The cooking equipment may be heated by gas, electricity, solid fuel, oil or steam. The principles that lie behind the process of heating by the first four of these are sufficiently widely known for satisfactory results to be obtained without extensive study. With steam, however, these principles are very different and not commonly appreciated, so that a knowledge of the fundamentals underlying the whole process is essential for the efficient use of steam heating equipment in the kitchen.
It may be argued that the whole subject of steam raising and utilization is one for the specialist only and is no concern of the caterer. The same argument might be applied to the knowledge of the internal combustion engine by the driver of a car, yet every experienced driver knows that he can run a car more efficiently if he knows something of the underlying principles and processes of the engine. With the use of steam by the caterer such knowledge is very much more important and although it is true that the study of steam in all its aspects is a specialized one, its application to kitchen equipment is only a very small part of the whole picture and is often not considered sufficiently important to be handled by the steam expert. Thus all too frequently it is found that the caterer has no specialist to appeal to when difficulties arise with steam equipment, and unless she herself understands the fundamentals of the system delays will occur in determining the cause of the trouble.
Properties of Steam.
Sensible Heat, Latent Heat and Specific Heat. Steam is used as a source of heat for several reasons. It is odorless, tasteless, readily distributed and easily controlled, and in addition will provide large quantities of heat at a constant temperature.
Before considering its practical applications we must examine some of the properties of steam. It is well known that when water is heated in an open vessel the temperature rises until it reaches a point known as boiling point (at atmospheric pressure 212°F.), and continued application of heat does not produce any further rise in temperature. Further heat applied to the water at this point, instead of raising the temperature, produces a change of state: the liquid changes into the vapor, steam. The heat which is applied in the first part of the process and produces a rise in temperature is known as sensible heat, and that applied in the second part, producing no rise in the temperature of the liquid, as heat of vaporization or latent heat.
When measuring quantities of heat exact definitions are of course necessary. The standard unit of heat is defined as the quantity of heat required to raise the temperature of 1 lb. of water 1°F. and this is known as the British Thermal Unit. When the centigrade scale is used the standard unit is the amount of heat needed to raise the temperature of 1 lb. water through 1°C, and this is known as the Centigrade Heat Unit (C.H.U.) or Pound Calorie (1 C.H.U. = 1.8 B.Th.U.). In this country the British Thermal Unit is used universally. It will be seen that 180 B.Th. U. are required to raise the temperature of 1 lb. water from 32°F. to 212°F. and so the Sensible Heat of water at boiling point and atmospheric pressure is said to be 180 B.Th.U. per pound. To convert boiling water into steam at the same temperature requires considerably larger quantities of heat and the recognized figure for this Latent Heat is 970 B.Th. U. per pound Let us now consider the reverse process, namely the conversion of steam into water, since it is the heat released from the steam during this change that is used in cooking. The total heat of steam at atmospheric pressure is 1150 B.Th.U./lb. (the sum of the Sensible Heat 180 B.Th.U./lb. and the Latent Heat 970 B.Th.U./lb.). When steam condenses and the Latent Heat is released for cooking purposes, the water or condensate remains at 212 F and of course still retains 180 B.Th.U./lb.
One other term which may arise in a discussion of steam problems is Specific Heat. Different substances have different heating capacities and in order to compare them a relative standard must be adopted. Water is taken as the standard and other sub. stances compared with it. Thus the Specific Heat of a substance is the amount of heat necessary to raise the temperature of one pound of the substance one degree, compared with that required to raise a pound of water one degree.
