Energy conservation in plastics processing: A review

Energy Conservation in Plastics Processing: A Review J. M. DEALY

Department of Chemical Engineering McGiEl University

3480 University Street Montreal, Canada H3A 2A7

The rapid recent growth in the relative importance of energy costs has directed the attention of machine manufacturers and processors to more energy-efficient designs and methods of operation. This paper reviews reports of advances in this field. The energy flows associated with the principal processes used in the plastics industry are enumerated, and the problem of defining a meaningful efficiency is addressed. Specific attention is given to recent advances in the areas of extrusion, injection-molding, and heat recovery. A method for energy analysis based on the second law of thermodynamics is described.

INTRODUCTION Scope of This Review

ere are opportunities for reducing energy con- Th sumption at all stages of the manufacture of plastics articles, including polymerization, compounding, melt processing, secondary forming operations, packing, and shipping. This review focuses attention mainly on the melt processing stage, but there are limits to the extent to which one can consider these steps as independent. For example, new resins (1-3) have been developed that require lower processing temperatures, thus reducing the energy needed for extrusion or injection. Further- more, the combining of steps is likely to be an increas- ingly important means of reducing energy consumption, for example, by compounding and extruding simulta- neously or by combining polymerization with forming. Hall (4) has noted the major savings possible by use of RIM in place of traditional injection molding; lower injection pressure and temperature are required, and the exothermic polymerization provides most of the heat needed for the mold.

On the other hand, it can happen that an energy-saving modification at one stage may simply increase the energy required at some later stage (4, 5). It is thus necessary to distinguish between local or tactical savings and overall or strategic savings.

Taking here the melt processor’s point of view, there are several levels at which energy conservation can be approached. The “housekeeping’ level involves mainte- nance and repairs, such as the periodic lubrication of mechanical devices and cleaning of heat-exchange surfaces. The second level involves modifications to the existing plant while retaining the same basic process. Examples are the installation of more elaborate control systems and the use of high-efficiency extruders. The present review is concerned primarily with this level of approach. A third level is the development of entirely

new processes for producing plastics products, for example, by using dielectric heating to fuse resin particles without bulk melting, as proposed by Erwin and Suh (6).

Energy Flows and Measures of Ef’ficiency

The major melt-processing operations are extrusion, injection molding, blow molding, film blowing, and melt spinning. In each case, the basic steps are the same; the resin is plasticated, heated, and pumped in one machine, and then formed into the desired shape and cooled to ambient temperature in subsequent operations. Virtually none of the energy that flows into the process ultimately appears as stored energy in the finished product; it is needed only for forming. Thus, the first law of thermodynamics tells us that all the energy flowing into the process ultimately flows out, although it leaves in the highly degraded form of low- temperature heat.

The first energy-consuming operation is often resin drying, required to remove absorbed moisture. Next, electrical power flows to a motor that produces the mechanical work needed to melt and homogenize the resin. A portion of the motor work goes into pumping the melt through a die or into a mold. Additional heat for melting is supplied by means of electric or steam heaters. All external surfaces that are at elevated temperatures lose heat to the environment. In addition, localized overheating in plasticating devices is controlled by the circulation of coolant.

Additional mechanical work is usually needed in the forming stage (for example, to close and open molds and to eject finished parts). Finally, the finished part must be cooled, and energy must be consumed to effect this massive rejection of heat. This will be used to operate circulating pumps, air compressors, or blowers. All of the heat removed (i.e., all the energy originally entering the process) ultimately passes to the environment, al-

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Energy Conservation in Plastics Processing: A Review

though in winter it may see intermediate duty for space heating.

