The following 4 tiers 72-in. Vento would prove bulky and engender difficult construction due to space in many cases. tabulation gives four selections with the friction and data comparatively from Table 20, as well as approximate space occupied. This was adjusted by Note that the 40 in. Vento is slightly under requirements. The next requirement is to determine whether with the head available the velocities will be realized. The method of determining the resistance and velocity is as follows: Assume for each flue a horizontal duct 30 ft. long with 3 elbows with a radius of the depth, a flue temperature of 125 deg., two registers with 75% of the flue velocity and an effective height of 20 ft. The cubic feet of air required per hour is 170,150 or 2820 cu. ft. per minute. The available pressure for 125 deg. and 20 ft. height (Table 26) is 0.287 lb. per sq. ft. Assume 300-ft. per min. flue velocity with 3 flues of 3 sq. ft. each, say 12 in. deep. From Table 27, 3 sq. ft. with 0.0045 lb. per sq. ft. drop in 10 ft. will give 942 cu. ft. per min. Assuming a length of 30 ft. in the cold air duct and 3 elbows with a radius of the depth, we will require 30 ft. additional of pipe or 30+ 90 + 20 = 140 ft. The ratio of the sides being 3 X 1, the actual friction will be for each unit flue Pipe friction 14 X 1.18 X 0.0045 0.0743 lb. per sq. ft. (Tables 27, 30) This shows there is ample power to overcome resistance at the assumed velocities, also that the Vento stack 4 deep has too great resistance when the outside tem FIG. 19.-Unit fan heater, perature reaches 50 to 65 deg. for the difference between the flue and outside. The available pressure then is 0.16 lb. per sq. ft. (Table 26). 260,360 The steam necessary at 220 deg. F. = 270 lb. per hr. Where the velocities are high, say around 1200 ft. per min., when fans are used, then proportions of heaters change; the number of stacks is increased and the sections per stack are decreased. There is very little data on indirect heating with low steam temperatures or with hot water but it is difficult to raise the temperature of the air with only one stack deep. There is no good reason or advantage in using indirect hot water, as, with the circulated air, a constant steam temperature may be used, varying the room temperature with that of the air flow and temperature. Fan coils are figured in exactly the same manner as for gravity circulation, only with higher velocities. An example is given in Art. 28d. 16d. Unit Fan Heaters.-Figs. 19 and 20 show a rather recent innovation called unit fan heaters. These are set near the floor with a disc fan and motor to circulate the air in shops and factories. They give a very effective distribution without ducts and the horsepower of the fans is only 1 to 11⁄2 hp. or less, per unit. One unit may handle 50,000 or 60,000 cu. ft of air per min., and they may be connected with a hot water or steam system of distribution, thus doing away with direct radiation. The type shown is not suitable FIG. 20.-Unit fan heater, for hot water, but they may be made up of a fan, casing, and a number of sections of short Vento placed about 6 ft. from the floor level. 17. Other Systems of Heating. American Blower Co. 17a. Vacuum Steam Heating.-A vacuum system may be applied to any steam heating plant by placing thermostatic traps on all radiators and drip connections between the supply mains and returns so the piping handling steam will be entirely separated from that taking care of the condensation, except through the thermostatic traps which are all designed to allow air or water to pass but to hold the steam. The system commonly takes the name from the name of the trap; the most common are Dunham (Fig. 204), Warren Webster, McLear, Hoffman and Marsh. The returns are all brought to a central point where there is a power vacuum pump operated by steam or electricity to mechanically remove the air and water from the system. The discharge of this pump leads to an air separating tank or receiver from which the water is pumped back to the boiler by the feed pump. 176. Air Line Vacuum Systems.-A modification of the vaccuum return system is the vacuum air line system used exclusively in connection with single pipe steam. The vacuum system proper has the thermostatic trap at the return of radiator, and air and condensation are removed through the return requiring no air valve on the radiator. Special air valves on the principle of the thermostatic trap are attached to the radiators, which allow air to pass but no water or steam. These are connected together with 2-in. pipe to a 34 or 1-in. main and led to the basement to a drain or sink. This obviates the nuisance of air valves with their smells and leaks. An ejector or other mechanical apparatus for air removal may be used on the end of the line if desired FIG. 20B.-Webster Type N modulation valve, sectional view. 17c. Vapor Systems.-Vapor systems are practically the same as vacuum systems with the mechanical air removal omitted. The same valves and traps are used. The air is exhausted by a large vent on the end of supply and return mains by means of initially raising the pressure on the boiler. After removing the air, the vent on the end of the main automatically seals and if the system can be kept air-tight, the steam will circulate somewhat below atmospheric pressure. Another method involves an entirely open system with a main vent connected to the boiler flue to obtain a pressure below the surrounding air. Automatic regulators on the boiler controlling the draft prevent any steam pressure above atmosphere. Very slight pressure drops and large pipe sizes for the mains are required to accomplish this. The same principles governing the flow of fluids are involved and special attention must be given to make sure that the pressures throughout the system will equalize quickly. Vapor systems are generally used for house heating and small plants where no exhaust steam is available. Thermostatic control is advisable as there is practically no variation in the temperature of the medium beyond a few degrees. Fractional valves are commonly used on the radiators for both vapor and vacuum systems (Fig. 20B). 