Temperature and Pressure Relationship
When water is boiled in an enclosed vessel the pressure inside increases and the temperature rises above 212°F. At the same time the Total Heat, Sensible Heat and Latent Heat are also affected. The increase in Sensible Heat is of course directly related to the increase in temperature. Thus at 5 lbs. pressure the temperature is 227°F. and the Sensible Heat 190 B.Th.U./lb. while at 12 lbs. pressure the temperature rises to 239°F. and the Sensible Heat becomes 207 B.Th.U./ib. The Latent Heat, however, decreases as the pressure increases. At 5 lbs. pressure it is 960 B.Th.U./lb., at 20 lbs. pressure 940 B.Th.U./lb., and at 65 lbs. pressure 901 B.Th.U./lb. Thus steam at 20 lbs. contains nearly 5% more Latent Heat than steam at 65 lbs.
As would be expected the volume of steam also undergoes a change with the change in pressure: it falls as the pressure rises. At atmospheric pressure the volume of steam is 26.8 cu. ft./lb., at 15 lbs. it is about half this volume, at 60 lbs. it is less than one quarter of its volume at atmospheric pressure. This point is important when considering steam raising and transmission, as of course no matter what the pressure, the volume of the boiler, the pipes and the steam using equipment remains unaltered.
Wet and Dry Steam and Superheat
When vapor is at a temperature corresponding to the liquid boiling point appropriate to its pressure it said to be saturated. Steam in this state is commonly called dry steam and is ideal for cooking purposes since it contains the maximum quantity of heat per unit volume. But steam is seldom dry. Under certain conditions steam may be a mixture of true vapor and liquid droplets and is then known as Wet Steam. In this condition it is obviously less efficient as a heating medium because the droplets have escaped from the boiler without carrying any latent heat. Dry steam may become wet when, for instance, it is passed through a cold pipe : some of it condenses on the inner surface forming droplets which are carried along with the steam. Another way in which steam may become wet is during its production in the high pressure boiler. If during the violent action of boiling, when steam is being produced at a high rate, droplets or foam are thrown up, these will be carried over with the steam, particularly when the water level in the boiler is high. The reduction in the heat content of wet steam compared with that of dry steam is illustrated by the following example. Wet steam which contains 90% dry steam and 10% droplets is said to have a dryness factor of 0.9; the total heat of 1 lb. of such steam at 60 lbs. gauge pressure is the sum of the sensible heat 278 B.Th.U. and 0.9 times the latent heat, 905 B.Th.U., i.e. = 1092 B.Th.U. Now the total heat of dry steam at 60 lbs. is 1183 B.Th.U., and thus wet steam of 0.9 dryness factor contains 91 B.Th.U./lb. less than dry steam at the same pressure.
Wet steam can be dried by the provision of a dryer in the steam space consisting of a series of baffles in the steam passage so designed that the droplets of water are deposited as the steam passes through. Alternatively it can be dried by heat applied either directly by passing the wet steam through a heated chamber, or indirectly by passing it through an expansion device known as a reducing valve.
Thus reducing the pressure from 60 lbs. to 15 lbs. releases 18 B.Th. U per pound and this is used to evaporate any droplets and convert them into steam. If there are no droplets present then reduction of pressure raises the temperature of the steam above its saturation temperature or superheats it. Superheated steam can also be produced by passing saturated or even wet steam through heated pipes, but as the use of superheated steam for the heating of cooking appliances is uncommon it calls for no further consideration.
The system of generating steam at a high pressure before use is very common in the steam heating of equipment for cooking as will be discussed below.
Steam Raising and Utilization
In many establishments the raising of steam for cooking purposes is only a small part of the work of the boiler house, the main output being required for water heating, space heating, and in some industrial concerns to provide power. There are large kitchens, however, where steam is raised only for cooking purposes and under such conditions the boiler house becomes part of the catering unit and consequently the concern of the catering supervisor. It is therefore useful to have at least a basic knowledge of the process of steam raising.
