Heat recovery from poultry processing scald water

J. agric. Engng Res. (1979) 24,325-330

RESEARCH NOTES

Heat Recovery from Poultry Processing Scald Water W. L. SHUPE*; W. K. WHITEHEAD?

Overtlow water normally wasted from the scald tank in a poultry processing plant, was used to heat incoming cold replacement water. In winter, the 5-plate bank heat exchanger distributed a heat gain of 102 kW to the cold water and the overall heat transfer coefficient was 474 Wm-* “K-l ; in spring and summer, heat gain dropped to 54.6 kW and the overall heat transfer coefficient was 472 Wm-* OK-‘. A ratio between actual heat transferred to the cold water to the maximum heat that would be transferred if the cold water were raised to the inlet temperature of the hot water, was a measure of effectiveness. The effectiveness of the system was 0542. With twoshift operation reclaimed heat could have paid for the system in 88 days under winter conditions or in 165 days under spring and summer conditions.

1. Introduction Since 1973 energy use in the U.S. has become increasingly important. In the search for alternative sources of energy, conservation must be foremost. In poultry processing, attention can be given to ways of reducing and conserving energy use. Several references cite concern for the use of energy in the poultry industry. Rogers, Benson and Van Dyne ’indicated that in 1974, 154,600 TJ (1465 x 1012 Btu) were used in poultry production and marketing. The Georgia poultry industry alone consumed 1435 TJ (1.36 x 1012Btu) just for broiler processing in 1974 according to Lowery and Combes*. In the poultry industry, processing of broilers takes such a large amount of energy that it is fairly easy to focus attention on those areas where energy recovery is possible. One such area is the scald tank. Poultry is scalded as an aid in feather removal as described by Klose, Meechi and Pool3 Once the bird has been killed, an immersion of the feathers in 325°K (126°F) water for at least 1 minute allows the feathers to be released easily during the picking phase of processing. For sanitary reasons Federal regulations require that 0.95 kg (one quart) of water overflow from the scalder for each bird processed. For reclaiming heat, one poultry processing plant installed a commercially available unit of the shell and tube type, with which Hayes and Spradling’ reported large dollar savings per year. Dixon5 described areas of energy loss points in poultry processing, one being the scalders. More recently Combes and Ebert6 used a plate heat exchanger to recover 468.9 kW (1.6 x lad Btu/h) from the scald tanks. We undertook this study to determine whether the heat from waste water could be reclaimed economically with no additional energy input, and minimal maintenance. The 5-plate bank heat exchanger that we tested differed from any of the above heat recovery systems and met the requirements of no added energy to recover heat, no pretreatment and minimum maintenance. 2.

Methods and materials

This study was conducted in a large poultry processing plant where 12,500 birds each hour were processed by two shifts of workers 5 days a week. Broilers were processed on two separate *Research Mechanical Engineer; tReearch Agricultural Engineer, EnvironmentalEngineeringLaboratory,RichardB.Russell Research Center, USDA-SEA-AR, P.O. Box 5677, Athens, Georgia 30604, U.S.A. Received 7 February 1979; accepted in revised fom

23 April 1979

325

Agricultural

HEAT

326

RECOVERY

FROM

SCALD

WATER

Scald tank make-up water line

- Flow meter (Rockwell Watermeter) Scold water overflow

5-Plate

bank-heot

exchanger

Fig. 1. Overflow holding tank with heat exchanger

and thermocouple locations

lines (6250 birds/h) that required identical equipment including a scald tank in each line. Prior to the start of the first shift each scald tank was filled with clean water and heated to 325°K (126°F) by injecting steam under water as described by Haynes and Dickens.’ Steam was produced in a 373 kW (500 hp) boiler fired with natural gas. A steam flow control valve and temperature sensor then admitted steam intermittently during the day to control the water temperature and heat the required replacement water. The overflow was only 80% of the replacement water because 20 % was carried out with the chicken carcasses. The experimental heat exchanger in Fig. I consisted of a catching tank in which the overflowing scald water was circulated around the bank of five plates in the tank to transfer heat to the cold incoming plant water. The scald water was then wasted at a lower temperature. Baffles in the tank tended to create a serpentine flow for the hot water. The end of the tank was removeable for cleaning and removing the heat exchanger. The bank of five roll bonded and welded stainless steel plates carried the warmed water to the scald tank make up water line. Flow through the bank of plates was in four passes+lirected alternately counter and parallel by two common headers, one an inlet with flow divided to the five plates and the other an outlet. Each plate of 1.55 mm (16 gauge) thick sheet type 304 stainless steel was 73.7 cm (29 in) long by 149.9 cm (59 in) wide. The effective area was 11.04 m2 (1188 ftz). The heat exchanger was designed by use of the manufacturer’s technical data for overall coefficient of heat transfer and pressure drop. The pressure of the incoming cold water was used and considered to be freely available at the inlet to the heat exchanger, while the wasted hot water was circulated by use of baffles, gravity and natural convection. With this system little external energy was used except that to provide the pressure of the incoming water. Thermocouples for temperature measurement were within 7.6 cm (3 in) of the plant water inlet and outlet and the scald water inlet and outlet. These temperatures were recorded on a multipoint potentiometric chart recorder. Plant water flow rate was measured with an inline flowmeter. Scald water overflow was determined by measuring the time required to catch a known volume in a bucket. Data collection was started on 1 January 1978 and continued to the end of July 1978. The 5-plate bank heat exchanger cost $5OO/plate and the open holding tank cost $300. With additional plumbing the system cost $3500.