Many types of energy ratios and efficiencies have been used to characterize processing equipment. For example, there is the “power drive economy” of an extruder, defined as the throughput in lbs/h divided by the motor horsepower. Kruder and Nunn (7) have pointed out that this is a poor guide to energy use, as a low motor power may be more than offset by large heater power requirements. A more useful quantity is the “overall power economy” or “specific power”, defined as lbs/h throughput divided by total electrical power input. A similar quantity is the “relative energy efficiency” of an injection-molding machine, defined by Johnson (8) as the power (kW) consumed by the extruder drive, barrel heaters, injection unit, mold clamps, and mold heaters, divided by the throughput in lbs/h.

Ratios such as these are useful in comparing different machines doing the same job, but they are not true efficiencies because they do not compare the actual machine with an ideal type of operation that uses the minimum possible energy. McKelvey and Bernhardt (9) proposed the definition of such an efficiency for extrusion. They suggested that the minimum power required to melt and pump the polymer is as shown in E9. 1.

TZ

Ti Pmin = h / C,dT f h v V P (1)

Assuming the polymer to be incompressible, this is simply the specific enthalpy increase in the extruder times the flow rate, and for adiabatic operation, the first law of thermodynamics tells us that this is the total power required.

The efficiency, then, is defined as follows

V E = P m i J p A (2)

where P A is the actual power used. In other words, we are comparing the actual extruder

with one in which there are no heat losses of any kind, either from the motor, the power transmission system, or the barrel. This type of efficiency is useful not only in comparing two machines, but also in comparing each with the ideal adiabatic extruder. Nunn and Ackerman (10) have used a similarly defined efficiency to evaluate injection-molding machines.

It is important to remember, however, that there is always some degree of arbitrariness in defining efficiencies. This results from the need to establish an “ideal” process with which to compare the actual one. For example, we note that the melt temperature, T,, is part of the definition of the efficiency defined in E9 2. Thus, the minimum power varies from one resin to another. This could pose a problem if there are several resins that can be used in the same machine to make the same product. Several resins recently introduced (1-3) were developed expressly to reduce T2 and thus reduce the energy requirements of processing. Furthermore, we note that the energy actually embodied in the polymer does not change significantly from the resin to the finished, cooled part, and this might lead us to conclude

POLYMER ENGINEERING AND SCIENCE, IUNF, 1982, Vol. 22, No. 9

that the minimum energy required is zero! This sounds a bit silly, but defining the ideal case in this way serves the important purpose of directing our attention to entirely new ways of forming that do not involve complete melting, for example, by the use of dielectric heating (6).

Finally, it should be mentioned that, when any energy is recovered (for example, for space heating), we will want to take a credit for this in calculating an efficiency. Now, a new factor needs to be taken into account, and this is the fact that not all forms of energy are equivalent. A method for taking this into account is described in the last section of the review.

For a commercial processor, of course, the key quantity is total cost rather than total energy input. The usefulness of energy analysis is not to identify “the” opti- mum machine configuration, but to direct attention to alternatives that, because of the prominent role now played by energy prices, are likely to give low operating cost.

REDUCING ENERGY CONSUMPTION

Efficient Use of Electricity

In this section, we will deal with the motor itself and with load management. The efficiency with which the processing machinery makes use of motor output power will be taken up in subsequent sections.

When an induction motor is used to supply mechanical energy at a variable load, significant electrical power is lost due to the low power factor associated with this type of service. Eickelberg (11) has described one solu- tion to this problem, the use of capacitors, and has given several examples of its use by commercial processors.

Another aspect of electric power utilization is load management. The main objective here is not to reduce power consumption but to reduce power costs by reducing the load factor. Since rate depends on peak demand, a brief period of high power usage increases the rate paid for all of the electricity used in a certain period. In order to get control of the load factor, one needs to monitor the total load and be able to quickly “shed load” by turning off carefully selected power-consuming units for short periods. In a plant of any complexity, this implies the use of a computer to monitor and shed loads. Security Plastics of Miami Lakes, Florida, not only avoided high peaks, thus reducing its electricity rate, but also reduced its total consumption by 15 percent by use of a computer for load control (11).