17d. Donnelly Positive Differential System. The positive differential system consists of the following essential parts: (1) a throttling valve for admitting the desired amount of steam to the radiators, (2) an impulse valve on the outlet of each radiator, (3) a positive differential valve located at each return riser for maintaining a standard difference in pressure between the steam supply and return risers. If the engine were exhausting at 4-lb. back pressure, which would be the pressure in the supply riser and in the radiators when turned on full, the valve on the radiator would be set at 1⁄2 lb. to reduce the steam pressure to zero while allowing the discharge of water by gravity through the valve. The differential valves in the branch return are weighted to about 5 lb. per sq. in. (area of the valve seat) which permits a vacuum as high as 10 in. to be carried on the main return. 17e. Vacuum Exhaust Steam Heating.-Where exhaust steam is utilized or where high pressure steam is reduced, a reducing pressure valve is required which opens and closes, governing the supply of steam automatically with the demand. This is composed of a diaphragm and spring which with the valve seat establishes an equilibrium so that a constant pressure is maintained on the low side. By-pass valves are essential so repairs may be made and operation maintained. A back pressure valve is required so if the engine supplies more steam than necessary, the pressure in the heating main will open the valve to the atmosphere and relieve the system. The back pressure valve also prevents air entering the system. These two pieces of apparatus keep the pressure constant in the mains, an essential point in proper operation and results. Vacuum systems are used where the exhaust steam is circulated at very low pressures in connection with its utilization from non-condensing steam engines. There is undoubtedly economy in the mechanical removal of air from any steam system. Much is claimed for vacuum systems, but except for removal of air, they are like any other steam systems. The vacuum in the returns cannot reach the steam in the radiators or mains due to the trap. If higher vacuum than corresponds with the return water temperature is carried, cold water must be injected back of the vacuum pump to reduce the temperature, or the condensation, being at a higher temperature than the boiling point at that pressure, vaporizes into steam. From the above it will be seen that a vacuum system can produce no pressure below atmospheric on an engine or turbine at the exhaust outlet. The best results are obtained when about 2 or 3 in. of vacuum are maintained on the return of the radiators or just enough to assure the rapid removal of the air leakage and condensation. 17f. High Pressure Steam.-When high pressure live steam is used for heating up to 20 lb., the water may be returned to the boiler by a return trap, a tilting tank that alternately connects the tank with the system, filling with water, and then by its added weight due to the condensation, tilting and connecting with the higher pressure steam in the boiler. As the tank is placed above the water line with checks to temporarily shut off the heating system, the water seeks its level in the boiler. When empty, the tank tilts back and an air valve relieves the residual pressure, shutting off the boiler steam. The pressure in the heating system forces in another charge and the operation is repeated. The same arrangement may be used for kitchen fixtures and high pressure drips on power plants, returning the condensation direct to a high pressure boiler. The circuit being sealed, it is very economical. 17g. Hot Water Heating in Connection with Condensing Reciprocating Engines. It is possible to operate a hot water heating system with partial vacuum on a reciprocating engine but the range in vacuum and steam rate is not very great, due to the necessary changes in compression. The valves have to be set differently for condensing and for noncondensing. There are engines provided with facilities so the valve rods may be changed quickly but there has been serious objection to this practice. The best method is to operate the engine on say 10 to 26 in. of vacuum and use additional live steam for the remainder of the heating requirements. Engines for this purpose should not have too large a ratio of cylinders as they are apt to be unable to carry the load on reduced vacuum; thus economy has to be sacrificed in summer due to engine design in order to utilize exhaust steam in winter. Many cases have arisen where steam has been tapped from the receiver between the high and low pressure cylinders. This will give good results if the engine is not too heavily loaded but just as soon as the receiver pressure drops, the interference is serious. There have been cases where reducing valve connections have been made to the receiver to help out the heavy intermittent draft for other purposes. It is obvious that this steam should be taken direct rather than by this method; the amount of steam generally that can be bled at this point is about the difference in steam rate between non-condensing and full vacuum, or about 25% of the engine's minimum full load requirements. The Bleeder turbine exhausting steam from between the stages involves the same principles. 17h. Combined Heating and Power.