Local Recirculation

Local recirculation systems differ from central systems in that all exhausted air is passed back to the room after cleaning and that the flow rate could be larger than the flow rate through the room.

Air cleaner FIGURE 8.2 Model of a local recirculating system (room air cleaner) used for calculating the con­nections between contaminant concentrations, airflow rates, contaminant source strength, qm, and air cleaner efficiency, tj. csup is the concentration in the supply (outside) air; c (equal to c,^) is the concen­tration In the room; crec is the concentration in the returned air; qalrtot is the total flow rate through the room; k is the ratio between the recirculated airflow rate, qairrK, and total airflow rate; is the flow rate from the general ventilation system; T is the time constant for the room; and V is the room volume.

One of the most common systems for cleaning air in homes, offices, schools, etc. is the room air cleaner. Figure 8.2 outlines a model of a local re­circulating system. Usually these units are situated inside the room if they are small and movable (see Chapter 10). For the model it does not matter if the unit is placed inside or outside the room with the contaminant source, as long as the exhaust and return air openings are inside.

The room air cleaner consists of a fan and some kind of air cleaner for particles or gases or both, usually mounted together as one unit. This is a local recirculating system and the equation for the contaminant concentration in the room, derived with the same assumptions and in the same way as for cen­tral systems, is the following:

TOC o "1-5" h z. . 4m.. 1 .. ,1 — a +K, T))f/7 ,n ,

C = —— X -7Z—— :X 1-e ) , (8.6)

‘Jairexh (1 + 1?)

where

C is the concentration in the room, mg m-3 qm is the source rate, mg s-1

‘7airexh’s the exhaust flow rate from the room, m3 s-1 kx is the local recirculation ratio equal to q^[cec/’J’airexh <?airrec’s the flow rate through the unit (cleaner), m3 s_1 ri is the efficiency of the cleaner (0-1)

T is the time constant for the room equal to V/gexh, s

V is the volume of the room, m3 t is time, s

By manipulating this equation for the steady state, in the same way as for central systems, the following could be achieved:

‘ («-7}

“Jairexh ‘Jairrec1?

Which is similar to the equation for central systems. This relation is often ex­pressed as

C =———- , (8.8)

L1airexh ‘Jairequ

Where (7airequ is the so-called equivalent flow rate for the cleaner, because the product of flow rate and cleaning efficiency has the unit of flow rate. In words this could be expressed as follows: The influence of a room air cleaner on the contaminant concentration is the same as if a flow rate of clean air (outside air) equal to gairequ were added to the ventilation flow rate in the room.

From this equation it is clear that if either flow rate or cleaner efficiency for the recirculation system is zero, there will be no change in contaminant concentration. Also, a low flow rate can only be compensated to a small de­gree by a higher cleaning efficiency, but a low cleaning efficiency can be com­pensated to some degree by increasing the flow rate.

This equation makes it quite easy to calculate necessary flow rate and cleaning efficiency for a local recirculation system (room air cleaner).

Local ventilation in industry usually differs from the description above in that it is connected to a local exhaust hood (Chapter 10), which has a capture efficiency less than 100%. The capture efficiency is defined as the amount of contaminants captured by the exhaust hood per time divided by the amount of contaminants generated per each time (see Section 10.5). Figure 8.3 outlines a model for a recirculation system with a specific exhaust hood. Here, the whole system could be situated inside the workroom as one unit or made up of sepa­rate units connected with tubes, with some parts outside the workroom. For the calculation model it makes no difference as long as the exhaust hood and the return air supply are inside the room.

The solution to the differential equation at steady state in this case is

C = -2ai-xjl-ffTl) , (8,9)

*7airexh (

Where a is the capture efficiency for the local exhaust hood (0-1). To get the time-dependent solution, the right-hand side is multiplied with the same term (the parentheses including the exponential term) as for the system without an exhaust hood.

This latter equation can also be used for systems without a local ex­haust hood by setting the capture efficiency to zero. It could also be used to show the result of recirculation from, e. g., a laboratory fume hood with immediate recirculation. In such a hood all contaminants are gener­ated within the hood and usually also all generated contaminants are cap­tured, so the capture efficiency is 1. The equation demonstrates that if the

C.

•sup

Exhaust air *?airexh

^irsu p — ^airexh

T V

A<lm

C

Recirculated air

Air cleaner

FIGURE 8.3 Model of a local recirculating system with a local exhaust hood, used for calculating the connection between contaminant concentrations, airflow rates, contaminant source strength, qm, air cleaner efficiency, rf and hood capture efficiency, a. csup is (he concentration in the supply (outside) air; c (equal to cexh) is the concentration in the room; crec is the concentration in the returned air; qJlralp is the supply flow rate to the room equal to the exhaust flow rate, qairexh; the recirculated flow rate (through the cleaner) is qair„ rK; T is the time constant for the room; and V is the room volume.

Cleaning efficiency is zero, there is no change in concentration, and the unit only works as a mixing unit.

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