boiler feedwater, No. 6 fueloil and combustion air, in addition to that of domestic water. .

I t is shown that implementation of this heat recovery system wiU reduce fuel consumption by approximately 5.5%. At today's fuel prices, annual savings for an 1170 apartment complex in New York City &e projected at $17,000. At a fuel cost of $0.525/gal, these savings would grow to 525.000.

The estimated cost of the proposed heat recovery system is 6100,000. Baked on the current price of No. 6 fuel oil of $0.3S/gal, the payback is six years. Should the price of fuel oil increase t o 52.5Wga1,' then the investment will be recovered in only four years, exclusive of any governmental incentives that might apply.

Given the present high fuel costs, with the prospect of still further increases, and the uncertainty surrounding future oil supplies, it has become important to quickly develop energy conservation systems for apartment houses which use sig- nificant quantities of fuel for space heating and domestic hot water.

The uniqueness of this application consists in presenting a heat recovery cycle utilizing flue gases from low pressure steam boilers serving 4883 rentable rooms and a population of approximately 3300 in ten buildings, ranging in height from six to eleven stories. The constructed area is 513,000 square feet and total volume is about 8,000,000 cubic feet. The buildings, completed in 1941, are essentially brick structures with single glazing.

Exhaust from low pressure steam o r hot water boilers, serving residential apartment houses or commercial installations, h& not been considered until now as a viable heat recovery source. The cost of fuel,oil and natural gas were such that extracting heat from the boiler flue gases, in small and medium installations, was not economical. Here, we investigate the feasibility of heat recovery from the boiler's flue gases, discharged into the atmosphere while still a t a temperature of approximately 460F.

Preheating of boiler feedwater and combustion *air with heat recovered froin flue exhaust has been practiced for many years. However, it was restricted to flue gases with temperatures above SOOF, encountered mainly in high pressure steam or water tube industrial boiler plants.

DECEMBER 1977lJANUARY 1978 49

T o develop an energy recovery cycle which will reduce fuel consumpti,on, we a have to: ' I

Investigate the available he& recovery equipment and hardware; Analyze the endineering ,feasibility of all equipment considered; . Develop heat reclamation systems; and Compute the anticipated savings from each of the heat recovery systems in- vestigated.

Descrlptlon of Plant The plant consists of five low-pressCre steak, fire tube; boilers recently reha-

bilitated. The heat specified'output of each boiler is 15,499, MBh gross and, . ; 12,033 MBh net, with burners rated:at 140.7 gph. using No. 6 oil. Boiler outlet, ' . ,

temperature is approximately 480F. The flue gas temperature in the stack is a p proximately 460F.

Computed stack draft losses are 1.205 in. W.C. a t the above gas temperature; computed stack gains are 1.693 in. w'.c. and, for each boiler, the quantity of flue gas is 6843 cfm at 460F or 3494 standard cfm. Present net available draft for the

, boiler plant is 0.448 in. w.c.. after deducting.all the losses.

Computed maximum heat losses for 'the apartment buildings are 37,976,000 Btuh. at OF outside temperature. Peak domestic hot water demand is 9,746.000 Btuh. Computed yearly fuel consumption is 906,425 gal. according to the ASHRAE formulae (Systems HandbOok, 43.8).

Systems Two basic types of commercially wailable gas-to-witer h a t exchanger: might

be applicable:

(a) The shell and tube type, similar to Industrial Steam Economizer model 100, in which the hot flue gases pass through the tubes and the heated fluid is in the shell.

(b) The finned tube economizer extended surface heat exchanger similar to Voss. Trane and Kentube, in which the finned tubes would contain the work- ing fluid and the hot gases would pass around the tubes.

In each case, a suitable heat transfer fluid must be selected which operates effi- ciently at gas stream temperatures of 450-480F and would transfer the reclaimed, I a

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heat from the boiler flue gases to the domestic hot water, feedwater entering tht , boilers, fueloil heater, or boiler room air (combustion air) prehater.

The heat exchangers must insure an exit flue gas temperature well above the dew point (approximately 2OOF with low-sulphur oil) to prevent condensation and formation of sulfuric acid. Modulating automatic controls on the transfer4fluid ,

circuit would insure that the leaving flue gas ,temperatures are within the preset range. An auxiliary domestic hot water storage type heat exchanger upstream of the existing storage tank would permit continuous heat storage from the flue gases during the off cycles of the boiler feed pumps. Piping loop would be such

! 50 BUILDING SYSTEMS DESIGN

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that the heat transfer fluid is pumped to heat exchangers for the fetdwata make- up tank and the domestic hot water preheat storage tank and to the fueloil pre. heater. Reclaimed heat may be used in any'cornbination of these, depending on the available heat and utilization requirements. The internal steam coils in the ex- isting boiler feedwater makeup tank and the domestic hot water storage tank would operate only when the recovered heat is not .sufficient. .