W.

L.

SHUPE;

W.

K.

327

WHITEHEAD

3.

Results and discussion

As previously stated, Federal regulations require 0.95 kg of scald water overflow per bird. At 6250 birds/h the scald tank overflowed 1a64 kg/s (26 gal/min) at a temperature of 325°K (126°F) during the coldest months of the tests-January and February 1978-when the average temperature of the incoming water was 279*2”K (42.8”F). The energy required to heat the cold water required for overflow was calculated by: . . . (1)

4 = h c, (G, -I,.)1 where q = rate of heat transfer in kW, riz = mass flow rate = I.64 kg/s, specific heat at constant pressure = 4.1868 kJ kg-l “K-l, = CP = temperature of cold outlet water = 325”K, and % t = temperature of cold inlet water = 279*2”K, c1

giving q = 314.5 kJ/s = 314.5 kW (17885 Btu/min). As a standard practice, this amount of heat is added through steam injection in the scald water. 3.1.

Winter conditions (January to end of February, 59 days)

During the winter the 5-plate bank heat exchanger gave an average heat gain to the cold incoming plant water of 102 kW (5800 Btu/min) with a range of 66.1 to 145.7 kW (3760 to 8340 Btu/min), shown in Fig. 2. Variation in plant water pressure, which changed the flow rate through the heat exchanger, accounted for much of the difference. Also, the amount of hot water overflow was always at least 20% less than the amount of replacement water, because of water carried off by the birds, which agrees with the results of other researchers.s Fig. 2 also shows a decline in heat gain due to the warming trend of the weather which reduced the cold water differential temperature. The overall coefficient of heat transfer (U) was determined for the heat exchanger. The log mean temperature difference (At,,) was calculated and used for counterflow that would give the best heat transfer. The equations used were as follows: u where U

. . . (2)

= qlAAt,,

= the overall coefficient of heat transfer in Wm’-2 “K-l,

A = the area of the heat transfer surface in m2, and At,,,, = the log mean temperature difference in K, given by:

.) At,,,, = (th.-tcJ-(th$ , I

Ih

In

I

.-& o

tho-tc.

where thi % t ci

%l

. . . (3)

1

= temperature of hot water inlet = 314_6”K,

= temperature of hot water outlet = 301.9”K, = temperature of cold water inlet =279*2”K, and

= temperature of cold water outlet = 298.4”K.

These are average values for the first 59 days of winter conditions. The log mean temperature difference is an integrated result from Fourier’s equation and is adequately described in heat transfer textbooks such as Chapman.9 A correction factor could be applied to the right side of Eqn (2) to account for plate fouling and crossflow characteristics.

328

HEAT

140-

;

RECOVERY

FROM

SCALD

WATER

-8000

Average heat gain 102 kW, winter conditions (Jonuary and February,

59 days)

- 7000

60 40 -

.

. 20 -

0

5-Plate

111,1111111111111111 20 40 Jan. Feb.

bank

heat exchanger-II.04

- 2000 -

m2

IO00 0

60

100 Apr.

80 Mar.

Time

(d

120

160 Jun.

140 MY

180

200 Jul.

1

Fig. 2. Heat gain by cold incomingplant water versus time, 1 January-19 July 1978

The correction was assumed to be unity because overall change in U over the test period was small as shown in Fig. 3. Substituting the average test values in Eqn (2) and (3) gives values for At, ,,, of 19.2 K (34*6”F) and a U value of 479 Wmm2 “K-l (84.4 Btu h-l ftm2 “F-l). This result agreed with the manufacturer’s data. 3.2. Spring and summer conditions (April to July, last 111 days) The heat exchanger was in continuous operation which included spring and summer. During the last 111 days the average heat gain by the cold incoming plant water was 54.6 kW (3103 Btu/ min) with a range of 35.6 to 75.0 kW (2022 to 4266 Btu/min). The average log mean temperature was 10.5 K and the average coefficient of overall heat transfer was 472 Wme2 ‘K-l (83.2 Btu h-l ftm2 “F-l). The plates were cleaned by passing a brush between them and the tank drained daily. The apparent degradation of heat transfer was small but could increase over a period of years if scale from the cold incoming water built up on the inside of the plates. 3.3.

