Heating and cooling

A good heating and air conditioning system provides more than creature comfort for those working in a laboratory. Conditions of reasonably constant temperature and humidity are important for the proper performance of many laboratory operations. 

Conventional hot-air furnaces, similar to those found in homes, are often used in laboratories. A university campus or an industrial plant may have steam available, which can be conveniently put to use for heating. 

Air conditioners are frequently combined with heaters. A convenient place must be found for the compressor, generally outdoors. Heat pumps have been used a great deal in recent years with satisfactory results. In very dry climates, laboratories often install evaporative coolers because of their reasonable cost of installation and operation. Their one drawback, sometimes serious, is that the inside air tends to become uncomfortably humid. 

Estimating the amount of heat, cooling, or air flow required is a job for a skilled professional. Such an expert should also be called in when it seems desirable to extend a system to areas other than those for which it was originally designed. 

A safe laboratory is the result of both good design and proper work rules. The laboratory operator, as the only person fully aware of the work to be performed, must be involved in all safety planning. He will be the one who can supply the safety experts with the information they need. 

In the peak-detecting mode, the heating procedme is more delicate than in the steady-state mode, because small changes in temperature may drastically modify the state of reaction achieved. 

Seawater samples are often collected at temperatures below the laboratory temperature. Consequently, small gas bubbles tend to separate out while warming up. These bubbles enter the analytical manifold and disturb the spectrophotometric detection. If the air segments are removed in front of the cuvette, the random micro-bubbles will also be removed. When a spectrophotometer is used which allows the air segments to pass through the cuvette, a bubble separator has to be inserted at the very beginning of the manifold just after the sample inlet. The bubble separator removes random and micro-bubbles before the defined air segmentation. 

A bubble separator fitted to a cuvette inlet is shown in Fig. 10-22. 

The use of heating and cooling phases are optional and not often used in Monte Carlo simulations. However, they may be useful for some applications. 

HyperChem offers four molecular mechanics force fields MM+, AMBER, BIO+, and OPLS (see References on page 106). To run a molecular mechanics calculation, you must first choose a force field. The following sections discuss considerations in choosing a force field. 

The units are also designed to effect a rapid (or crash ) cooling of the injection unit in the event of an un-programmed interruption in the operation of the cycle. The rapid cooling ensures that the rubber does not begin to cure in the injection unit before it can be safely removed or replaced with a purging compound. 

Cold runner systems have a similar temperature control requirement to the plasticising and injection unit, since the cold runner acts as an extension to the nozzle, controlling the rubber temperature to preclude any build up of scorched material while it awaits injection into the mould. 

The heating method should provide the required metal temperature, metal temperature uniformity, and temperature control, and may include an enclosed furnace, local flame heating, electric resistance. 

As an illustration of irreversible heat transfer, consider a system that is a soUd metal sphere. This spherical body is immersed in a well-stirred water bath whose temperature we can control. The bath and the metal sphere are initially equilibrated at temperature T = 300.0 K, and we wish to raise the temperature of the sphere by one kelvin to a final uniform temperature T2 = 301.OK. 

One way to do this is to rapidly increase the external bath temperature to 301.0 K and keep it at that temperature. The temperature difference across the surface of the immersed sphere then causes a spontaneous flow of heat through the system boundary into the sphere. It takes time for all parts of the sphere to reach the higher temperature, so a temporary internal temperatme gradient is established. Thermal energy flows spontaneously from the higher temperature at the boundary to the lower temperature in the interior. Eventually the temperature in the sphere becomes uniform and equal to the bath temperature of 301.0 K. 

The reverse of the reversible heating process is a reversible cooling process in which the temperature is again uniform in each state. The sequence of states of this reverse process is 

In any real heating process occurring at a finite rate, the sphere s temperature could not be perfectly uniform in intermediate states. If we raise the bath temperature very slowly, however, the temperature in all parts of the sphere will be very close to that of the bath. At any point in this very slow heating process, it would then take only a small decrease in the bath temperature to start a cooling process that is, the practically-reversible heating process would be reversed. 

