The Heat Is On Part II
Boiler flue gas heat recovery in the laundry process

By Lee R. Kemberling

Editor's Note: Click here for the first part of the article.

As was stated in the May 2001 introduction of this series, the use of heat remaining in exhaust gasses from the boiler has been practiced for many years. The main reason has been to reduce the primary fuel requirement of the boiler or of another separate process by preheating fluids used in the process. This technique has been carefully applied and studied and the application is well documented with respect to fuel savings and hardware. Recent increases in fuel costs have resulted in a renewed emphasis on flue gas heat recovery. Many linen supply boiler operators are unaware of new direct contact stack economizers and avoid stack gas heat reclaimers based on outdated information. Previously it was not uncommon for old technology with limited heat recovery capacity to develop mechanical and corrosion problems. Based on today's costs, stack economizers' return on investment runs as high as 100% per year and returns over 50% per year are common.

Tubular vs. direct contact stack economizers

An analysis of the heat recovery (Btu/hour) and fuel savings ($/year) provided by the various systems is set forth below, so a comparison of each system's effectiveness may be made in a typical laundry application. Calculations are based on a 350 BHP boiler, producing 10,350 lbs. of steam per hour (300 BHP), operating at a combustion efficiency of 80% and, therefore, using 12,900 cu. ft. per hour of gas, and having a stack temperature of 395ºF. Total water heating system efficiency is taken as 75%. Fuel savings are based on a plant operating 10 hours a day, five days a week, 52 weeks a year, with fuel at $6 per million Btu. Results are set forth for 50ºF (cold) and 100ºF (preheated) process water as a cooling medium.

The heat recovered from flue gas consists of the sensible heat (which is equal to the weight of flue gas, times its specific heat, times the temperature change) plus the heat of condensation (which is equal to the volume of gas fired, times the heat of condensation per ft3 fuel). This can be expressed as follows:

Where

Q	=	Heat to be recovered in Btu/Hr
Wg	=	Weight of flue gas in Lbs./Hr
Cp	=	Specific heat of flue gas as Btu/LB°F
TFG	=	Temperature drop in flue gas in°F
VG	=	Quantity gas fired in Ft3/Hr
hFG	=	Heat of condensation of water vapor in the cooled flue gas at exit temperature per cu. ft. of gas fired in Btu/Ft3

Then

Q

=

(Wg x Cp x DTfg)+ (VG x Hsg)

In the example

VG	=	12,900 Ft3 gas/hr
Wg	=	(1.2# flue gas/# steam/hr) (10,350# steam/hr) = 12,420# flue gas/hr
Cp	=	.278 Btu/lb.°F
T	=	285°F with 100°F preheated inlet water, 335°F with cold inlet water, 50°F
h fg	=	85 Btu/ft3 at 60°F
h fg	=	52 Btu/ft3 at 110°F
h fg	=	0 Btu/ft3 at 130°F and above

HEAT RECOVERED - LAUNDRY PROCESS WATER
DIRECT CONTACT CONDENSING TYPE

With cold inlet water

Stack gas in	............................................	395°F
Stack gas out	............................................	60°F
Process water in	............................................	50°F
Process water out	............................................	105°F
Process water volume	............................................	140 GPM

Q	=	(12,420 x .278 x 335) + (12,900 x 85)
Q	=	1,156,675 + 1,096,500
Q	=	2,253,1755 Btu/hour

% of fuel heat value recovered = 2,253,175 Btu/Hr

              12,900,000 Btu/Hr x 100=17.5%

HEAT RECOVERED - LAUNDRY PROCESS WATER
DIRECT CONTACT CONDENSING TYPE

Stack gas in.	..............................................	395°F
Stack gas out	..............................................	110°F
Process water in	..............................................	100°F
Process water out	..............................................	133°F
Process water volume	..............................................	100 GPM

Q	=	(12,420 x .278 x 285) + (12,900 x 52)
Q	=	984,037 + 670,800
Q	=	1,654,837 Btu/hour

% of fuel heat value recovered = 1,654,837 Btu/Hr

              12,900,000 Btu/Hr x 100=12.9%

HEAT RECOVERED - LAUNDRY PROCESS
WATER TUBULAR TYPE STACK ECONOMIZER

Stack gas in	.............................................................	395°F
Stack gas out	.............................................................	270°F

(minimum allowed to avoid cold end corrosion)

Process water volume	.......................................	100 GPM
Process water in	.......................................	100°F or 50°F

(may cause cold end corrosion)

Inlet cooling water mix temperature

.....................

