Re: Positive pressure home unit
First, I question the rarity of circumstance that would necessitate such extensive protection, but see value in the concept of safe-room air flow and temperature control issues in general, so here are some thoughts for consideration and possible advancement of the concept.... I very much enjoyed the 'vanilla extract and smoke' test methods discussed, :tiphat: I'll have to try that (anyone else do this yet?).
As with most engineering exercises, success or failure of a design is often more dependent on the governing assumptions than on the quality of execution.
Anyone seriously considering this type of protection would be well served by emphasis on the planning side of the effort.
Some of the
assumptions made in the referenced article are;
1) a need to enclose the fan shroud and motor to prevent infiltration caused by the pressure gradient the fan creates.
1 A: the design can be much simpler if the
inflow pipe is what penetrates the 'barrier' (plastic sheet, wall, etc). Simply flange and seal the 4" plastic pipe directly to the intake opening of the fan unit. That would put the fan, shroud, motor, power cord, and switch access on the 'inside' with the uncontaminated air (no sealed box or welding and cutting required). Both designs require perforating the 'living envelope' for a duct, and in both designs that perforation is 'protected' by the positive-pressure as long as the motor is running. Minimizing the amount of equipment that is 'outside' getting contaminated could be an advantage in case the motor needs replacement, or for multiple 'episodes'.
2) the article recommends leaving significant cracks and leaks for escaping air, because they will be protected by the constantly-running device. This assumes that the air under pressure is at a desirable temperature, and increases
dependency on the sustained alternate power.
2A: if it's cold out, and the plastic bubble defines the 'heated volume' of the emergency shelter, then any airflow greater than necessary for oxygen replenishment (and moisture management) is undesirable. An expensive (in dollars and watts) option is an ERV (energy recovery ventilation) unit. A less tech-dependent approach is to seal everything as well as possible, then install an adjustable outflow mechanism and a motor speed controller. Adjustment and 'balancing' of these two 'controls' allows
optimal airflow, with maximum air pressure and minimum heat loss ( or cooling loss if the thermal scenario is reversed). Some motors can be controlled by inexpensive 'router speed controllers' that use an efficient SCR device (solid state electronics), I don't know about the specific fan unit he used.
3) that contaminated air is not entering during the '
burping' of the 'overpressure' device.
3 A: unless the pressure relief device is carefully designed, it may allow eddy currents and transient reverse flows to contaminate the 'protected volume'. An overpressure device (especially a valve, or 'flapper') should not be necessary if the controls in (2) are adjusted properly, and is likely a frailty in the design intent, and should be eliminated. Even the 'adjustable outflow' mentioned in (2) should proceed outward thru enough 'snorkel-length' of small diameter ducting that any momentary reverse flows do not traverse the entire length of the snorkel (such as opening / closing a door in the enclosed volume).
4) that 2 -3 rooms sealed in this way is superior to an entire section of the house.
4 A: In my mind, the best system is analogous to the chain with the fewest links. Having PPE, decon supplies, staging areas, etc. are wonderful if you need them, but better yet to not need them. Put the filter outside the house, or poke it through to another level of the house, but minimize ANY traffic thru the envelope by having enough space 'inside'. It is horrendously difficult to assure sterile technique even in the most controlled environments, and impossible to do so in lesser conditions. Far better to carve out more space in the early design which obviates the need to do all the 'fancy' stuff, and don't go out till it's safe. Try to use the walls you have instead of creating new ones that are made of plastic and duct tape. With proper controls (see 2A ) and attention to sealing around windows and doors, 'Warlord' may be able to positive-pressurize a larger volume at the same or less watts.
experimentally yours,
Read