Resin Drying

Many polymers absorb moisture from the air, and this moisture must be removed before processing. If it is not, it will accelerate equipment corrosion and cause problems in the forming stage. Combining data of Cal- land (12) and Glanville (13), Schott and Derby (14) show that the energy used for drying can often be of the same order, per lb ofresin, as the total of all other energy used in processing.

Part of the drying energy is lost in the water vapor, and part goes to heat up the resin. If the drying step is separate from processing, the heat in the resin is also

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lost. One way of preventing this loss is to use a hopper dryer so that the resin enters the plasticating machinery immediately after drying and carries the drying heat with it. Another approach is to vent the plasticating device to allow the outflow of water vapour and thus eliminate the separate drying step.

Lord (15) suggests eliminating devolatilization in the plastication stage and modifying the mold to allow the water to escape in the forming stage without affecting product quality.

Extruder Design and Operation The functions of an extruder are to melt resin and

then to homogenize the melt and pump it through a die. At the same time, it is necessary to minimize the degradation of the resin, and this may require the removal of heat by circulation of a coolant. At steady state, the actual energy flowing into the extruder, P A , must be balanced by the absorption of energy by the resin plus all losses. This can be expressed as shown in E 9 3.

PA = w + O H = ?h IT’ C,dT

where: P A W

QH = power used for heating of barrel 7iz = mass flow rate of resin TR = resin inlet temperature To = desired melt outlet temperature C, = heat capacity of resin u = specific volume of melt h P = pressure at die entrance Q L = rate of heat loss from barrel by natural convec-

tion Oc = rate of heat removed from barrel by forced-

convection cooling Pa,, = power consumed by water pumps, compres-

sors, blowers, and control systems PL = losses in main drive motor and hydraulic sys-

tem. (This energy flow appears as heat losses from motors and hydraulic devices.)

The first two terms on the right-hand side of E q 3 are the ones identified by McKelvey and Bernhardt (9) as the minimum power required to accomplish melting and pumping. Kruder and Nunn (7) suggest that typical values of the terms in E9 3 , expressed as percentages of P A , are shown in Table 1 . We note that the efficiency of this “typical” extruder, using the definition given by E q 2 , is 61 percent. It is clear that reductions in heat outflows and mechanical losses could result in substantial

TR

+ ?hvAP + Q L + + Pa,, + PL (3)

= total actual power input =power used by motors (extruder drive,

pumps, fans, blowers) and controls

Table 1. How Energy is Used in an Extruder (7).

Melt polymer and heat it to desired outlet temperature 56%

Heat losses 8% Generate desired outlet pressure 5%

Forced-convection cooling 9%

drive system 19%

Auxiliary power 3% Mechanical losses in extruder

improvements in this efficiency. We will now consider the potential for savings associated with each of the terms listed above.

The first term, the energy required for melting, is one that the processor has little control over, although the use of new resins designed to have a lower value of To (1-3) will reduce its magnitude. It is useful to note, in addition, that the efficiency will always be adversely af- fected when the melt is heated above the required melt temperature, To.

With regard to the pumping term, it is clearly disad- vantageous, from an energy point of view, to produce an exit pressure that is higher than needed for die flow. Rice (16) suggests that the combination of an extruder for melting and homogenizing, together with a gear pump for pumping, leads to improved energy efficiency for the entire operation, because each compo- nent can be designed to do its specialized task more ef- fectively than when plastication and pumping are done in a single device. The use of gear pumps with extruders to improve energy efficiency has also been discussed by McKelvey and Rice (17) and by Schuler (18).

With regard to heat outflows, QL and Qc, one obviously wants these to be as small as possible for maximum efficiency. Heat losses could be reduced by insulation, but cooling is generally necessary to prevent overheating and thus degradation of the melt. This implies that more energy is flowing into the resin than is necessary to raise its temperature to To, and most, if not all, of this excess energy is transferred to cooling water and room air in the form of heat.

