Incomplete - pictures to be added and text cleaned up (1 June 1998).

HEAT DISTRIBUTION

By A. Margolis, Dipl.-Ing.

(Proceedings of the Instution of Mechanical Engineers, Vol 135, p. 359-366. April 1937)

District heat distribution has been developed in Germany since the War, and now forms a special branch of engineering. To-day heat distribution is not merely a question of erection of pipe lines, trench work, expansion joints, insulation, etc.; but it is, especially for combined power and district heating plants a question of economy which has to be investigated in each individual case. For the purpose of the paper, a short outline of the main problems of heat distribution is perhaps best afforded by considering examples of the district heating plant of Hamburg, of which the author was the manager for 12 years.

The first stage of the Hamburg district heating plant was erected in 1921 in connexion with the oldest station of the Hamburg electric works situated at the Poststrasse, with boilers having a total heating surface of 24,000 sq. ft. In 1924 the Carolinenstrasse station, with a boiler plant of 43,000 sq. ft heating surface was put into service for the extensions of the system and in 1929 the Bille Station with a boiler heating surface of 73,000 sq ft also commenced working. Finally, in 1933, it was decided to increase the heat supply from the comparatively modern Tiefstak station, which had three boilers with a heating surface of 154,000 sq. ft. The first three stations had small units - 51 boilers in all - and were obsolescent.

The system of heat distribution employed steam, because both steam- and hot-water heating plants could then be served. The working pressure of the boilers at Poststrasse was 160 lb.per sq. in., and the pressure was about 150 lb. per sq. in. at the engine stop valves. A very low back pressure of only 7 lb. per sq. in. was chosen for the first system.

The Carolinenstrasse and Poststrasse stations were connected by a steam main initially 25 inches in diameter and 7,000 feet in length, and the Bille and Poststrasse stations by a steam main initially 32 inches in diameter and 14,000 feet in length. The back pressure of the turbines varies between 15 and 50 lb. per sq. in., according to thesteam demand. The Tiefstack and Bille stations were connected by a steam main, 28 inches in diameter, for conveying 250 tons of steam per hour, at a pressure of 210 lb. per sq. in. and a temperature of 660 deg. F. The length of the main was 11,000 feet

The last extension was necessitated by the decision, after thorough investigation, to install the new boiler plant under consideration, with 1,700 lb. per sq. in. pressure, not in the Bille power station but in Tiefstack, because at the latter station two 11.000 kW. back-pressure sets could be utilized in conjunction with the existing condensing turbines. In addition to the steam distribution system, Hamburg has a special hot-water distribution line for a number of buildings. Heat distribution by steam has the great advantage of affording a supply either for a steam-heating or a hot-water system, and of enabling the measurement of the heat supply to be made reliably by means of cheap condensate-water meters. Contrary to the general American practice the condensate-water is returned from the buildings to the boiler plants and used as feed water. By using the closed system, as in Hamburg, ordinary commercial steel pipes have proved quite satisfactory. The loss of condensate as a rule amounts to a small percentage only, so the purifying costs for the make-up water are quite small. Hot-water distribution is more expensive than steam distribution, but the connexions are cheaper and it has the great advantage of cheap storage of heat in the form of hot water. The higher the pressure drop, the greater the electrical output. This means that for a given initial pressure the distribution pressure should be as low as possible; on the other hand, by decreasing the pressure the capital cost increases. Therefore, to ascertain the most economical scheme for a combined power and heating plant, the distribution system must be calculated for different back pressures. The amount of calculation involved is considerable, but it is the only way to make a thorough investigation.

The superheated water-heating system is not represented in Hamburg. This system has many advantages and it is especially suitable for factories. As a comparatively high temperature can be chosen for the water, it is possible to apply this system, instead of steam, not only to heating, but also to process work. Fig 15 is a diagram of a superheated hot-water system for textile works supplied by The Strutevant Engineering Company, Ltd. For district heating plants, superheated hot water has the disadvantage of decreasing the electrical output, due to the increased back pressure; moreover, calorifiers are required in all buildings. These factors increase the capital costs as compared with a steam distribution system. The conclusions derived from the development of the Hamburg district heating plant is that the transmission of heat is possible even at comparatively long distances. The author recalls that in the early days of his district-heating practice, the transmission of heat over a long distance was so unusual that when he suggested to the manager of a brewery the taking of heat from a power station at a distance of a mile, he was asked whether he hoped to carry the heat in sacks!

