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Lecture 8: Incubator Design

Incubator = tempertaure controlled environment

Temperature Control:

temperature measurement and temperature reference section (Omega)

accuracy (sensors, control system)

gradient  (control throughout incubator, heat sources, ..)

cooling / heating rate (heat capacity, power)

Typical Tempertaures:

4°C: preservation without freezing of aquous samples

37°C: typical cell culture tempertaure for human / animal cells

20-25°C: ambient temperature range for variety of systems (plants, crystal growth, bacteria)

Thermal Calculations:

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 x DT x K) / (DX)

Where:

Q = Heat (Watts)
A = External surface area of container (m²)
DT = Temp. difference (inside vs. outside of container) (Kelvin)
K = Thermal conductivity of insulation (Watt / meter Kelvin)
DX = Insulation thickness (m)

2) Time required to change the temperature of an object:

t = (m x C_p x DT) / Q

Where:

t = Time interval (seconds)
m = Weight of the object (kg)
C_p= Specific heat of material (J / (kg K))
DT = 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 DT is changing. An approximation for average Q is:

Q _ave = (Q (DTmax) + Q (DTmin)) / 2

3) Heat transferred to or from a surface by convection:

Q = h x A x DT

Where:

Q = Heat (Watts)
h = Heat transfer coefficient (W / (m² K))

(1 to 30 = "Free" convection - gases, 10 to 100 = "Forced" convection - gases)

A = Exposed surface area (m²)
DT = Surface Temperature - Ambient (Kelvin)

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Conversions:

Thermal Conductivity	1 BTU / hr ft °F = 1.73 W / m K
	1 W / m K = 0.578 BTU / hr ft °F
Power (heat flow rate)	1 W = 3.412 BTU / hr
	1 BTU / hr = 0.293 W
Area	1 ft^2 = 0.093 m^2
	1 m^2 = 10.76 ft^2
Length	1 ft = 0.305 m
	1 m = 3.28 ft
Specific Heat	1 BTU / lb °F = 4184 J / kg K
	1 J / kg K = 2.39 x 10^-4 BTU / lb °F
Heat Transfer Coefficient	1 BTU / hr ft^2 °F = 5.677 W / m^2 °K
	1 W / m^2 °K = 0.176 BTU / hr ft^2 °F
Mass	1 lb = 0.4536 kg
	1 kg = 2.205 lb

Power Minimization: Insulation

thermal conductivity

insulators: foam, vacuum panels / aerogels, inert gases

Vacuum Panels:

http://www.vacupanel.com/tech-vip.html

Gas-Filled Panels high performance insulation

http://gfp.lbl.gov/default.htm

The LBNL Windows and Daylighting Group has extensive experience with highly insulating windows that use gas fills and multiple layers of glazing materials. An outgrowth of this research is the development of an opaque thermal insulation technology known as Gas-Filled Panels. Gas-Filled Panels, or GFPs, use thin polymer films and low-conductivity gas to create a device with extraordinary thermal insulation properties. GFPs are essentially hermetic plastic bags that can take on a variety of shapes and sizes. Inside the outer barrier is a cellular structure called a baffle. Argon gas filling provides an effective thermal resistance level of R-7 per inch, krypton gas provides R-12.5 per inch, and xenon gas provides R-20 per inch.

Aerogel Insulation:

http://eande.lbl.gov/ECS/aerogels/satoc.htm

http://www.glacierbay.com/ultra-r.asp, comparisons: http://www.glacierbay.com/Instresult.asp

Heater: Resistive Heater

typical designs, controller, flexible heater foils

safety hazards and controls

Insulation - Aerogel:

Glacier Bay, Aerogel vacuum panels: http://www.glacierbay.com; http://www.ultra-r.com/index.html

Vacupanel, foam core vacuum panels, http://www.vacupanel.com

Superinsulation, Aspen System, Marlborough MA, (blanket, clam-shell): http://www.aspensystems.com/

A Partial Bibliography for Silica Aerogels: http://eande.lbl.gov/ECS/aerogels/sabib.htm

NASA web site: http://technology.ksc.nasa.gov/WWWaccess/Opport/aerogel.html

NASA web site: http://technology.ksc.nasa.gov/WWWaccess/93report/msf/ms55.htm

Insulation - Others (gas-filled, foams):

Gas-filled Panels: http://gfp.lbl.gov/default.htm

Dow Chemical, Styrofoam (expanded), Styrofoam Square Edge (extruded) : http://www.dow.com

The Celotex Corporation, Thermax: http://www.celotex.com

Fomo Products, Inc.: Handi-Foam Quick-Cure Model II-12: http://www.fomo.com

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