Because, in existing boiler plant installation as is generally the dase, s p a a is very tight, the shell and tube economizers are difficult to ,accommodate. Their application is best suited to new boiler plants, where, one can provide a suitable layout. Conversely, the more compact finned tube economizer can be installed with limited modifications of the breeching.

u t i t l m t i ~ ot Recovered at - ~ o n n r ) t c ~ o t watw The following order of priorities has been established for'the utitimtion of re- , . ,

covered heat. First, is domestic hot water (DHW);, : ,

The maximum recovered heat per hour occurs at the maximum boiler load and . - is equal to maximum building heat load plus domestic hot water peak demand load divided by the maximum net output per boiler, times flue gas flow (sefm), times the constant 1-08, times the temperature difference of entering and leaving flue gases:

361620100*9'746p00 x 3494 x 1.08 x (460-300) - 2,337,000 Btuh . 12.033,ooO

at an outdoor temperature range of 0-5F. This is not sufficient to satisfy the peak DHW demand. Thus. DHW needs will be met by, one boiler operating year- round to supplement the above-computed recovered heat. However, when outside ' - '

. . temperature is above 70F, which occurs in New .York during 2145 hours (approx- , # , ; I .

imately, the three summer months), one boiler'(12,033,000 Btuh) is sufficient to cover DHW demand.

The recoverable heat during these three summer months is 491,000 Btuh, which represents about 5% of the DHW demand. Total recoverable heat during that time, based on 9 hours per day of DHW demand,' is 491,000 x 9/24 x 2145 = 395,163,000 Btu, representing a savings, based on 100,000 Btu/gal, of 3952 x .

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0.35 = 51383 at current fueloil prices. Moreover, maximum DHW demand occurs daily ,from 6 a.m. to 10 p.m., and

from 10 p.m. to 6 a.m. we estimate that only 10% of recovered heat is necessary. Then. 90% of available recovered heat is available.for boiler feedwater heating., oil preheating or storage in the DHW preheating tank.

Boiler reedwater Preheating , , . .

Each boiler has a potential maximum heai reibve&'rate of 607,000 Btuh. This . .

heat could raise feedwater temperature 38 degrees. This. represents the equivalent' of 6 gal of oil per hr from each boiler. If we were to dedicate the recovered heat to this task, there would be a saving of (8760 hr/yr x 6 gal/hr) 52,560 gal of oil, which amounts to $18,396 at $0.35/gal.

-DECEMBER I ~ ~ ~ I J A N U A R Y 1978 5 1

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Combustion Air Preheating ,-

If all the reclaimed heat were used to preheat boiler room supply air (i.e.. com- bustion air) the temperature rise of the preheated air would be theoretically equal to the temperature drop of the flue gases, 160F. In fact, the temperature rise of the supply air must be limited to a maximum 75F, so that boiler room temper- ature does not exceed 90F. The air preheating requirements are much lower than the available recovered heat, and therefore could be easily satisfied.

Fueloll Preheating Fueloil temperature rise obtainable with recovered heat with only one boiler ,

operating could reach 99 deg, which exceeds the acceptable level. Therefore, only a fraction of the reclaimed heat can be used for heating fueloil returning to the storage tank. For illustration purposes, we have computed, also, the oil temper- ature rises possible with two, three and four boilers operating and utilizing all of the reclaimed heat just to heat the fueloil. Thus, with two boilers operating, we' obtain a temperature rise of 197F. With three boilers, the rise would be 318F and, with all four boilers on line, the fueloil temperature would increase by 415 degrees. Obviously. such temperatures are not acceptable. See graph, Fig 2.

When using the reclaimed heat to preheat fqel oil in the existing 20,000 gal and 40,000 gal storage tanks, the following temperature rises can be obtained:

I , Temperature Rise in Oil Tanks ~ ..... .............. 20,000 gal tank only: 'With one boiler operating : 7.0 deg ! W,ith two boilers operating ................... 14.0

............................ . : - With three boilers .: 21.0 '

............................... '.. With four boilers 28.0

.................... 40.000 gal tank only: With one boiler operating With two. boilers ................................

- . ....:........ ............... With three boilers : With four boilers ................................

, . .................... Both.tanks at one time: With one boiler operating

................................ With two boilers With three boilers ..............................

................................. With four boilers 4 1 When using part of the recovered heat for preheating fueloil by 30 deg. re-

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quired by New York City's Department of Air Resources, maximum recovered . ,

heat consumption would be: ' . ,

! Q = 1407 gph x 8 Ib/@l x 0.5 Btu/de&lti 30 deg = 168,840 Btuh .