A plasticating extruder serves three essential functions; it melts the resin, homogenizes the melt, and heats it to the desired processing temperature. Ideally, homogenization would be completed just as the temperature reached To, the required outlet temperature, so that no excess energy would have to be removed as heat. Twin screw extruders, for example, are efficient in this sense and usually require little cooling.

In general, for a given screw of traditional design, it appears that slow speeds provide adequate homogenization with a minimum of excess viscous dissipation. Moreover, the results of Chung, et al(19), suggest that, even in the melting section, the maximum energy efficiency is obtained when the extruder speed is rather low. The actual trend over the past 10 years has been to use higher screw speeds in order to increase production rates and thus minimize capital costs. Recently, however, several manufacturers have introduced new screws designed to give improved efficiency at moder- ately high speed.

In the design of high-efficiency extruders, it may be useful to keep in mind that extensional flows are much more effective for mixing than simple shear. Erwin (20) has cited a simple example in which the energy required to accomplish a given level of mixing is many times greater for simple shear than for simple extension.

In the light of the above discussion of the efficiency with which the polymer is homogenized in an extruder, something should be said about the way in which energy is introduced into the polymer (i.e., drive power vs. di-


Energy Conservation i n

rect heating). Heaters are used mainly for rapid heating at start-up and to facilitate melting for high-speed operation. Obviously, it is advantageous to accomplish as much homogenization as possible for a given amount of power consumption. This implies that it is generally bet- ter to supply energy to the polymer as drive power (W) rather than heater power (OH). McKehey (21) pointed out many years ago the advantages of adiabatic operation. Shearing serves both to heat and to mix, while direct heating serves only to add thermal energy. In most cases, the net effect of the direct heating is to increase the amount of cooling required in the metering section. Therefore, from an energy efficiency point of view, it is desirable to minimize direct heating. Of course, there are situations in which the melt viscosity is low and the desired outlet temperature is high (for example, in extrusion coating), and in such a case, barrel heating is unavoidable.

While extruder developments in North America have been evolutionary in nature, in Europe, where energy costs have been a much bigger problem, there has been a marked trend toward the use of a continuous-solids pump in place of a conventional extruder. Menges (22) has reported that such a machine has twice the energy efficiency of a traditional extruder.

To close this section on extruder design and operation, we consider the final item in Table 1 , mechanical losses in the drive system. We note that, typically, 19 percent of the power flowing to an extruder operation is dissipated in the drive system before it can have any im- pact on the energy balance inside the extruder. Kruder and Nunn (23) suggest that this 19 percent can be further broken down as follows: 15 percent in the DC drive motor, 4 percent in the gear box, and 1 percent in the SCR rectifier. They note that DC drive efficiency falls off as either speed or load decreases below rated values and suggest that this fall-off is especially rapid in the case of decreasing speed. Thus, it is clearly advantageous to operate the drive system as closely as possible to design load and speed. The availability of a wide range of sizes permits improved matching of machine capacity to production rate, and this will reduce the con- tribution of the drive system to overall inefficiency.

Hall (24) quotes F. R. Nissel of Welex as suggesting that: belt drives increase motor power consumption by 5 to 10 percent; worm gear reducers use 15-25 percent more energy than herringbone or helical gear reducers; and hydraulic drives use 25-35 percent more energy than d.c. direct drives.

Injection Molding Power is used in the injection-molding process in the

Resin dryer Extruder drive Barrel heaters Injection unit Mold clamps Mold heaters Chiller for cooling water Scrap grinder

components listed below:

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Plastics Processing: A Review

Resin drying has already been mentioned, and heating, cooling, and scrap handling are covered in a later section. We will focus attention here on the remaining operations.