The Boiler House
The boilers commonly used for steam production for kitchens are of the high pressure type and usually operate at 50-60 lbs. pressure. Low pressure boilers operating at a few pounds pressure are sometimes found but are less satisfactory for cooking purposes. Solid fuel in the form of coke is the fuel most commonly used but boilers consuming gas, oil or electricity are also available and have some very definite advantages in the saving of labor and case of operation. They can be operated automatically and do not require constant attention for firing and removal of ashes; there is no coke or ash in the boilerhouse to cause dirt and steam can be raised much more quickly. Whereas with a coke fired boiler the operator must be in his boiler house by 5.30 a.m. to ensure the required steam pressure by 7.0 a.m., an electrically heated boiler reaches full pressure within 10-15 minutes.
Gas and oil fired boilers are basically similar in principle to coke fired boilers. The boiler using electricity, however, known as the Electrode Boiler, is different and merits a brief description. The principle on which the water is heated to produce steam in an electrode boiler differs from that employed in any other system: the current actually flows through the water. When a current of electricity is passed through any substance the loss of energy due to the resistance causes a rise in temperature. Thus if water is used a rise of temperature takes place and the water may be boiled. This is the principle employed in the generation of steam in the electrode boiler and because the heat is generated within the water itself and not transmitted from an outside source such as an immersed element, the operation is quick and highly efficient. One manufacturer of boilers of this type claims a 95 to 98% efficiency, the only heat losses being due to conduction through and radiation from the boiler casing.
Electrode boilers are made in a range of sizes varying in loadings from 30 kW. with a steam output of 100 lbs. per hour, at 212°F., to 1,000 kW. with steam output of 3,350 lbs. per hr, at 212°F. The current must be alternating, of course, since with a direct current electrolysis of the water would take place, with a consequent loss of efficiency in the heating of the water, generation of an explosive gas and corrosion of metal parts.
To determine the size of electrode boiler suitable for steam generation in a particular kitchen the general average figure of 1 lb. steam per meal served can be taken as a rough guide. It is of course possible to calculate steam requirements more exactly by considering the consumption of each piece of steam equipment employed. It is a fairly simple matter to calculate the amount of steam necessary to raise the temperature of a certain quantity of water from 50°F. to 212°F. in a boiling pan, but thermal losses due to convection and radiation must also be taken into account and such figures are not readily available. In any case such calculations are only of interest to few caterers and are primarily 2 matter for engineers. There is, however, one simple test to determine adequacy of steam output which any caterer can apply after the boiler is installed and in full production. The pressure gauge fixed at the outlet from the boiler shows the pressure of the steam being generated by the boiler. If this remains steady at the generating pressure of say 50-60 lbs. when all equipment is in use the caterer can be assured that the boiler is adequate for the work for which it is intended. If, however, it falls to 15-20 lbs. and remains there during the working period of the steam equipment the matter should be carefully investigated since it is evident that the boiler is not equal to the load placed upon it.
Whatever type of boiler is used, there should be least one employee responsible for the satisfactory operation of the boiler. When once the pressure gauge indicates that the required steam pressure has been raised, the duty of the boiler attendant is to maintain this pressure; it must be realized that during a period when steam is continually drawn of the sold fuel fired boiler will need constant attention. The maintenance of a hot fire with solid fuel calls for replenishing at least every half hour, and the water level in the boiler must be kept fairly steady. If the level sinks too low the boiler may become dry and explode, while if too much water is introduced wet steam may be produced with resulting loss of efficiency in the cooking equipment. A gauge glass is always provided on the boiler to show the water level so that these two extremes can be avoided.
Water used for replenishing the boilers is obtained from two sources. The main and preferable source is from the tank holding the returned condensate from the cooking equipment, as this is hot and therefore more economical in the use of fuel. It has another advantage also in that it is free from impurities which cause hardness and will not therefore cause scaling of the boiler. The other source of supply is from the mains from which water at about 50°F. is forced into the boiler by a pump or an injector. This method of replenishing must be used from time to time in steam systems where all the condensate is not returned to the boiler, as for example where steaming ovens utilizing live steam are in use and exhausted through the steam trap to the atmosphere. There are two different systems used for forcing water into a boiler against the internal pressure. One is the injector, a simple device without moving parts operating by a simple cock, and the other an electrically driven pump switched on and off in the usual way. It is usual to use the former for the introduction of mains water and the latter for returned condensate.
