Thermoelectric Handbook by Melcor

More catalogs by Melcor | Thermoelectric Handbook | 16 pages | 2008-06-10


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® structure and function since thermoelectric cooling systems are most often compared to conventional systems perhaps the best way to show the differences in the two refrigeration methods is to describe the systems themselves a conventional cooling system contains three fundamental parts the evaporator compressor and condenser the evaporator or cold section is the part where the pressurized refrigerant is allowed to expand boil and evaporate during this change of state from liquid to gas energy heat is absorbed the compressor acts as the refrigerant pump and recompresses the gas to a liquid the condenser expels the heat absorbed at the evaporator plus the heat produced during compression into the environment or ambient a thermoelectric has analogous parts at the cold junction energy heat is absorbed by electrons as they pass from a low energy level in the p-type semiconductor element to a higher energy level in the n-type semiconductor element the power supply provides the energy to

® parameters required for device selection there are certain minimum specifications that everyone must answer before the selection of a t.e device can begin specifically there are three parameters that are required two of these parameters are the temperatures that define the gradient across the t.e device the third parameter is the total amount of heat that must be pumped by the device the gradient across the t.e device actual t is not the same as the apparent t system t the difference between these two ts is often ignored which results in an under-designed system the magnitude of the difference in ts is largely dependent on the type of heat exchangers that are utilized on either the hot or cold sides of the system unfortunately there are no hard rules that will accurately define these differences typical allowances for the hot side of a system are 1 finned forced air 10 to 15°c 2 free convection 20 to 40°c 3 liquid exchangers 2 to 5°c above liquid temperature since the heat

® design/selection checklist the information requested below is vital to the design/selection of a thermoelectric device to achieve your desired performance please attempt to define as many of your application s existing conditions and limiting factors as possible please indicate units on all parameters i ambient environment temperature k air k vacuum k other ii cold spot temperature size insulated type thickness desired interface k plate k fins k fluid flow parameters k other iii heat sink k finned free convection k finned forced convection k liquid cooled maximum heat sink temp -or-heat sink rating °c/w iv heat load at cold spot if applicable above should include active i2r passive radiation convection insulation losses conduction losses e.g leads transient load mass time v restrictions on power available indicate most important k current k voltage k power k no restrictions vi restrictions on size vii to ensure the most effective

® thermoelectric multistage cascade devices a multistage thermoelectric device should be used only where a single stage device does not fill the need figure 4 depicts t vs c.o.p max vs number of stages c.o.p is defined as the amount of heat absorbed in thermal watts of heat pumped at the cold side of the device divided by the input power in electrical watts this figure should help identify when to consider cascades since it portrays the effective t range of each cascade a two-stage cascade should be thought of somewhere between a t of 40°c tc -5°c where the c.o.p bars of the one and two-stage devices begin to diverge and a t of 65°c tc -30°c where a single stage device reaches its maximum t and also heat pumping shutoff qc 0 similar decisions must be made as to the number of stages to be considered at larger ts the two important factors again are t and c.o.p figure 4 t vs c.o.p max as a function of of stages melcor offers a line of standard cascades though there are no

® assembly tips the techniques used in the assembly of a thermoelectric t.e system can be as important as the selection of the proper device it is imperative to keep in mind the purpose of the assembly ­ namely to move heat generally a t.e device in the cooling mode moves heat from an object to ambient all of the mechanical interfaces between the objects to be cooled and ambient are also thermal interfaces similarly all thermal interfaces tend to inhibit the flow of heat or add thermal resistance again when considering assembly techniques every reasonable effort should be made to minimize thermal resistance mechanical tolerances for heat exchanger surfaces should not exceed 0.001 in/in with a maximum of 0.003 total indicated reading if it is necessary to use more than one module between common plates then the height variation between modules should not exceed 0.001 request tolerance lapped modules when ordering most t.e assemblies utilize one or more thermal grease interfaces

