Grantville Gazette, Volume 40 (35 page)

Cooper says, "The first commercial ice houses were built below the surface of the ground, but at present all are constructed above ground, for the reason that drainage is more easily secured, and the ice is more easily removed from the house. The protection afforded by the earth is of comparatively small value when the disadvantages of storing below ground are taken into consideration." (491). Doors should be as high as possible to minimize the escape of cold air. (495).

In 1914, the USDA weekly newsletter warned that "excavations are expensive," and also reiterated Cooper's points about drainage and ice removal. It also pointed out that the thermal stability of earth was a disadvantage in winter, the stored ice having to be protected against the earth heat.

Building-Grade Insulation

What we want is
low
conductivity. Conductivity is the amount of heat energy passing through a unit thickness and area of the material in a unit time, under the influence of a unit temperature difference. Conductance is defined similarly, but for a specified thickness. Resistivity is the reciprocal of conductivity and resistance (the "R-values" used by builders) that of conductance. Resistance should be proportional to thickness (and conductance inversely proportional) but they aren't exactly so.

There's plenty of conductivity information in various standard engineering handbooks; there should be multiple editions in Grantville, covering different materials. Just looking at pre-RoF editions of the handbooks that are in my personal library or my local library, I find . . .
CRC Handbook of Chemistry and Physics
(69th, 1989) provides conversion factors for units of thermal conductivity (E2) and the density and thermal conductivities (BTU/ft
²
-hr-
^o
F; one inch thick) of several dozen materials (E6).
Machinery's Handbook
(24th, 1992)(MH) provides yet another conductivity/conductance table (2445).
Perry's Chemical Engineers' Handbook
(8th, 1963) gives thermal conductivities (BTU/ft
²
-hr-
^o
F; one foot thick) for numerous building and insulating materials (3
–
211ff), formulae for predicting thermal conductivity (3
–
223), heat transfer equations (section 10), and a detailed discussion of cryogenic-grade insulation)(12
–
33ff). An even richer source is
Marks' Standard Handbook for Mechanical Engineers
(9th, 1987), with a chapter on heat (4), sections on air conditioning (12.4), mechanical refrigeration (19.1) and cryogenics (19.2); several conductivity tables are provided. I will use the CRC values/units in the following discussion, unless otherwise stated.

Conductivity is not a constant for a material; it increases (slowly) with increasing mean temperature. For example, extruded polystyrene might be 25% more conductive in summer (44
^o
C) than winter (4
^o
C).(Kirk-Othmer, 14:656).

Standard construction materials, generally speaking, are not good heat insulation; brick and glass (3
–
6), concrete (6
–
9), stone (4
–
28). Poured concrete is worse (12; MH) than concrete blocks. Wood is the best of the lot: oak (1.02), white pine (0.78), and especially balsa (0.33
–
0.58).

Air- or kiln-dried wood is a good insulator, but a filler that traps air in tiny pores is more efficient than solid wood. This was not fully appreciated in the nineteenth century, and there are instances of "the use of from six to ten thicknesses of boards in one wall." (Cooper 70). Note that nails are good conductors of heat.

Up through the nineteenth century, the best insulating materials were inorganic or bioorganic particles or fibers. These could be provided as a loose fill, or sealed into flexible batts or rolls, or cemented with a binder to form semiflexible or rigid sheets. Conductivity is in the 0.25
–
0.5 range.

In the new time line the materials that will be available first are probably local cereal plant chaff and straw (straw bales ~0.4
–
1), local plant leaves, sawdust (0.41), planer shavings (0.42), charcoal (0.36
–
0.39), regranulated cork (0.30
–
0.31), hair felt (0.26), "Linofelt" (flax fibers between paper)(0.28), and perhaps felts using other fibers.

The problem with the organic materials is that they initially contain water (increasing conductivity, unless they are dried, which increases cost), and are vulnerable to fire (especially if dry), biological attack (fungi like it damp) and wicking (drawing in more moisture). Insulation may be given a protective "finish" or additive, at additional cost.