Nunn and Ackerman (10) classify energy consumption in injection molding under two major categories: shot preparation and forming. They suggest the use of an efficiency based on a minimum theoretical energy input per shot, defined as follows:

T2

Ti 0 (4) Emin = M C,dT + lti p6dt

where: M = mass of shot Ti, T2 = screw inlet and outlet temperatures P = injection pressure (function of time) 6 = volumetric injection rate ti = injection time Based on previously published data (24, 25) they esti-

mate that typical efficiencies of machines now in use range from 12 to 22 percent, where the efficiency is defined as:

q~ = E m i J E A (5) Much of the inefficiency of injection molding is re-

lated to its being an intermittent rather than continuous operation. For this reason, motor selection is especially crucial.

Olmsted (25) has emphasized the importance of selecting the correct motor. He points out that the load varies significantly over the course of a molding cycle and that selecting a motor on the basis of the peak load generally leads to substantial waste. This is because an electric motor operating with no load still draws 40 to 50 percent of its rated power. Olmsted suggests the use of the root mean square cycle power requirement, rather than the peak load, to size motors. This is possible because motors can often operate at 200 percent torque overload over 25 percent of the cycle without damage.

Lafreniere (26), and Schott and Derby (14), have also recommended the selection of motors based on the RMS cycle power. The latter authors also compared several schemes for supplying mechanical power for both plastication-injection and for clamping:

One hydraulic motor for plastication-injection; second hydraulic motor for clamping.

A single hydraulic motor for both operations. Hydraulic motor for plastication-injection; me-

chanical clamping system. They find that the third option gives the minimum

power consumption but note that there are certain applications where this may not be a feasible choice. When hydraulic clamping is required, they recommend the use of a single hydraulic motor, as in the second option listed above. The use of two pumps is less efficient, because only one is in use at any given instant, while the other is idling, and an idling pump can draw several hp. Schott and Derby also emphasize the importance of op- timizing the cycle settings.

As much as 50 percent of the power consumed by the hydraulic pump is dissipated in the pump and removed

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J. M. Dealy

as heat, so it’s clear why it is worthwhile to concentrate efforts on this stage of the process. First, it is desirable to match the size of the pump to the demand (25). Also attractive are hydraulic systems that develop pressure according to demand rather than maintain full volume and pressure at all times (24, 27) and variable displace- ment pumps that provide only the volume of oil needed in high-pressure, low-volume stages, such as clamp holding (24, 28).

One convenient way of providing for variable hydraulic flow and pressure is by the use of a chip-based microprocessor (29). Such a microprocessor can be used as the basis of a process control for the entire injection- molding operation that not only provides for “digital hy- draulics” but makes rapid adjustments to compensate for random variations in operating conditions.

As a guide in the development of more efficient injection-molding machines, the major manufacturers have carried out extensive studies of how various design features affect energy consumption. For example, John- son (8) has reported on the tests carried out by Husky Injection-Molding Systems. He uses a “relative energy efficiency” to compare different machine designs and operating modes. This quantity is defined as the kWh of electricity consumed by components two through six in the above list divided by the polymer throughput in Ib.

A general result of the Husky tests was that the relative energy efficiency of the motors, taken as a unit, decreases with throughput, while that of the heaters varies little with throughput. This reflects the fact that the motors consume significant power even when idling so that shorter cycle times generalky improve the energy efficiency. In another series of tests, different clamping mechanisms were compared, and it was concluded that mechanical toggle clamps are significantly more efficient than hydraulic clamps. It was suggested that this results from the pressure losses in the pumps, lines, and valves that are essential elements of a hydraulic circuit. In addition, toggles are faster in operation so that they can contribute to a reduction in the cycle time. It is pointed out, however, that toggles are not suitable for all applications.

Other conclusions of the Husky studies were as follows:

D.C. extruder drives are more efficient than hydraulic drives, especially in smaller machines.

The use of stack (multiple) mold cavities improves efficiency markedly by allowing more throughput per cycle.

Hot runners are more efficient than cold runners because of shorter cycle time and reduced scrap recycle.

The “connected load” (kW) is not a useful measure of energy efficiency.

Because of “load factor” pricing of electricity by utilities, the savings rsulting from a reduction in kWh/lb are reduced significantly, and this reduces the incentive for the investment in energy-efficient equipment.

