Piston Strategy

8.6.3.1 Description

The highest effectiveness can be achieved with the piston strategy. The contaminant concentration, temperature or humidity, and the local effec­tiveness are functions of the location and the power of the sources in rela­tion to the supply and exhaust openings. With a homogeneous distribution

Strategy

Stratification	Zoning	Mixing
Piston

Room dimension EX

~T

To provide uniform Conditions through­out the ventilated space

Description

Heat, humidity, and contaminant distribution*

To create unidi­rectional air­flow field over the room area by supply air

To support flow field crcatcd by density differ­ences by replacing the airflow out from the room area with supply air

Room dimension EX

R

To control air condi­tions within sclccted zone in the room by the supply air and allow stratification of heat and contami­nants in the other room areas

Room dimension EX Room dimension EX 1—

" /
	/
	———- і

Main characteristics

Low-momentum unidirectional supply airflow, strong enough to overcome disturbances

SU T. C.x SU T, c,x

Room airflow pat — Room airflow terns controlled by patterns controlled mainly by buoyancy; supply air distribution with low momentum

SU T, C, x Room airflow patterns controlled partly by supply and partly by buoyancy

SU T, C, — V

Room airflow patternscon trolled typically by high-momentum supply airflow

■*——————- Oo			——————— ► 1
Ј 3 Ј S ^ У* з	Ј |C Fft ■> ■ і—	T ^ R* *N №	I~r Л T f "1V

Ideal efficiency Typical application (example of a general room air dis­tribution method)

F ; ~ ‘^c t0Wol ‘і: 1 fc= (Ccx|,—CoK(C02— Co)

’.v-axis: “C, mg/m5, g1<g; у-axis: room dim. (e. g., height); SU = supply, FX = exhaust

Of sources, the contaminant concentration and temperature change linearly between supply and exhaust openings located at opposite ends of the room. With local sources, the concentration upstream of the sources is very low.

8.6.3.2 Advantages and Disadvantages

Advantages include; the whole flow pattern can be controlled, areas up­stream ol sources can be kept clean, and high contaminant removal and tem-

It K

(kI Horizontal piston

Ib Vertical piston

(c) Partial piston

FIGURE 8.10 Examples of air distribution and exhaust methods for the piston strategy.

Perature effectiveness. Disadvantages include the need for high supply airflow rates and large supply areas.

8.6.3.3 Design Criteria

The design criterion of the piston strategy is to overcome al! the air cur­rents opposite to the directional airflow created within the room.

8.6.3.4 Application

Piston air conditioning is an expensive strategy due to the high airflow rate that is needed to create a desired airflow pattern inside the room. Thus it is usually used only in applications where it is required, like in the semicon­ductor industry, where up to abour 400 air changes per hour are used. An­other example of its application is horizontal piston flow in reinforced plastic plants. y Schemes of different ways to apply the piston air conditioning strat­egy are shown in Fig. 8.10.

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