Heat transfer need to be thought about in situations when reactions are catalyzed by immobilized enzymes v high reaction rates, highly reduced substrates are oxidized, thermal sterilization and subsequent cooling that the bioreactor room required, and much more generally as soon as temperature regulate is required.

You are watching: The movement of matter due to differences in density

From: comprehensive Biotechnology (Third Edition), 2011

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Heat carry occurs through one or much more of the complying with three mechanisms.

(i)

Conduction. warm conduction wake up by move of vibrational energy in between molecules, or motion of totally free electrons. Conduction is specifically important with metals and occurs there is no observable activity of matter.

(ii)

Convection. Convection requires movement on a macroscopic scale; that is therefore confined come gases and liquids. Natural convection occurs once temperature gradients in the device generate localised density differences which result in circulation currents. In forced convection, circulation currents are set in movement by an external agent such as a stirrer or pump and also are live independence of thickness gradients. Higher rates of heat transfer are possible with forced convection contrasted with organic convection.

(iii)

Radiation. power is radiated native all products in the type of waves; once this radiation is took in by matter it appears as heat. Due to the fact that radiation is important at much higher temperatures than those typically encountered in biological processing, it will certainly not be pointed out further.


The heat transfer scale-up strategy can be adopted when the extrusion process is restricted by warmth transfer, and also the preferred melt temperature might not it is in achieved. Thermal warm transfer in TSE is dependence on the degree of fill, barrel surface ar area, temperature gradient between product and also barrel, and also residence time. Straight scale-up for warmth transfer is based upon the heat transfer area, the is, a square law (Eq. 31.15). Based upon this strategy, the warmth transfer coefficients have to be constant among scale to preserve the exact same melt temperature. Similar to other scale-up assumptions, the level of fill and also screw diameter ratio need to be the same.


(31.15)Q2=Q1×(D2D1)2

For extruders with different screw diameter ratios, Eq. (31.15) should be modified based on the true worth of the inside barrel surface ar area.

A warmth transfer border is not typical in pharmaceutical applications, as product temperature is largely dependent top top the viscous dissipation produced by convert SME rather than barrel heating or cooling. In general, the very same barrel temperature profile can be employed initially throughout scaling. However, due to the circular geometry of the extruder within barrel surface, we have to realize that boosting the extruder size results in the decreased thermal warmth transfer per unit mass. Thus, as soon as scaling with this strategy, a big extruder could be activate at lower degree of fill as compared to the small-scale device due to increased melt temperature. Reduced fill at a constant screw speed also increases the average shear rate and residence time. Scaling based upon heat transport limitation is a complicated process; therefore, caution have to be taken when using this strategy. Added experiments and modeling could be compelled to yield ideal scaling results.


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G. Ding, ... K. Cheng, in substantial Biomedical Physics, 2014

10.15.3.3 Moxibustion warm Transfer in APs

Heat transfer into the tissue during moxibustion theatre a key role in the curative effect of moxibustion. The warmth transfer procedure during indirect moxibustion and also warm needle moxibustion at Zusanli (ST36) was numerically simulated making use of the finite aspect method. The results indicate that the moxibustion heat can reach deeply into biological tissue, such as the IOM between the tibia and fibula. The numerical simulation also showed that warm is very first absorbed and also then emitted by the tissue, which forms heat oscillation (Cheng et al., 2008).

The numerical simulation reflects that the warm from the warm needle moxibustion transfers right into a deeper component of the tissue 보다 indirect moxibustion. The simulation likewise proves that due to the fact that silver is highly conductive, the tissue temperature 7 mm listed below the skin during moxibustion is 320 K as soon as using silver- needles, while gold and also stainless‐steel needles only generate 300 and also 290 K, respectively. This may be why ancient Chinese acupuncturists used heat silver needle moxibustion in deficiency syndromes for the best curative result (Cheng et al., 2007).


For the exponent, c, a value of 0.14 is frequently found. The hatchet sets the mean bulk viscosity, η, in relation to the viscosity directly at the wall, ηw, whereby heating and also cooling have the right to be distinguished. Therefore, heating would support heat transfer by to reduce the viscosity in the sublayer and also vice versa. However, the impact is less pronounced during standard operation, other than for heavy steam sterilization, and also can be considered smaller 보다 ±5%. In chemistry engineering, wherein viscosity of polymer solutions have the right to be magnitudes higher compared come water, close-clearance or wall scraping impellers are offered under partial or completely laminar conditions, where most of the approaches displayed here don't use directly and also therefore, particular models should be used, depending strongly on the given procedure conditions and also fluid properties. Return some process fluids present in biotechnology present highly viscous, non-Newtonian behavior, e.g., some mycelial fungi cultures or throughout xanthan fermentation, the bulk of warm transfer tasks have the right to still be covered by the equations provided in this section. Together a basic rule, before using models indigenous the literature, the parameter ranges need to be evaluate upfront, because empiric models are just valid in ~ the arrays they were initially set up.