This would replace the heating of oil by steam in the existing heat exchangers, , upstream of the fueloil pumps. ,

I BUILDING SYSTEMS DESIGN 52 f

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Annual fuel consumption for heating was 'calculated on the basis'of drstside temperatures and the number of hours of their occurrence. The heating load was computed for each 5-deg F temperature interval. By multiplying this load by the number of hours of their occurrence, the total heating load is obtained.

Total annual requirement for heating is 108,853,813,000 Btu. This represents theoretically, 756.000 gal of No. 6 oil.

The DHW heat requirement was calculated on the basis of the previously es- tablished peak demand load of 9,746,000 Btuh, spread. over nine hours a'day us- , .

age. The resulting annual consumption is 40,0'19,923.006 Btu and is equivalent to 278,000 gal of No. 6 oil. The theoretical annual fucl consumption (No. 6 oil @ 144,000 Btu/gal) would therefore be 1,034,000 gal. . Number of Boilers In Simultaneous Operation ,

Total demand (heating + DHW) will vary according to the outside temper- ature (DHW demand remains constant). The number of boilers in simultaneous operation is obtained by dividing the total 'demand by the net boiler output . (12,033 Mbh).

Available Flue Gases The variable flue gas quantity (in scfm) is bbtained by the ratio of the Actual

boiler load and the specified capacity of t h e boiler, multiplied by 3494 scfm. ' I

which is the maximum volume of the flue'gases. for one boiler.

Avallable Recovered Heat I The available recovered heat per hour =

scfm x 1.085 x (460 - 300) Btuh where (460 - 300) is the 'temperature drop of the flue gases. For each outside tem- perature range, the Btuh was multiplied by the number of hours for that temper- ature range to yield the available recovered heat. The total available recovered heat is 5,454,979,000 Btu per year or 54,549 equivalent gallons of fueloil per year. This yields a reduction in consumption of 54,549 , 100 j13%

' 1,034,000 ,

This represents a saving of $19,091 at $0.35/gal . '~e have computed the projected savings at assumed escalation rates of 10%. 20%. 30%. 40%. SO%, and 100% for fuel costs.

Heating Requirements B a d on Degree-Day , - ,

(or ASHRAE) Method..-. , .. '. . . . _ . , . . . . . . ' . I .

With this method, the annual fuel consumption for heating is lower, i .e. , ' f

628,509 gal of No. 6 oil. Consumption computed on the basis of outside temper- ature range method is 755,929 gal.

Heat recovered from the: DHW load is the saine with both computation meth- ods. The heat recovered from the heating load is lower for the Degree-Day (DD) method. Thus, the projected reclaimed'heat for the DD method is 4,807,513,000

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BUILDING SYSTEMS DESIGN . .

.- ' Btu per year. represynting the equivalent .of 48,000 gal of No.6 oil.: This repre-. sents an annual savings of 516,829 at S0.35/gal. ,We computed the available &v- ings at projected cost escalation rates of 10V0, 15%. 2070, 30V0, 40%. 50%. and 100%.

Following is a comparison of the projected savings, resulting from computing the heat losses by the two different methods:

Fue 1 E q u i v a l e n t Savings, Consumption, Recovered, at Gallons Gallons $0.35/Gal

Outaide Temperature 1,033,845 54,549 $19,091 Range Degree -Day Method 906,436 48,077 $16,829

The equivalent recovered heat and the savings in dollars are approximateiy pro- portional to the total fuel consumption. Thus, on the basis of actual fuel con- sumption of 868,171 gal/yr, the equivalent recovered heat is 45,808 gal/year and the projected annual savings are $16,032.

Conclusions .

At today's prices for No. 6 fuel oil (3% per gallon), the projected k w i l sav- ,

ings for this application, achievable.thr,ough heat recovery from, boiler flue gases '

are $16.800. When considering potential fuel cost escalation to $0.7O/gal. the projected annual savings will reach $33.600. At the present fuel oil cosi, the pro- jected annual savings of 516.800 couId yield,S1,230,300 after 25 years, com- pounded at 8%. ' I

Should one assume .that, over the next 25 years, fuel oil prices will ri& min- imally, and the average cost will be only S0.52/gal, a 50070 increase above today's price, then the projected annual average savings will be 325,240. This will yield 51,845,000 after 25 years, compounded at 8qo interest.

Projected annual fuel savings amount to 48,000 gal and represcnt'4.54Vo of the average fuel consumption recorded by the owner of this project for the last 10 '

years. The estimated first cost to install a heat reclamation system which will yield the ,

savings shown above is estimated at-$140,000, when using shell and tube'heat ex- , changers, and 3 100,000. when using finned tube economizers.