Historically, the natural ice industry created a market for sawdust; it sold in Maine at $3/cord (Hall 22). It deteriorates, especially when damp, so Cooper recommends its use only as the immediate packing material around the ice blocks, inside the ice house. Shavings replaced sawdust as the preferred nineteenth century ice house insulation; they are somewhat resistant to rot as the cell structure is preserved. Wood shavings may be treated with lime and other chemicals for improved life. Charcoal was used in Europe OTL for insulation, but blackens everything.

To make hair felt, cattle hair was obtained from tanners, washed and air dried, deodorized, and put into a felting machine. A waterproof paper was sometimes applied to it.

Within a few years we are likely to also have the opportunity to choose "Cabots quilt" (0.25
–
0.26), rock wool (0.26
–
0.29), mineral wool, glass wool (0.29), powdered diatomaceous earth (0.31), powdered gypsum (0.52
–
0.60), corkboard (0.25
–
0.34), "Insulite" cemented wood pulp (0.34), and perhaps the tropical plant-derived "Dry zero" (kapok between paper) (0.24
–
0.25) and "Celotex" cemented sugar cane fiber (0.34). There's also the possibility of "air-entrained" (foamed) concrete (0.4; Wikipedia/Building Insulation Materials).

In the Pierce House (1635), in Dorchester, Massachusetts, eel grass (Zostera marina) was used as an insulation. Cabot's Quilt (1891) was an eel grass mat, sandwiched or stitched between sheets or waterproof paper. Despite its organic origin, thanks to its iodine content it's resistant to fire and vermin.

To make rock wool, granite and limestone are crushed, mixed with coke, and fused in a furnace at about 3000
^o
F, creating a slag. The slag is run out through a high pressure steam blast, which blows it into a fibrous form. (Or you can use the slag from a blast furnace and add limestone to it, to make mineral wool.) It's not subject to decay, but it is brittle (shouldn't be packed more closely than 12 pounds/cubic foot) and should be handled with gloves. It's possible to have the mineral wool compressed into slabs or sheets that can be used much like lumber. (Cooper 57). A natural mineral wool was collected by Hawaiian islanders from volcanic deposits, and used as insulation. The artificial version was "first commercially produced as a pipe insulator in Wales in 1840, and artificial rock wool was first produced in 1897. (Bynum 4).

Glass wool fibers are made by extruding molten glass through an array of holes, or by drawing out molten glass.

The solid (unfoamed) rubbers and plastics likely to appear first in the 1632 universe (see Cooper, "Industrial Alchemy, Part 5: Polymers,"
Grantville Gazette
29) include natural rubber, rayon, cellulose acetate, cellulose nitrate, phenol-formaldehyde, and polyacetal. Polystyrene and polyurethane will come somewhat later. Rayon is the best of the lot; 0.054
–
0.07 W/m-
^o
K; 0.37-0.48 BTU/ft
²
-hr-
^o
F, inch). This isn't good enough to justify its use! But the up-timers might not know this in advance. . . .

Synthetic polymers may be foamed to improve their performance. Foaming takes advantage of the relatively low thermal conductivity of air, especially air confined to small voids; closed cell foams are preferred. Of the "expected early" polymers, the ones with lowest conductivity foams are phenol-formaldehyde (
0.029
–
0.040
W/m-
^o
K; 0.2 BTU/ft
²
-hr-
^o
F, inch), urea-formaldehyde (
0.026
–
0.030
; 0.18
–
0.21), and natural rubber (
0.036
–
0.043
; 0.25
–
0.30). Nice, but still not better than the materials cited earlier. Also, the various formaldehyde based foams were banned because improper installation could result in over-exposure to formaldehyde.

The lowest conductivity foam is polyisocyanurate (
0.012
–
0.02
; 0.08
–
0.14), a polyurethane (0.016
–
0.040; derivative. I forecast lab quantities of polyurethane (
0.016
–
0.040
; 0.11
–
0.28) in 1635
–
37; it's dubious whether Grantville Literature says how to make polyisocyanurate. It's also uncertain whether it reveals that polyurethane foam can have a conductivity lower than 0.25.

As of the early 1990s, foamed polymers were more expensive than traditional fibrous insulation, although perhaps stronger and more durable. (Kirk-Othmer 14:659).