166°F

(to avoid cold end corrosion)

Q	=	(12,420 x .278 x 125) + 12,900 x 0)
Q	=	431,595 Btu/hour (The same for both 50°F & 100°F cooling
		water because of cold end corrosion limitations) limiting cooling to flue gas \ 270°F

% of fuel heat value recovered = 431,595 Btu/Hr

                                 12,900,000 Btu/Hr x 100 = 3.4%

HEAT RECOVERY IN BTU PER HOUR WITH VARIOUS TYPES
OF STACK ECONOMIZERS VS. COOLING WATER

Cooling Water Inlet Temperature °F	Tubular	Direct Contact Condensing Packed Mass Heat Exchanger
Process water 100°F (in)	431,595	1,654,837
Process water 50°F (in)	431,595	2,253,175

Core of calculations

The calculations indicate the effectiveness of each type of stack economizer available to the laundry industry. The tabulations show that the tubular type of exchanger recovers less than 4% of the fired fuel. The only exception to this would be if the stack temperature were to be reduced below the critical temperature and this is not recommended. The direct contact condensing heat exchanger recovers four to five times as much stack exhaust heat depending on inlet water temperature.

The efficiency improvement of the boiler system from the application of direct contact stack economizer can be easily visualized by referring to the heat recovery curve set forth. This recovery is somewhat understated since it is based upon 350ºF flue gas, although most boilers have flue gas temperatures 40-100ºF higher. Note that two vertical scales are given, one, which gives the actual percentage of fuel recovered in the process, and the second, which gives the percent of fuel savings for various flue gas exit temperatures. In general, boilers, which are fired with an 80% combustion efficiency, will have an overall system efficiency of 70% or less. This 10%+ difference in efficiency is made up of losses that do not show up when efficiency is determined by measuring the stack temperature, O2, or CO2 and consist of loss to blowdown, the loss by radiation, convection and conduction of the condensate, steam system (including piping and the boiler), the flash to ambient and other small losses.

Value analysis

The annual value of heat recovered (calculated below) is based upon the 350 BHP boiler at the conditions previously noted.

The cost of stack economizers varies considerably with the type of equipment used. The tubular type heat exchanger may be less

Where

$	=	Value of recovered heat per year
Q	=	Stack heat recovered in Btu/hr
HRS	=	Operating hours per year (2600)
EFF	=	System efficiency (%) = .70
F	=	Fuel cost ($6.00/1 x106 Btu)

     $ Heat Value/Yr = (Q) (HRS) (F) (EFF)

For Stack Gas Recovery with Cold Water (60°F)

$ Heat Value/yr	=	(2,253,175) (2600) ($6.00/1x 1x106 )(.7)
$ Heat Value per year	=	$50,213.00

For Direct Stack Gas Recovery with Preheated Water

$ Heat Value/yr	=	(1,654,837) (2600) ($6/1x106)(.7)
$ Heat Value/yr	=	$36,879.00

For Tubular Type Stack Gas Heat Recovery
(both temperatures)

$ Heat Value/yr	=	(431,595) (2600) ($6/1x106)(.7)
$ Heat Value/yr	=	$9,618.00/yr

expensive for a given size boiler, but recovers only a fraction of the available heat and is subject to a short life expectancy from "cold end" corrosion.

The direct contact condensing stack economizer recovers the most heat and has a long life. In most laundry applications about 15% or more of the heat in the fuel fired can be recovered and up to 20% reduction in fuel bills are common. The cost of all stack economizers varies with the fuel, type of boiler, type of boiler auxiliaries, type of heat sink and physical layout of the equipment. It would be improvident to give specific costs but generally the installed cost would be approximately $50-$75 per boiler horsepower. Recent studies indicate that on single shift operations with boilers of reasonable efficiency, direct contact condensing flue gas stack

HEAT RECOVERY IN ($) DOLLARS/YEAR BASED ON 350 BHP
BOILER FOR DIRECT CONTACT AND TUBULAR TYPE
STACK ECONOMIZERS

Cooling Water Temperature °F	Tubular	Direct Contact Condensor
Process water 100°F	$9,618.00/YR	$36,879.00/YR
Process water 50°F	$9,618.00/YR	$50,213.00/YR

economizers return their own cost in one to three years. In most linen supply plants, the installation of a stack economizer can equal the members'
LaundryESPSM