Based on its own studies, the Cincinnati Milacron Company has disputed the above finding regarding the advantage of toggle clamps except when the mold-cooling cycle is long (27). In any event, this company be-

lieves that the emphasis should be placed on the plastication-injection unit and its associated heaters, as these consume 80 percent of the electricity used in injection molding. Among the measures suggested to reduce the power consumed in this operation are the use of insulated heater bands and faster cycle times. Olmsted (25) has also discussed the advantages of insulated heater bands.

The question of heat loss from the injection unit has also been raised by Hall (24), who recommends insulation. However, Nunn and Ackerman (10) warn that, un- less the screw has been designed especially for adiabatic operation, the simple addition of insulation can lead to overheating in the feed zone and to irregular flow.

Catic (30) has analyzed heat transfer from the mold and has shown how, depending on the temperature of the cooling medium, it can be quite advantageous to in- sulate the mold.

Blow Molding In blow molding, the clamping pressure is much

lower than in injection molding, so clamping does not involve a great deal of energy consumption. A number of machine manufacturers have recently announced new designs that are said to be more energy efficient (4, 31).

In injection blow molding, a preform is injection molded and transferred to a second station for blowing. If the preform is allowed to cool too much before reach- ing the blowing stage, it must be reheated, and this provides opportunities for saving energy. A new blow- molding process that eliminates reheating is the “Dis- placement Blow Molding’ process developed by Saum Systems of Waltham, Massachusetts (32).

Thermoforming As in the case of injection blow molding, thermoform-

ing normally involves the reheating of a previously pro- duced object, in this case a sheet, above the softening temperature. One way of reducing energy consumption is to eliminate the reheat process, as in the Thermoline TL-1601 thermoforming system made by Brown Ma- chine, a Leesona Company. This machine produces finished forms, starting from resin in the form of pellets.

Two new thermoforming processes that involve significant reductions in energy consumption are the “scrapless forming process” developed by Dow Chem- ical Company and the “solid-phase forming process” of Shell Chemical Company (11). Both these processes use low forming temperatures and are able to produce thin- ner parts while maintaining good strength so that less resin is used (and heated). In addition, in the case of the Dow process, the scrap is eliminated, leading to additional savings.

Waste Heat Recovery Virtually all the energy that flows into a polymer pro-

cessing operation must be removed as heat. Until recently, this heat was most often transferred to the outside air by means of cooling water loops, cooling towers,


Energy Conservation in Plastics Processing: A Review

and air blowers. However, there is now a growing recog- nition that this heat can be used to reduce the amount of oil or gas needed for building services (33).

In an injection-molding plant, the principal areas of heat removal are the hydraulic pump and the mold. This heat is generally removed by cooling water at 80 to 95°F. Although municipal or river water can be used for this purpose, most plants use a recirculating water system with heat removal by means of a cooling tower, usually located on the roof of the building. There is now a strong incentive to use a water-to-air fan coil heat exchanger to transfer this heat to the building air for winter space heating.

Cooling water for the mold needs to be in the range of 40 to 55”F, so a refrigeration machine is needed (34). Heat at elevated temperatures must be removed from the refrigerant condenser, and chiller systems are now available that allow this heat to be used for heating building air in winter months. In the summer, a vent system directs the hot air outside and aids building ven- tilation at the same time (35).

A major problem in the use of waste heat is that it is normally available at temperatures much too low for direct use in the process itself. Schuler (18) has proposed the use of a heat pump to upgrade this heat so that it can be used for resin preheating.

Scrap Handling

Most polymer processes involve a finishing or trim- ming step that produces plastic scrap. This scrap may be discarded, incinerated, recycled for use in a lower grade product, or recycled for use in the same product. Clearly, these options are listed in order of increasing economic desirability. From an energy point of view, scrap always involves the waste of energy, since this material requires just as much processing energy as the finished product. Thus, it is always desirable to minimize scrap (36, 37).