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The shaded areas in Fig. 6.24, known as pockets, represent areas of additional process-to-process heat transfer. Remember that the profile of the grand composite curve represents residual heating and cooling demands after recovering heat within the shifted temperature intervals in the problem table algorithm. In these pockets in Fig. 6.24, a local surplus of heat in the process is used at temperature differences in excess of AT ,in to satisfy a local deficit. ... 

The dominant heating and cooling duties associated with a distillation column are the reboiler and condenser duties. In general, however, there will be other duties associated with heating and cooling of feed and product streams. These sensible heat duties usually will be small in comparison with the latent heat changes in reboilers and condensers. 

Figure B.l shows a pair of composite curves divided into vertical enthalpy intervals. Also shown in Fig. B.l is a heat exchanger network for one of the enthalpy intervals which will satisfy all the heating and cooling requirements. The network shown in Fig. B.l for the enthalpy interval is in grid diagram form. The network arrangement in Fig. B.l has been placed such that each match experiences the ATlm of the interval. The network also uses the minimum number of matches (S - 1). Such a network can be developed for any interval, providing each match within the interval (1) satisfies completely the enthalpy change of a strearh in the interval and (2) achieves the same ratio of CP values as exists between the composite curves (by stream splitting if necessary).

To find the boundary between heating and cooling we set Tr(/ ) = 0- Figure Al.6.33 shows isocontours of Tr( P ) as a fimction of the parameters P22 IP12I region corresponds to 0 that is,... 

When using the heating and cooling features of IlyperChem ii should he remembered that ii is accomplished through rescalingof the velocities, so if the velocities are zero no tcmperaiurc can occur. This happens, for example, if you start with an exactly opti-mi/ed stni ctii re an d heat from a startm g lem perature Tj of/.ero.or use the restart option when velocities are all zero. 

Use Equipment Only When deeded. Start morning warm-up no earHer than necessary and do not use outside air for ventilation until the building is occupied. Use minimum amounts of outdoor air according to reference 8. Supply heat at night only to maintain a temperature above 13°C. Supply Heating and Cooling from the Most Efficient Source. 

Sequence Heating and Cooling. Do not supply both at the same time. The 2oning and system selection should eliminate or at least minimi2e simultaneous heating and cooling. 

Heat Transfer. One of the reasons fluidized beds have wide appHcation is the excellent heat-transfer characteristics. Particles entering a fluidized bed rapidly reach the bed temperature, and particles within the bed are isothermal in almost all commercial situations. Gas entering the bed reaches the bed temperature quickly. In addition, heat transfer to surfaces for heating and cooling is excellent. 

If food can be heated quickly to a temperature of I3I°C a lethaUty equivalent to 6 min at I2I°C can be accumulated in 36 s. This rapid heating and cooling of hquid foods, such as milk, can be performed in a heat exchanger and is known as high temperature—short time (HTST) processing. HTST processing can yield heat-preserved foods of superior quahty because heat-induced flavor, color, and nutrient losses are minimized. 

Neopentyl glycol can be used for thermal energy storage by virtue of its soHd-phase transition, which occurs at 39—41°C, a temperate range useful for solar heating and cooling (28—31). 

Manufacture, Evaluation, and Safety. The manufacture of shampoos is a relatively simple operation requiring a suitable stainless steel kettie with provisions for heating and cooling and equipped with appropriately sized mixers. Although shampoos are easily handled during preparation, precautions should be taken to not aerate the product. Cream shampoos are particulady sensitive to aeration and require more special care in their manufacture. 

Marlotherm Heat-Transfer Fluids. Two heat-transfer fluids are manufactured by HbIs America Madotherm S is a mixture of isomeric diben2ylben2enes intended for Hquid-phase systems, and Marlotherm L is a mixture of ben2yl toluenes that are suitable for both Hquid- and vapor-phase appHcations. Marlotherm L can be pumped readily at temperatures as low as —50° C and can be used in vapor-phase systems at temperatures from 290—350°C. The low temperature characteristics of Marlotherm enable it to be used in processes involving both heating and cooling. 

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