-It is generally admitted that when current may be purchased at rates below 1.5 c. per kilowatt-hour, that exhaust steam heating combined with power generation is questionable as a paying investment. The heating system is a dissipator of heat and no steam engine utilizes more than 15% of the heat of the fuel for actual power, the balance being discharged into a lake or pond in the process of producing a low terminal pressure and high vacuum. Reciprocating engines for electric power generaton due to low speed, space required, and costliness, are being rapidly displaced by the small turbo-generator with high speed and vacuum although this machine is exceedingly uneconomical under non-condensing operation. Heating is required about 8 months of the year and while exhaust steam heating would prove profitable during that period, additional condensing equipment would be required for summer operation to maintain the economy and prevent the loss in summer of the saving in winter. The requirements of an economical combined heating and power generating system may be stated as follows: (1) Power cannot be generated under non-condensing conditions in competition with the public service plant with either reciprocating engines or turbines, due to the constant steam power rate. not. (2) In all cases the engine must of necessity deliver its full rated power load whether there is a heat balance or (3) This means an almost constant quantity of exhaust steam due to the fixed terminal pressure at which it must be used on any steam system. (4) The heating and power can only balance at one outside temperature as the heating requirements will vary from 100% in zero weather to less than 50% in moderate weather. Live steam will be required in colder periods and steam will be wasted to the atmosphere during the warmer periods. Therefore a variable steam rate for power is required to balance the heating and maintain the constant power load if the combination is to be a paying investment. (5) The turbo-generator under variable vacuums has a steam rate varying 100% between no vacuum and full vacuum, and hot water forced circulation enables steam temperatures below atmosphere to be used, whereby the variation of the vacuum so produced at the exhaust outlet of the turbo-generator will cause the proper variation in the steam power rate. Therefore, the condensing turbo-generator and forced hot water heating system have all the essential features for an economical combined heating and power system. The same economical and low cost condensing turbo-generator is employed that is used by the public service companies, with high vacuum at all times summer and winter when heating is not required. When heating is required, the heating system takes its portion of the condensing load, the variable vacuum producing the variable steam power rate, constant power load, and perfect heat balance. Thus a maximum power recovery is obtained from the heating fuel. 17%. Evans' "Vacuo" Hot Water Heating System Combined with Power. The writer has perfected a system of vacuum control on which letters patent are about to be issued, whereby the vacuum on a condensing turbo generator may be varied at will from 3-lb. back pressure to 28 in. of vacuum, independent of the power or heating loads without stopping the machine or opening the relief valve. The system has been in operation for several years in a large railway terminal in the East and the relations given are from actual test data, and are therefore reliable. The saving of 70% in steam as indicated, is not dependent on skill of operation, but is inherent in the physics of the problem. It is one of the easiest means of conserving the fuel of community at a profit by utilizing the heating fuel, steam, and boilers for combined power generation. It is applicable to any heating plant of over 500-hp. capacity in zero weather, such as office buildings, factories, or institutions, as long as there is use for the available electric power during the period the heating system operates. The arrangement enables the plant to be placed on a proper accounting basis whereby all heat and current can be metered and apportioned according to different departments and operations. There is a large plant known to the writer wasting about $100 per day in exhaust steam from two non-condensing nozzle turbines used as auxiliaries for a turbo-generator whose entire steam load does not aggregate twice the steam these auxiliaries require. By purchasing 200 hp. in motors, this waste could be eliminated, yet this condition has continued several years. All fuel and power plant expense is lumped and divided by the number of units per day of the manufactured article. There is a very large machine shop receiving power and heat belonging to a subsidiary company manufacturing presses for outside customers. This expense is all lumped in the articles manufactured by the main plant. They actually do not know what it costs only in a very general way or where the expense should be charged. Diagram 7 was deduced from an actual problem and reduced to percentages for convenience. If a plant used 100,000 lb. of steam per hr. in zero weather for heating, and a steam rate of 16 lb. per kw-hr. could be obtained = with 200-lb. initial pressure and 28 in. of vacuum, the average steam rate, hourly power load, and total recovery 100,000 would be respectively 16 lb. X 140% 22.4 lb., X 45.3% 2800 kw. load average, and 2800 X 4900 16 13,720,000 kw.-hr. X 1.2c If plant operates 12 hr. instead of 24 hr., $164,640 per heating season of 8 mo. the saving would be 1⁄2 or $82,320. If the turbine condenser and plant cost $50 per kw. for 6000 kw., the investment would be $300,000, or the debt on the plant would be amortized in less than 2 or 4 yr. hr. = = This same turbine Hourly percentages of the 100% of steam required for zero weather heating 50 10 DIAGRAM 7.-Relative fuel saving and mechanical power recovery from the heating plant under condensing and non-condensing operation. and condenser is available for power under full vacuum in summer and there would be no idle heating boilers in summer to be covered by interest and depreciation. If boiler operation and fuel is 0.7¢ per kw.-hr. and it costs 1.2¢ per kw.-hr. to purchase current, the summer power load would involve a saving of 0.5¢ per kw.-hr. in addition. The 7% area is the actual heat converted into mechanical energy, the 27 % area the increase in steam rate due to the reduction in vacuum during colder periods of higher circulating temperatures. The ordinates are the hourly percentages of the 100% of steam required for zero weather heating. The abscissas are relative percentages |