The skill of the boiler house attendant lies in his ability to maintain a steady pressure of steam by careful attention to refueling and introduction of water. If pressure is allowed to fall too low either by inadequate firing or the introduction of a large amount of cold water cooking operations will be delayed. If on the other hand the pressure rises above a certain fixed limit, usually about 15-20 lb. above the normal working pressure, the steam blows off through a safety valve and is dissipated and fuel is wasted.
The steam generated by a high pressure steam boiler at about 50-60 lb. pressure is conducted through iron pipes to the point where it will be utilized. These iron pipes are heavily lagged to prevent excessive heat losses, the material used being either asbestos or 85% magnesia, i.e. 85% magnesium carbonate with 15% asbestos to bind the magnesia crystals together. The lagging is applied in the form of a plaster which usually is finished with a protective coating of paint.
As has already been stated, steam is generated at approximately 50 lb. pressure and used for cooking at either 15 lb. or 5 lb. To obtain steam at these lower pressures a reducing valve is introduced into the pipes. These valves may be located either in the boiler house or in the kitchen, but the usual method is to reduce the pressure to 15 lb. before piping the steam away to the kitchen. A gauge fixed on the delivery side of the valve records the pressure and it is as well to glance at this from time to time to ensure that a steady pressure is maintained. Steam at 15 lb. pressure is used for all the equipment except steaming ovens where 5 lb. pressure only is required. Practice varies in the location of the reducing valve controlling the supply of steam to the ovens. In some systems it is fixed in the boiler house, in others immediately above the point at which steam is admitted to the ovens. In the former case, if the boiler house is at any considerable distance from the ovens it is unlikely that steam will be at 5 lb. when it reaches the ovens, since a drop in pressure of about 1 lb. is usual for every 100 ft. of piping. In this system there is an added possibility that steam may not be dry when it reaches the oven owing to the condensation that takes place in the pipes during transmission. It would therefore seem that it is preferable to fix the reducing valve above the steaming ovens. This has the added advantage that the gauge fixed with this valve will also be in the kitchen and therefore accessible to the kitchen staff for verification of the normal pressure.
(a) Heating by Steam Jacketing and Steam Coils.
Jacketed boiling pans are heated by steam which is admitted to the space between the inner and the outer shell by opening a control cock on the steam pipe linked to it. The quantity of steam admitted can be regulated by this cock thus providing for fast and slow boiling. When equipment is brought into use after a resting period, as for example at the beginning of the morning’s work, the pipes through which the steam passes will be cold so that there will be more condensation and heating up will be slower than later on. One other point should be borne in mind when equipment is brought into use after a resting period, particularly if a number of appliances are used simultaneously at full pressure. The amount of steam drawn from the boiler in these circumstances will be much greater than at any subsequent period when all the equipment has been thoroughly heated, and unless there are great reserves the pressure in the boiler will fall too low. It is therefore advisable, if possible, to bring equipment into use over a reason. able period of time and not to turn the cock to full pressure immediately. This is sometimes described as cracking the valve gently.
When steam gives up its latent heat it becomes water and the condensate thus produced must be removed from the equipment or inefficiency will result. As has already been pointed out in describing the operation of boiling pans, the accumulation of water in the space between the two shells of a jacketed boiling pan is an all too frequent cause of slow heating. If the condensate were allowed to drain away through a pipe to atmosphere it would get away easily because of the steam pressure behind it, but when all the water had been pushed out steam would follow it and this is clearly not desirable. A valve must therefore be installed which will allow water and not steam to pass. This valve, known as a steam trap, is a device which distinguishes between steam and water, opening to allow the latter to pass but closing to the former and hereby trapping it, thus ensuring that all the steam entering an appliance delivers up its latent heat before leaving as condensate.