However, some scrap is inevitable in most processes, and it is of interest to look at the energy needed for its re-use. Scrap recycle systems often involve multistep processes and consume significant energy. Several recently announced systems are said to use less energy than previous processes (38). This is usually accom- plished by reducing the number of steps involved (39).

SECOND-LAW ANALYSIS OF ENERGY UTILIZATION Basic Concepts

The first law of thermodynamics tells us that energy can be neither created nor destroyed so that, in any process, total energy is conserved. When we speak of the ‘‘use’’ or “consumption” of energy, we are actually re- ferring to a transformation of energy from one form to another, less valuable form (i.e., a form in which it is less “available” for useful purposes).

There are three forms in which energy may flow into or out of a system. One of these is heat, which flows as a result of a difference in temperatures between the system and its surroundings. For example, in space heating, energy flows from the hot radiator into the warm

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room. Then, since the room is imperfectly insulated, this heat flows to the outside, where it comes to rest in the cold outside air. We know from the law of conservation of energy that, at steady state, the rate at which heat flows into the room from the radiator is exactly equal to the rate at which it leaks through the walls and windows, SO that no energy disappears. However, it is clear that the usefulness of this heat decreases each time that its temperature decreases, until it is virtually worthless when it reaches the ambient outdoor temperature.

A second way in which energy can flow is as work. Work can be either mechanical, in the form of a rotating shaft or a moving piston rod, for example, or electrical. Work is not subject to the same variation of its usefulness as heat, and all forms of work are equally valuable. Furthermore, work can be converted into heat at any temperature. Thus, work is a more valuable form of energy than heat and, as has been explained by Dealy and Weber (40), it is convenient to adopt work as the basic form of energy for analysis purposes, with other forms being converted, for calculational purposes, to their “work equivalent.” For example, if an amount of heat, Q, is available at a temperature, T, where the environ- mental temperature is To, the work equivalent of this heat is given by:

T - To W . E . H . = 0

The temperatures must be in absolute units. We see that, as expected, the work equivalent, or rel-

ative usefulness, of the heat increases with temperature and becomes zero when its temperature is equal to the ambient temperature.

A third way in which energy can flow is in association with the flow of mass. For example, the natural gas that flows into a furnace carries with it a large amount of chemical bond energy, which is converted to heat by the combustion process. In fact, most changes in the phys- ical or chemical state of matter involve changes in the amount of energy carried with that matter, and the energy gain or loss must occur by means of heat or work so that total energy is conserved. It has been shown (for example, by Dealy and Weber (40)) that such changes of state of matter have associated with them a change in work equivalent, which in this case is called “exergy,” E ,

and is defined as follows:

A€ = AH - T0AS

Finally, the laws of thermodynamics can be used to show that the rate at which work equivalent flows out of a steady-state system is always less than the rate of in- flow, with the two rates approaching equality only in the case of an ideal, reversible process that can never be achieved in practice. This concept forms the basis for a universal definition of the efficiency of energy utilization of any process, as will be demonstrated in the fol- lowing examples.

Example-Pipe Extrusion

An extruder for manufacturing plastic pipe is equipped with an eight-horsepower motor and barrel- heating elements having a total electrical load of 17.2

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kW. The tubular molten extrudate is solidified and cooled to room temperature in a water trough. Cooling water comes from the water main at 45°F and is heated to 120°F in the trough. In winter, the hot water passes through an air heater, where it is cooled to 80°F with the warmed air being used for building space heating. The water flow rate is 947 lb/min. Some heat leaks directly into the room air from the hot surface of the extruder barrel and the water trough at a rate of 7890 Btu/h. The plastic pipe leaving the trough has the same temperature and composition as the resin fed to the extruder. The average winter outside temperature is 18”F, and the building temperature is 68°F.

