TM Vacuum..sometimes less is more...

    This is one of my best articles I wrote on how vacuum works in a TM. Remember this was written 2 years before I brought out teflon glides, so some of the figures mentioned at the wands WCI have been exceeded since I wrote this:

TM Vacuum- Sometimes less is more!
Posted By Kenneth Harris on 6/28/2001 at 10:15 PM
With all the talk we hear/see on this board about TM power levels and cfm output, i would like to tell you what it takes to have a great vacuum system for a TM system without all the marketing info that seems to accompany these numbers.

Vacuum can be measured in two ways; inches of lift and air flow(cfm).A brief discussion of these 2 terms is needed so that you can understand the relative importance of each and ways that you can manage both to arrive at an efficent vacuum system.


1) AIR FLOW (CFM)

This is the volume of air flow that actually takes the soil out of the carpet.Without enough air flow,carpets are left wetter and soil removal is not maximized.Carpets take longer to dry and customers are left unhappy!So clearly this is an important to get right. How much is enough? To get the answer to that question, you have to know where the most restrictive part of the air flow system is.

On any TM system , that will be found on inlet side of the system, because you have to rely on atmospheric pressure of 14.7 lb/sq. in. to fill the system whereas the exhaust side moves the air out under pressure of the blower.The inlet side consists of a vacuum tank,100+ ft of hose,and usually a cleaning wand.

a) Vacuum Tank : Not too much restriction here. Most tanks have a 2" outlet on them that will allow 300-400 cfm of air flow to enter the tank in open flow(no hose attached).Most important consideration here is to size the tank right in relation to the blower to keep the rise time of the system to a reasonable value.

b)Hose : Here 2" hose is what you need to use to keep cfm up and minimize frictional losses.Most 2" hose will handle 300 cfm of airflow well at 12HG(164" WL)before these losses get too great. But since vacuum hose losses about 25" WL/100 ft; it is wise to limit the length that you use.Since in residential cleaning only 100 ft or so is needed,this is not a problem.The 10' whip hose that is often used with the 2" hose to fit on to the 1.5" tubed wand is likewise not too restrictive as it will allow up to 250 cfm of air flow thru when used at this length.

c)Cleaning Wand : This is where the real restriction lays.When the wand is on the carpet, a tremendous restriction exists to the air flowing into the wand at the interface with the carpet.Wands with the standard 1/4" deep slot at the lips will only allow about 12.5 cfm/inch of wand width into the wand lips when on the carpet.The following figures show what this means in terms of real world cfm:

12" wand......150 cfm *
14" wand......175 cfm *
16" wand......200 cfm *

* - measured at 12"HG at 110 ft.
Now, since most truckmounts come with 12" wide wands you can see just how much this restiction really is.But it shows one other thing. If you can't flow more than 150 cfm; why do you need a large blower for your TM? Below are some of the cfm ratings for the most common blowers:

#33..........175 cfm
#36..........280 cfm
#45..........320 cfm
#47..........405 cfm

As you can see, all but the #33 will max out the capacity of the above wands when they are on the carpet.The need for for cfm for WD and dual wanding will be dealt with later.

2) VACUUM LIFT

Once the cfm capacity of the wand has been satisfied(200cfm),the next thing to get right is the vacuum's lift.This measures the pressure drop in the system and measures that systems ability to do work (clean carpets), by overcoming the resistance to flow(the wand/carpet interface).Say you have a vacuum reading of 12"HG showing on your gauge when the wand is on the carpet.Does this meanthat all this vacuum goes to work to cause air/water/soil to enter your wand? No. To find out why you must find out what "working vacuum " is.

The resistance of your system is determined in the main by how much air your tring to flow thru it. What this means is a larger blower will cause a higher resistance in your system just by tring to drawing air thru the hose without a wand attached!This running vacuum does no work. Say your reading is 12HG with the wand on the floor.Now lift it off and the reading is 8"HG.The difference between these two numbers is your working vacuum,or 4"HG. You want this number to be as large as possible to have an efficient vacuum system!But the larger the blower you use ,the less this number becomes. the following are some real readinds i obtained from various blower equipped TM's :

#47.......12"total.....8"RV.....=4"WV
#45.......12"total.....5"RV.....=7"WV
#33.......12"total.....3"RV.....=9"WV

The above illustrates how the smaller blower equipped TM's are more efficient with there vacuum and can still dry carpet in a couple of hours.Because the #47 equipped TM has less working vacuum than the #45 TM ,i often see the carpet dry in less time than the identical job done by the #47!The reason is they are both at 12"HG,but the wand will not let the additional cfm it produces get to the carpet; so the more efficient vacuum of the #45 wins out.


Now that the limitations of trying to move much more than 200 cfm past the wand have been shown,what do you do if you want to dual wand, or need more cfm for WD? Get a true dual wand system that uses 2 separate vacuum tanks; like the Split-Steam sold by Advanced Cleaning. You will then have all the efficency that a small blower equipped TM provides(with it's high working vacuum),plus the ability to double your production by putting another man on the other wand!In a WD situation this set-up will also suck the water out of those basements fasterthan a large blower equipped TM could with 1 hose .

It is really a case of win-win.


Ken Harris Turboteck Supplies

13.8" WL = 1" HG.