Efectiveness

and economic evaluation

In order to express the heat exchanger capacity of the immersed plates in a holding tank, we defined an effectiveness ratio as the ratio of the actual heat transferred to the plant water to the heat that would be transferred to the plant water if raised to the inlet temperature of the overflow water. The scald water decreased in temperature as it entered the holding tank from the scald tank. The effectiveness (E) of a heat exchanger is determined by: e=- 4 actual 4 mar: where 4 actual 4 max

. . . (4)

= the average heat gain calculated in the results, = the maximum possible heat transfer, given by: 4 max

= (%)

cold

(tht-tc

i).

. . . (5)

For winter conditions, where average measured cold water flow was 1.26 kg/s, qmax = (1.26) = 186.7 kJ/s = 186.7 kW (10,617 Btu/min). Therefore, E = 102/187 = 0.542.

(4-1868 x 103) (314.6-279.2)

W.

L.

SHIJPE;

W.

K.

329

WHITEHEAD

5-Plate

10

I

I

20 Jan.

I

I

t

40 Feb.

I

60

bank heat exchangerI

1

80 Mar.

I

I

100 Apr. Time

I

I

120

II*04 I

I 140

m2

I

May

I 1,;~,~]605 160 Jun. Jul.

(d)

Fig. 3. Overall coefficient of heat transfer versus time. I .Ianuary-19 July 1978

Cost was estimated on the basis of data reported by Whitehead and Shupe’O who found that using natural gas to fire a boiler, to produce steam, which in turn was injected under water in the scald tank, required 644 therms of natural gas/1000 head. Georgia Natural Gas Company officials reported that natural gas cost 19#/therm in 1977. We calculated the estimated cost savings for the heat exchanger system by dividing the heat saved by the heat needed and using the above values. For winter conditions those calculations were : heat saved lo2 kW = 0.32 ’ heat needed = 3145kW (0.32) x

6.44 therms x $0.19 x 6250 birds ~.= $2*48/h. h 1000 birds therm

By use of equipment cost as $3500 and time of operation/day as 16 hours, time required to pay for the system under winter conditions was calculated : $3500 = 88 days. $2.48/h x 16 h/day The time required to pay for the system under spring and summer conditions was similarly calculated as 165 days. 4.

Conclusions

The 5-plate bank heat exchanger performed satisfactorily as an immersion-type heat transfer system for wasted scald water in a poultry processing plant. During the winter conditions, 32 % of the heat needed for the scalder was recovered by the heat exchanger. The system effectively conserved energy because it used the waste water directly without pumps, screens or filters. REFERENCES

Rogers, G. B.; Benson, V. W; Van Dyne, D. L. Energy use and conservation in the poultry and egg industry. USDA, ERS, Agricultural Economic Report No. 354, 1976 Lowry, J. F.; Combes, R. 3 cents energy cost per bird can be reduced 15-40x. Broiler Ind., January 1976 44,47,48, 52-53 Klose, A. A.; Meechi, E. F.; Pool, M. F. Feather release by scalding and other factors. Poultry Sci., 1962 41 (4) 1277-1282 Haynes, J.; Spradling, J. Reclaimed heat from overflow scalder water saves poultry plant $11,366 per year. Fd Proc., 1976 39 (7) 120-121 Dixon, J. Here’s how to cut energy cost in sixplant areas. Broiler Bus., 22 May 1977 44, 52 Combes, R. S.; Ebert, T. Scalder heat recovery saves energy at Mar-Jac. Poultry Proc. Market., 3 FebruaryL1978 27-29

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’ Haynes, B. C. ; Dickens, J. A. Attenuation of noise from steam injection in poultry scald tanks. USDA, ARS, ARS-S-153, 1976 a Georgia Institute of Technology. Energy conservation in thepoultry processing industry. Georgia Institute of Technology Engineering Experiment Station, Research Project A2012,l December 1977-1 March 1978, Unpublished progress report 9 Chapman, A. J. Heat Transfer, second edition. New York: MacMillan, 1967 lo Whitehead, W. K.; Shupe, W. L. Energy requirements forprocessingpoukry. ASAE Paper No. 78-3039, ASAE, St. Joseph, Mich. 49085, 1978