Further improvements are possible by replacing the air with a lower conductivity gas (e.g., trichlorofluoromethane), and by giving the polymer a foil facing to stave off radiation. Still, I suspect that by the time we develop these fancy foamed polymers, and get the production costs down enough to be competitive with sawdust etc., the window of opportunity for the natural ice industry will have closed.

Air is an excellent insulator— but only if the air was still. The problem is that if a large volume of air is trapped between hot and cold surfaces, convection currents will develop. Cooper "considered the one half inch air spaces formed by battens and paper . . . to be efficient until practical experience and the tests conducted by him proved otherwise." (92). He concluded that "any space over one-half inch in width, if it can be kept dry, will be of greater value if filled with an insulating material as good as mill shavings, than if left as an air space." What granulated and fibrous "fills" do is confine the air into many tiny air spaces, small enough so that the viscosity of air inhibits convection (45).

Cryogenic-Grade Insulation

The best possible insulation is a vacuum—it blocks both conduction and convection. It's overkill for an ice house, of course, but I think this article is as good a place as any to talk about high-performance insulation.

Vacuum insulation was pioneered by Dewar (1893); his first Dewar flask has a high vacuum between double glass walls, with the walls are joined at the top of the flask. One of Dewar's papers notes that the problem of constructing glass that "would withstand the atmospheric pressure and bear considerable oscillations of temperature without cracking" was "only gradually overcome by . . . improvements in the blowing and adequate annealing of the glass." (Dewar 1115). The evaporation rate of liquefied gas in the vacuum vessel was only one-fifth that in an ordinary vessel. Dewar then silvered the inner wall, to reflect away heat radiation, and that reduced the rate to one-sixth of that before silvering. (EB11/Liquid gases).

To significantly reduce heat flow, the vacuum must be good enough so that the mean free path of air molecules is greater than the distance between the facing surfaces. For air, the mean free path at atmospheric pressure is 93 nm (0
^o
C), and the path length will be inversely proportional to the pressure. For a vacuum of 10
^-3
mm Hg (torr; normal atmospheric is 760) the mean free path is one micron. (Perry 12
–
34). In 1960s, a typical Dewar flask would use 10
^-4
or 10
^-5
.

In the Thermos bottle, the glass was protected with a metal shell and spacers were placed between the walls. Still, glass is brittle and of limited compressive strength (needed by the outer wall) and so people naturally wanted to replace it with metal. That didn't work, and in 1905 it was realized that the problem was that gas molecules are "occluded" inside metals, and released for a long time if the metal is exposed to hard vacuum. That, of course, ruined the vacuum. In 1906, the problem was solved by placing charcoal (an excellent gas absorbent) in a recess communicating with the "interspace." As noted by EB11/Liquid Gases, that meant that Dewar flasks could be "formed of brass, copper, nickel or tinned iron, with necks of some alloy that is a bad conductor of heat."

The use of vacuum insulation for an ice house was suggested as early as 1903 (Cooper 46); the proponent figured that if we could construct a single-walled steam boiler to resist 400
–
600 psi, a double-walled vacuum-insulated refrigerating room could be built to resist normal atmospheric pressure. I don't see this as cost-effective, but we will certainly find uses for vacuum insulation.

Sometime before 1911, it was recognized that one could reduce the effective spacing—and thus achieve low conductivity with a poorer vacuum—by filling the space between the walls with a powder before evacuating the air. (The "fill" would also allow use of thinner walls, as they would help resist outside air pressure.) Tests showed that "silica, charcoal, lampblack and oxide of bismuth all increase the heat insulations to four, five and six times that of the empty vacuum space." (EB11/Liquid Gases). This evolved into the modern "evacuated powder" insulation with fills of silica, silica aerogel, expanded perlite, diatomaceous earth, fused or laminated alumina, expanded mica, lampblack, synthetic calcium silicate, and even charcoal peach pits. Typically, the evacuated powders are 10
–
30 fold better insulators than the powders alone. For perlite and diatomaceous earth, the conductivity curve levels off, as the vacuum approaches 10
^-2
, at a little above 10 uW/cm-
^o
K (
0.0069
). (Perry 12
–
38ff). Evacuated lampblack conductivity is more sensitive to pressure but at 10
^-2
isn't much inferior to perlite and kieselguhr and lampblack of course is universally available. I suspect that many other particulate and fibrous fills will work, too.