As the temperature of LPG drops, so does its pressure. The Engineering Tool Box offers a handy reference showing the pressure of different mixes at different temperatures:
http://www.engineeringtoolbox.com/propane-butane-mix-d_1043.html
Here is a summary of its contents:
| Vapor Pressure in PSIG | ||||||
| Mixture | Propane % | 100 | 70 | 50 | 30 | 0 |
| Butane % | 0 | 30 | 50 | 70 | 100 | |
| Temperature (F) | -44F | 0 | 0 | 0 | 0 | 0 |
| -30F | 6.8 | 0 | 0 | 0 | 0 | |
| -20F | 11.5 | 4.7 | 0 | 0 | 0 | |
| -10F | 17.5 | 9 | 3.5 | 0 | 0 | |
| 0F | 24.5 | 15 | 7.6 | 2.3 | 0 | |
| 10F | 34 | 20.5 | 12.3 | 5.9 | 0 | |
| 20F | 42 | 28 | 17.6 | 10.2 | 0 | |
| 30F | 53 | 36.5 | 24.5 | 15.4 | 0 | |
| 40F | 65 | 48 | 32.4 | 21.5 | 3.1 | |
| 50F | 78 | 56 | 41 | 28.5 | 6.9 | |
| 60F | 93 | 68 | 50 | 36.5 | 11.5 | |
| 70F | 110 | 82 | 61 | 45 | 17 | |
| 80F | 128 | 96 | 74 | 54 | 23 | |
| 90F | 150 | 114 | 88 | 66 | 30 | |
| 100F | 177 | 134 | 104 | 79 | 38 | |
| 110F | 204 | 158 | 122 | 93 | 47 |
When running subzero temperatures for extraction, tank pressure nor the push pull from the pump are adequate for injection at some point, because of the dramatic increase in viscosity accompanying the drop in pressure.
You can increase the Propane content of the LPG to lower its viscosity at subzero temperatures, but alas a 100% propane mix has not only a potential for high pressures if something goes wrong, but by itself doesn’t produce as flavorful a product as a mix with Butane.
We typically use a 70% n-Butane/30% n-Propane or a 50/50% blend at -50C and below, but even that mix can need help as the temperature falls, so we typically use a hot LPG vapor assist to add the required boost.
To that end, we set a 50# refrigerant tank of the same LPG mixture that we are using into a pot of hot water, and connect its vapor port to the vapor port of the injection tank.
The extra pressure head, in conjunction with the pumps push pull, is enough to force the liquid in the tank through the heat exchangers and subsequently through the column, after which it is easily recovered.
That contrasts with an alternate process using N2 to pressurize the tank head space, which must be burped off before recovery, lowering the LPG recovery percent, so we don’t typically use it.
Hot vapor assist diagram