The shell and tube economizers present no measurable pressure drop and are likely to have a longer useful life when compared with the finned tube exchang- ers. However, they need more space and require significant breeching modifica-

, . I . .. . . ..=. : .. . . - '.' .. tions. They also cost more. The first cost of the heat reclamation system can be recovered in about six

years, on simple payback when compu,ted ,on the basis of S0.35/gal fuel cost. When fuel cost escalation is taken into account, the investment can be recovered in less than 'four years, based on U).52/gal. ,

The proposed heat recovery system does not require any additiopal minte-

DECEMBER 1977lJANUARY 1978 ' 4 . 55

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nance or operating personnel beyond the rkular boilet room staff. With the pre- sent fueloil shortage and its inevitabk cost exalation, heat reclamation is a "must". I

Extraction of useful heat from the boiler flu$ gases at this boiler' plant has been shown to be technically feasible and economikally justified. ' ,

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Table 1. Heatin Based on Tern

llions of Btu o

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mges, Br of Occurance :onsumption: Btuh) *

DHW ( Total 4.61 56.9

50-55 377.1 145.2 522.3 5;223 55-60 261.5 140.9 ' 402.4 4;024 60-65 158.7 142.0 * 8 300.2 3,,002 65-70 57.59' 155.1 ' , . ,212.7 2 1.27 7 0 6 ~ ~ --- , 345.2' ,:395.2' 3,952', . . TOTAL 3841.147 1613.832 5454.979 54,549

BUILDING SYSTEMS DESIGN , I

T a b l e 3. T h e o r e t i c a l F u e l C o n s h i p t i o n and E q u i v a l e n t Recove red F u e l Based o n T e m p e r a t u r e ~ b n g e s

I 1 E q u i v a l e n t I , I Recovered

5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-6 5 65-70 70sup TOTAL

Temp. Range

0-5

T a b l e 4 . P r o j e c t e d S a v i n g s f o r P r e i e n t .a:d ~ s c a l a t e d ' ~ u e 1 C o s t s , , ,

Based o n T e m p e r a t u r e Ranges - I I E q u i v a l e n t S a v i n g s , i n D b l l a r s , b a s e d o n t h e Fol low- !

Temp. * 5-10

10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70

F u e l Consumption, Gal H e a t i n g I DHW I T o t a l

363 1 32 1 395

706up TOTAL

F u e l , . ,' . Gallons

2 0

Equ iv Hecov

Gal 20

- i n g ~ s c a l a t i o n R a t e s / F u e l o i l . p r i c e 4 p e r G a l . ' - i 1 0 % i 1 5 s i 20% i 30% 1 40% 1 50% 1100% 35e I 3 8 . 5 ~ 1 40.75q 4 2 ~ 1 4 5 . 5 ~ 1 4 9 ~ I sz.sel 7oc

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T a b l e 5. R e c o v e r a b l e H e a t , Based o n Degree Days I I A v a i l a b l e Recove red I Equ iv .

Month J a n u a r y F e b r u a r y March A p r i l Mav

H e a t , KBtu, f rom

580,252 , 714 ,738 498,295 632 ,781 267,506 401,992

77.367 211 .853

G a l 1,81(5

7 - 1 4 7 .

August

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ANNUAL

DECEMBER 1977lJANUARY 1978 . . . . . . . .

Table 6. Theoretical Fuel Consumption and Equivalent

ANNUAL

Table 7. Projected Savinqs for Present and ~scaiated Fuel Costs, ~ a s e d on Degree-Days

I IEsuivalent Savinqs, in Dollars, Based on the Follow-

Month Jan Feb Mar Rpr . May Jun . Jul *UY

. Sep oct Nov D Z TOT

Equiv - ing ~ s c a l a t i o n ~ ~ a t e s / ~ ~ Recov -- 1 10% 1 15% 1 20% Gal 35C ] 38.5C 1 40.25CI 42C 1,810 2,734 i 3,001 13,144 13,280

?loil Prices per Gal 30% 1 40% 1 50% 1 100%

45.5C 1 49C 1 .52.5C 1 70C 3,554 1 3,827 1 4,100 1 5,467

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Table 8. No. of Boilers in Simultarkous Operation No. of Boilers in Oper.

4 4 3 3 3 3 2 2 2 2. . 2 1 1 1 1

BUILDING SYSTEMS

Temp. Range 0-5 5-10

10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 , 50-55 55-60 60-65 65-70 706up 2145 --- 3,655

140-of Oper. Hrs.

22 69

139 262 507 804 825 814 788 765 771 842

Load, MBtu per hour

Heating

i",:",,': 31,195 28,482 25,769 23,057 20,344 17,632 14,919 12,207 9,494 6,781 4,069 1,356

AV . DHW Demand 3,655

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. Total 40,215 37,562 34.850 32,137 29,424 26,712 23,999 21,287 18,574 15,862 13,149 10,436 7,724 5.011