® procedure for assembling lapped modules to heat exchangers important when two or more thermoelectric devices are mounted between a common plate the thermoelectric devices thicknesses should vary no more than 0.0015-in contact our engineering department for more information on close tolerance lapped thermoelectric devices step 1 prepare cold plate and heat sink surfaces as follows a grind or lap flat within 0.001 in module area b locate bolt holes as close as possible to opposite edges of module 1/8 clearance recommended 1/2 maximum in the same plane line as the heat exchanger fins this orientation utilizes the additional structural strength of the fins to prevent bowing drill clearance holes on one surface and drill and tap opposite surface accordingly see sketch in assembly tips if a spacer block is used to increase distance between surfaces performance is greater if the spacer block is on cold side of system c remove all burrs chips and foreign matter in thermoelectric

® device performance formulae heat pumped at cold surface voltage maximum current optimum current optimum cop calculated at iopt maximum t with q 0 qc 2n [a i tc i2 p 2 g k t g v 2n i p g a t imax k g a 1 2 z th 1/2 1 iopt [k t g 1 1 z tave1/2 a tave copopt tave t 1 z tave1/2 1 1 z tave1/2 1 1/2 tmax th 1 2 z th1/2 1 z miscellaneous expressions notation th tc t tave g n i cop zsrk definition hot side temperature kelvin cold side temperature kelvin th tc kelvin 1/2 th tc kelvin area length of t.e element cm number of thermocouples current amps coefficient of performance qc iv seebeck coefficient volts kelvin resistivity cm thermal conductivity watt cm kelvin figure of merit 2 kelvin-1 device seebeck voltage 2 n volts kelvin device electrical resistance 2 n g ohms device thermal conductance 2 n g watt kelvin 10 tel 609-393-4178 · fax 609-393-9461 · web · email for a list of global sales offices

® heat transfer formulae note due to the relatively complex nature of heat transfer results gained from application of these formulae while useful must be treated as approximations only design safety margins should be considered before final selection of any device 1 heat gained or lost through the walls of an insulated container q a xtxk x where q heat watts a external surface area of container m2 t temp difference inside vs outside of container kelvin k thermal conductivity of insulation watt meter kelvin x insulation thickness m 2 time required to change the temperature of an object t m x cp x t q where t time interval seconds m weight of the object kg cp specific heat of material j kg k t temperature change of object kelvin q heat added or removed watts note it should be remembered that thermoelectric devices do not add or remove heat at a constant rate when t is changing an approximation for average q is qave q tmax q tmin 2 3 heat transferred to or from a

® typical properties of materials 21°c material name air alumina ceramic-96 aluminum nitride ceramic aluminum argon gas bakelite beryllia ceramic-99 bismuth telluride brass bronze concrete constantan copper copper tungsten diamond ethylene glycol glass common glass wool gold graphite iron cast kovar lead molybdenum nickel nitrogen gas platinum plexiglass acrylic polyurethane foam rubber silicone undoped silver solder tin/lead stainless steel steel low carbon styrofoam teflon thermal grease tin titanium water 70°f wood oak wood pine zinc density kg/m3 1.2 3570 3300 2710 1.66 1280 2880 7530 8490 8150 2880 8390 8960 15650 3500 1116 2580 200 19320 2560 7210 8360 11210 10240 8910 1.14 21450 1410 29 960 2330 10500 9290 8010 7850 29-56 2200 2400 7310 4372 1000 610 510 7150 thermal conductivity w/m-k 0.026 35.3 170-230 204 0.016 0.23 230 1.5 111 64 1.09 22.5 386 180-200 2300 0.242 0.80 0.040 310 85 83 16.6 35 142 90 0.026 70.9 0.26 0.035 0.16 144 430 48 13.8 48 .029 0.35 0.87 64 20.7

® reliability mean time between failures mtbf thermoelectric devices are highly reliable due to their solid state construction although reliability is somewhat application dependent mtbfs calculated as a result of tests performed by various customers are on the order of 200,000 to 300,000 hours at room temperature elevated temperature 80°c mtbfs are conservatively reported to be on the order of 100,000 hours field experience by hundreds of customers representing more than 7,500,000 of our cp type modules and more than 800,000 optotectm type modules during the last ten years have resulted in a failure return of less than 0.1 more than 90 of all modules returned were found to be failures resulting from mechanical abuse or overheating on the part of the customer thus less than one failure per 10,000 modules used in systems could be suspect of product defect therefore the combination of proper handling and proper assembly techniques will yield an extremely reliable system historical

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