Steam traps of two main types are used in conjunction with kitchen equipment, one being operated mechanically and the other thermostatically. The former distinguishes between water and steam by means of the difference in density between the two. A vessel which will float on water, either a bucket or a hollow metal ball is employed. When the condensate enters the trap this float rises until is closes the valve, then fills with water and opens the valve through which the water is then blown by the pressure of the steam. As the water escapes the float rises again and closes the outlet valve. The thermostatic steam trap differentiates between the temperature of steam and the temperature of the condensate formed from it. The temperature of steam at a given pressure is constant and when the steam condenses it forms water at the same temperature; this water then gives up some of its sensible heat, causing the temperature to fall, and the thermostatic steam trap is designed to operate on this difference of temperature.
It is obvious that steam traps must be maintained in good working order or defects in the heating system will occur. Skilled maintenance service with replacement of worn parts when necessary is of course required, but one maintenance task does not need skilled labor and should be undertaken by the kitchen staff. One of the causes of defects in steam traps is the presence of pipe scale, rust and other forms of muck in the pipes which may partially block the valve and damage the mechanism. This condition is particularly common in new pipes which may contain Casting sand, packing, jointing, solder and similar materials. To prevent these substances entering the trap a strainer consisting of a wire gauze cylinder is fixed in the pipe immediately in front of the trap. The dirt screened by this appliance can be removed from time to time by taking out the strainer and cleaning it. This is a job which only takes a few minutes and should be done periodically, care of course being taken to turn off the steam before unscrewing the opening to the strainer. A complete description of steam trap design and operation is outside the scope of this book, but those wishing to know more are referred to a useful book on this subject by Northcroft.
The condensate, having passed through the steam trap, then returns to the boiler house where it is collected in a tank before being pumped back to the boiler. Where the boiler house is situated at a level lower than the kitchen the condensate will gravitate back to the tank, but it will also be forced back by the pressure of the steam even if the boiler house is not at a lower level. This steam pressure will raise the condensate 2 ft. for every lb. sq. in. pressure difference between the steam trap and the condensate return main, and therefore, even if the condensate tank in the boiler house is above the level of the steam trap, the condensate will be forced into the tank without any additional mechanical action.
(b) Heating by Direct Contact Steam.
The main use for direct contact or live steam in the kitchen is to heat the steaming oven, but it is also used to heat liquids (by being blown into them through a nozzle) and for sterilizing. The advantages of direct contact steam are that the heating up process is very quick and an exact unvarying temperature is obtained.
When the cock admitting steam into the oven is first opened a spray of water comes from the pipe. The reason for this is that condensation has taken place in the pipe while the cock has been closed. This water is removed from the oven by a drain in the base. It would clearly be an advantage if this water could be prevented from entering the oven, and in some steam pipe designs for ovens this is effected by including a trap before, and at a lower level than, the point at which steam enters the oven, so that the condensate gravitates to the trap instead of being forced into the oven.
When steam begins to enter the oven it forces the air out either through the drain outlet or through a specially designed thermostatic air vent at the top of the oven. It is important that no air should remain in the space or it will slow up the cooking. As the oven fills with steam the temperature rises and some steam condenses, giving up heat to the surrounding surfaces. The condensate so produced falls to the bottom of the oven and runs away through the drain outlet to the trap which is usually a combined steam and grease trap. This trap in the mechanical float-type is a large iron box of about 1½’xl’xl’ with a removable cover held firmly in place by ring nuts on hinged bolts. Inside are two compartments separated by a partition which does not reach to the bottom of the vessel. The grease in the condensate which naturally floats on the top of the water is retained on the inlet side of the vessel and when cold can be removed.
The need for careful cleaning of all steam grease traps cannot be emphasized too strongly. The frequency with which the operation need be performed varies according to the use made of the oven, but generally it is carried out as a routine matter once a week. If this is not done the valve admitting water to the trap will become blocked and the drain outlet and eventually the heating space in the oven itself, will gradually fill up with water.