Typically, an appropriate temperature difference for heat exchange at moderate and high temperatures is 10C, however, in several processes which include refrigeration systems, heat exchange must be done as low as -90 C, -40 C, etc. What temperature difference is appropriate in such cases?
In many chemical process applications this difference for refrigeration is usually small (on the order of 2 - 3 deg C) maybe 5 max. The approach temperature is an economic consideration for capital and operating costs and is especially important for cooling at these levels since energy costs (power and fluid) and material costs (material of construction) for these applications can be high.
Pinch temperature difference isn't a function of being above or below ambient temperature. By choosing this parameter, the amount of heating surface is determined. In fact it has to be decided in thermoeconomic optimization. As far as refrigeration is more costly than heating, which also depends on the energy intensity (exergy), pinch temperature difference is smaller. this parameter is also dependent on local energy or fuel cost versus investment cost.
The temperature difference will be similar for the same rate of heat transfer, independent of where you are, anywhere above absolute Zero.
The heat flow rate is proportional to the temperature difference between the two mediums. The reason for imposing large temperature differences relates to the time in which one would like to cool / warm something,
We usually use refrigeration to preserve foodstuffs and we want this to happen quickly, (otherwise it spoils and we will get sick) therefore the large temperature difference.
@David Johnson
Time is only important when the process is considered transient which is important at start up period. Pinch temperature is a matter of cost.
Dear David, as Rasool pointed out "Time is only important when the process is considered transient". At steady state, the important parameter is the exchanging surface area. For a given heat rate, the smaller the temperature difference, the larger this parameter.
The temperature difference would depend on what you want to accomplish,
and how quickly you want to accomplish it. For a low difference in incoming
and outgoing temperatures, the residence time would be long, and the
length of the apparatus would be great ( long contact period ).
For a great difference in temperature, the residence time is short,
and the apparatus can be smaller ( shorter contact period ).
Or as in house refrigeration, you cycle the same air thru the system
continuously until it stabilizes such that heat flow in ( solar ) = heat flow
out via refrigeration.
One method you could apply would be similar to the cold temperatures
at 39,000 feet outside an airplane moving at 540 MPH where the outside
temperature is minus 57 C. In short , very low density atmosphere,
or a partial vacuum, superior insulation, removal of humidity, combined with
a heat exchanger.
At temperatures of -90 C to -40 C you will encounter the problems of the
brittle nature of materials.
Device (Heat Exchanger) where heat exchange takes place the temperature difference is not so wide generally in normal case means at atmospheric condition is 3C to 7C more temperature at given heat transfer rate cause irreversibility but below atmospheric condition at very low temperature this difference should not more and lie between 2C to 3C to prevent sudden change in fluid property. also too low temperature can attained in 2 or 3 successive processes by choosing appropriate refrigerant.
above problem is also analyzed by Entransy Extrimum Principle by keeping temperature difference minimum at a given heat transfer rate. and this difference should not be more than 5C.
For experimental work the range of measurable parameters are fixed based on the allowable errors.
The heat transfer is obtained as a product of mass flow rate, specific heat and the temperature difference. As specific heat is a property; mass flow rate and the temperature difference govern the heat transfer between the hot and cold fluids. Both mass flow rate and the temperature difference are measurable quantities. For a given quantity of heat to be transferred, which is a data, on increasing one of them other parameter will decrease. Traditionally, during the experimental work, the maximum error in the measurable parameters is expected to be in the order of 1 to 2 percentage only. Using the least count of the instruments, which are proposed to be used in the experiments, you can estimate the error and fix the measurable range of a parameter may be fixed suitably.
Regards,
Dr.S.Ravindran
Temperature difference is not the design rule, but temperature difference is process criteria. If process demands I don't think we follow such 10 degree delta. If we follow this, it will lead to very bulky equipment. Practically I found ammonia refrigeration cycle with more than 10 degree delta. Could you please mention the reference of this 10 degree delta. For my understanding. Thank you.
Temperature difference is required to transfer the heat from one fluid to another fluid. keeping surface area constant if we want increase heat transfer, then, we have to increase temperature difference and/ increase mass flow rate. i.e following the law mCpdt or area*LMTD.
So this is the basics of heat transfer, and it is followed at high temperature as well as low temperature.
temperature difference depend upon many factors.
1. the size of H.E
2. types of two fluids exchange
3. the range and limit of temperature difference finally
4. the mass flowrate and heat capacities of the fluids
Thank you all for adding to my answer. I like to keep things simple, so I ignore a lot of the finer details (and retain my sanity).
The most disheartening thing I've learned after graduate school is that most constants are variable e.g. the thermal conductivity of materials are not constant over all temperatures. To split hairs, the thermal conductivity of metals are reasonably constant at low temperatures, but decreases at increased temperatures. For liquids and gasses, the thermal conductivity increases with increased temperature.
To answer the original question, the higher the temperature difference, the better the heat transfer per unit area. So, one can use any reasonable temperature difference one may expect (say between 10 to 40 degrees C), but the H.E. gets larger for smaller temperature differences. Also, is the flow laminar or turbulent?The next problem is that the environmental conditions change e.g. between summer and winter and where in the world you are. The trick is to find something that will be economically feasible for the manufacturer and that will work under all conditions.
My approach would be to make a rough calculation, using constant thermal conductivity, simulate it in a multi-physics FE package e.g. ELMER (allowing for variable thermal conductivity) and then fine tune the design for manufacturing. Next up, construct/obtain one and verify the analytical and simulation results.
Most of the time it is good enough to just assume constant values for thermal conductivity, unless it is an academic exercise, in which case one would follow the analytical-simulation-verification method.
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