INTERNAL DIAMETER OF STEAM PIPE-INCHES

Fig. 16. Relative Economies for Various Sizes of Steam Mains

Length of main, 1 mile; average pressure, 20 lb. per sq. in. gauge; pressure drop, 0.5 lb. per sq. in. per 100 feet; air temperature in conduit,, 95 deg. F.; conductivity coefficient 0.365 B.Th.U lb. per sq. ft. per in. thickness per deg.F. temperature diference per hour; thickness of insulation, 2 inches

The curves illustrate the relation of the rate of heat delivery and the rate

of heat losses

Through a 4-inch pipe, heat delivery is 2.2 x l0 B.Th.U. per hour.

" "24-inch ,, ,, increases to 190 x 10 B.Th.U. per hour.

" "4-inch pipe, heat losses are 11.4 per cent.

" "24-inch ,, ,, ,, decrease to 0.59 per cent.

To-day heat is conveyed in Hamburg from the Tiefstack station to the Carolinenstrasse station, a distance of 6.2 miles. Fig. 16 shows how the conditions for heat transmission improve with the increasing heat demand. Curve a shows the heat transmitted in millions of British Thermal Units per hour for low-pressure steam at an average pressure of 20 lb. per sq. in. gauge for various pipe sizes, all other conditions being the same. Curve b shows the heat losses. In this connexion, the transmission of heat is more satisfactory than the trans-

Fig. 17. Industrial and Agricultural Scheme for Port Talbot District

--------Steam mains for I0O-15O lb. per sq. in. pressure.

_.__._._._" "atmospheric or subatmospherie pressures.

The diameters of the mains are reduced as their distance from the supply increases. The figures against the various mains and branches indicate the internal diameter in inches. Scale, 1 inch =500 yards.

mission of electricity. Of course the radius for heat supply is much smaller, but it is large enough to supply even a city the size of London from power stations situated outside the town.

The second conclusion is that in the future it will be possible to weld the generation of electricity by steam with the heat supply of a town into one complete system. The power station should be located in the outskirts of the town and in the vicinity of the industrial quarter. The super high-pressure turbines, exhausting at a pressure of, say 170 lb. per sq. in. This exhaust steam is delivered to industrial users. At some convenient place, suitable for a centre of power distribution, a substation is provided. At the substation the steam, now having a

pressure of, say, 150 lb. per sq. in., is used for power generation by driving turbines working against a back pressure of, say, 40 lb. per sq. in. gauge. The steam is distributed at this pressure to supply administrative buildings, offices, and the like. Under favourable conditions the remaining energy in the steam at 40 lb. per sq. in. can be further used for power generation in back-pressure turbines having a back pressure of 7 lb. per sq. in. abs.; the water used for condensing the steam in these secondary substations is utilized for heating purposes by circulating it through a hot-water distribution system. To balance the different loads due to electrical and heat demands, large-capacity hot-water accumulators, such as those developed by Rudolf Otto Meyer in the Hamburg district-heating scheme, can be applied.

Fig. 19. Steam Mains, 28 inches Internal Diameter

Referring to development in this country, Fig. 17 shows a scheme designed by Mr. F. W. Shilstone for heat distribution from a combined power and heating plant. An interesting feature of this scheme is theof the management. He had experienced that personally. It was often very difficult to get the management to consider any improvements which would affect only, say, 1 or 2 per cent of their total turnover, though it might eventually be possible to show that the 1 or 2 per cent might in turn result in further savings. He had been extremely interested in Mr. Margolis's paper, and he asked what inspired the installation of the Hamburg system. He found it difficult to conceive of a similar system in this country. Was it installed from purely aesthetic considerations, to eliminate smoke in the city, or had it an economic basis? No figures were given for coal costs, and so on. It was surprising that steam could be transmitted such long distances with so small a loss in large pipes. He would like some information on the thickness of the insulation and the materials used. The prevention of leakage was a big problem. His own experience with expansion joints of the gland type, which he gathered were used in the scheme described, had not been satisfactory; it was most difficult to keep them tight. Presumably that had not been the experience in Hamburg. In reply to Mr. Fitt, the Hamburg District Heating Company (Fernheizwerk Hamburg G.M.b.H.) was started in 1921 by Rudolf Otto Meyer and the Hamburg Electricity Company, because it was found from a thorough investigation that it would be a sound commercial proposition. Elimination of smoke was a minor consideration. Rudolf Meyer also installed at his own expense the district heating system of Kiel purely on the basis of the commercial return it showed, in spite of the comparatively short licence of 22 years for which the concession could be obtained. The gland type expansion joints used in the Hamburg installation were absolutely tight and required little attention. These joints were troublesome in the initial stage but by a series of experiments a suitable packing was devised which completely overcame the troubles.