Heat transport by convection occurs together a result of the movement of liquid on a macroscopic scale, in the type of eddy and circulation currents. This convective movement deserve to take two forms:

1.

Natural convection: these currents arise from the heating procedure itself, since of the differences in density between the boil fluid and the surrounding cooler fluid, which reasons the previous to rise, and also the last to sink to take the place of the former.

2.

Forced convection: in this type of convection, the currents are developed by an exterior device, e.g., a circulation pump bring about turbulent circulation in a pipe.


When warm transfer wake up from a surface right into the human body of a fluid, herbal convection currents room weakest at the surface, which is extended by what is successfully a static film. Consequently, warm transfer throughout this film can only occur by conduction, and, as stated above, heat conductivity in fluids is low. Hence, the main resistance to heat transfer into fluid in a pipeline is this film surrounding to the pipeline wall. Boost in the velocity the the fluid moving through the pipe will minimize the thickness that this static film and give climb to an in its entirety increase in the warm transfer into the fluid.


In theory, the warm load transferred across this film is defined as in eqn (1). However, in practice, the is challenging to calculate the film thickness, X, and also so the adhering to relationship is used:


4Q=−hAT,

where h is the heat-transfer coefficient.

Thermal resistance is because of this the mutual of the heat-transfer coefficient, i.e., 1⧸h.

Forced convection is of more importance than herbal convection in commercial food processes and equipment, wherein the fluids are under turbulent flow conditions. It need to be noted, however, the evaporators deserve to be classified as either ‘natural circulation’ or ‘forced circulation’ systems. In the previous case, the eddy and circulation currents described above are greatly magnified by the currents led to by the rising bubbles generated in the cook processes.

Whenever possible, rationalization flow problems should be avoided or at the very least minimized, as the heat transfer coefficients of fluids (in convective warm transfer) are much greater than thermal conductivity components (in conductive warm transfer). With really viscous fluids (e.g., food pastes and slurries), turbulent circulation can just be created by a high-pressure drop throughout the heat-transfer an equipment (e.g., by the intake of a large quantity that pump energy).

In a tubular warm exchanger (e.g., an evaporator), where probably one liquid is flowing within a pipe and being heated (or cooled) by one more outside, the factor to consider must it is in of warmth transfer both inside and outside the tube. In the latter case, circulation can be one of two people lengthwise follow me the tube (in either direction) or at appropriate angles come the solitary tube or pipe bundle.


Also, those fluids passing follow me the size of the warm exchange tube (either within or outside) will suffer either an increase or diminish in temperature, obviously since of warm transfer. This way that in stimulate to appropriately quantify this warm transfer, it is first necessary to specify the difference in temperature in between the fluids, provided that it will not only vary at various points follow me the length of the tube, however will also vary relying on whether cocurrent or countercurrent circulation is gift employed. The is crucial to calculation an ‘average’ value for the temperature difference, and the aspect normally work is the logarithmic average temperature difference (LMTD):


4aLMTD={(T1−T3)−(T2−T4)loge(T1−T3)/(T2−T4)(cocurrentflow)(T2−T3)−(T1−T4)loge(T2−T3)/(T1−T4)(countercurrentflow)

where T is the temperature (see number 3 because that a an interpretation of subscript numbers)


*

M.J. Lewis, in physical Properties of Foods and Food processing Systems, 1996

10.1 INTRODUCTION

In plenty of heat transfer situations, one is interested in the time taken to heat, cool, freeze, cook or dry food. Options to these troubles may be discovered by considering unsteady-state warmth transfer equations which define how the temperature in ~ a resolved location changes with time.

In the first part the this chapter, unsteady-state heat transfer including only cautious heat alters will be considered, complied with by a conversation of latent warm changes, associated in freezing and also thawing time calculations. Dry operations, in i beg your pardon both heat and mass transfer processes occur, will be considered in section 13.9.


Joel M. Press, Deborah A. Bergfeld, in Clinical sporting activities Medicine, 2007

Techniques that application

Heat have the right to be transferred to tissue in 3 ways.

Conduction

Direct heat transfer native one surface ar to another due to direct contact. This is a type of superficial heat. Examples encompass hydrocollator packs, paraffin baths, electrical heating pads and hot water bottles.

Convection

Heat transfer due to the activity of wait or water across a body surface. This, too, is a type of superficial heat. Examples include hydrotherapy and also fluidotherapy.

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Conversion

Transfer of heat as result of a adjust in the form of energy. Instances of superficial warm conversion incorporate radiant energy such as that produced by infrared lamps. Deep heating, additionally known together diathermy, is due to conversion v the use of quick waves, electromagnetic microwaves and ultrasound.