We wish to analyze this process with regard to energy utilization, and to do this, we need to evaluate the flows of both energy and work equivalent. We will use units of kW for all quantities and will consider first the case of winter operation. The mechanical work flowing into the extruder from the motor is completely available, and we need only convert horsepower to kW to obtain the work equivalent for our analysis.

8 hp = 5.968 kW

The electrical work flowing into the barrel heaters is also 100 percent available, so that its work equivalent is 17.13 kW.

The heat leaking directly to the room has a work equivalent that depends on the room and outside temperatures.

Heat loss rate = 7890 Btu/h = 2.31 kW

h

Work equivalent = 2.31 (g;:;) = 0.219 kW

We note that the work equivalent of this heat is less than 10 percent of its energy content. The amount of heat transferred to the air from the cooling water is:

Q = hC, AT = 947(1.0) (120 - 80) = 3.79 . lo4 Btu/h = 11.1 kW

The associated rate of flow of work equivalent is:

11.1 ( 68 - 18) = 1.05 kW 68 + 460

The net amount of heat carried away by the water, after heating the air, is:

Finally, we take note of the plastics material flows. The energy used in this process is used exclusively to melt and homogenize the resin, and there is no net change in its energy content, since the pipe leaves the trough with the same temperature and composition as the resin. Thus, there is no net change in the work equivalent of the plastics material.

The results of the analysis are tabulated in Table 2. We note that the total energy flow in is balanced by the total energy flow out, as is guaranteed by the first law of thermodynamics.

Table 2. Analysis of Pipe Extrusion.

Energy Work Eq. Energy Work Eq. in in out out

~~ ~~

Motor work 5.97 5.97 Heater power 17.15 17.15 Direct heat loss 2.31 0.219 Air heater 11.1 1.05 Water heating 9.71 not

used

Total 23.12 23.12 23.12 1.269

Now we can calculate efficiencies for this process. First, we will calculate a traditional “first-law” efficiency that takes no account of the concept of work equivalent. Thus, we take the total useful energy flow out and divide it by the total energy flow in. Since the water leaving the heater goes to the sewer, its energy is not used, and we cannot take credit for it as useful energy. Thus, this efficiency is:

2.31 + 11.1 = 0.58 or 58% ’* = 23.12

Thus, 58 percent of the energy flowing into the process is recovered for secondary use in space heating.

However, a more meaningful efficiency is one that takes account of the difference between energy and work equivalent, or exergy. In the present case, this “second-law” efficiency can be defined as the total useful flow of work equivalent out of the process divided by that which entered.

o‘219 + + 0.055 or 5.5% “ = 23.12 We note that, from this point of view, our use of energy is much less efficient than we would have been led to believe from the first-law efficiency.

Regarding summer operation, since there is now no opportunity to use waste heat for space heating, the efficiency, whether based on the first or second laws, is zero.

One of the principal benefits of this type of analysis is that it directs our attention to the possibilities ofincreas- ing the efficiency. For example, a substantial improve- ment could be made by using a heat pump to boost the temperature (i.e., the work equivalent) of the heat removed in the trough for use in barrel heating.

Which Efficiency to Use A number of definitions of efficiency have been

given in this paper, ranging from the overall power economy, which is simply the power consumed divided by the throughput, to the second-law efficiency used above in the analysis of a pipe extrusion operation. These various quantities are not directly related, and each provides a different basis for the evaluation of a machine or process.

Such quantities as the energy per Ib of output are useful for comparing several machines performing the same task, while the efficiency defined in E 9 2 is more useful in the design of process machines to accomplish a cer-

534 POLYMER ENGINEERING AND SCIENCE, JUNE, 1982, Yo/. 22, No. 9

Energy Conservation in Plustics Processing: A ReGiezc;

tain generally-defined objective. Finally, the thermo- dynamic efficiencies defined in this section are useful in directing attention toward energy recovery possibilities and completely new forming methods. The choice of efficiency depends on the objectives of the analysis.

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Energy conservation in plastics processing: A review