Dometic 39626.501, 39726.506, 39726.501, 39626.506 AIR Flow and Friction, Economics of Duct Design

INSTALLATION INSTRUCTIONS

2.1.5 AIR FLOW AND FRICTION

Air flows in a duct system from a region of high pressure to a region of lower pressure. The blower creates the pressure differential which causes the air flow through the duct system. The point of highest pressure in the system is at the outlet of the blower. The point of lowest pressure in the system is at the return opening of the blower. The air pressure constantly decreases as the air flows through the system. The pressure ultimately diminishes to zero as it passes through the register and is diffused into the conditioned space. As air moves through a duct, a pressure drop occurs due to the friction between the air and the walls of the duct. Another factor in pressure drop is the turbulence within the air stream itself. Air moving within a duct does not flow in a placid stream. Rather it moves in a churning and mixing path, or “turbulent flow.” The cumulative effect of rubbing friction and turbulence friction is friction loss. Air turbulence in a duct system becomes substantial whenever there is a change in the direction of air flow.

2.1.6 ECONOMICS OF DUCT DESIGN

In order to match to the system air delivery capacity, elbows for turning the air must be kept as large as allowed by the unit construction. Duct depth for Dometic air conditioners may vary with each model series. Confirm the system to be installed and refer to Figure 3.4, for specific duct configurations required. High loss elbows must have their resistance lowered by the use of splitters or turning vanes. Refer to Figure 2.3 through 2.7.

Another factor in duct losses is air leakage. Although leakage is not considered in duct design it should be an installation consideration. Cold air leaking into the surround- ing cavity will cause condensation to form under high humidity conditions. Sealing all joints will assure moisture free cavities and maximum distribution of air to the outlets.

In most cases of high resistance encountered in duct systems, one or more of the following points have been overlooked by the installer:

A. Small Diameter

Pressure losses increase as diameter of a duct is reduced. Good design practice is that which enables the installer to put in the smallest size duct that will do the job of delivering required airflow rate with pressure available. No single size of duct will prove to be ideal for all jobs.

B. Length of Duct

Pressure loss increases as duct length is increased. This is almost obvious; a duct which is 6 ft. long has twice the pressure loss of one that is 3 ft. long, provided that both ducts are the same size and both are carrying the same airflow rate.

C. Changes in Direction (Figure 2.7)

Pressure losses increase when direction of air flow is changed. When air is forced to make a 90° turn in a duct system, pressure loss is much greater than for a straight run of the same length. (Refer to FIG. 2.7)

Item 1. This sharp-angled bend causes a large pressure loss. A simple way of visualizing such pressure loss is to imagine that these diagrams represent highways on which you are driving a car. As you approach a bend you are forced to slow the car speed to 15 mph. In so doing, a considerable part of energy of the fast-moving car has had to be absorbed by the brakes. The analogy holds for air particles flowing around a sharp bend.

Item 2. This sharp-angled bend on the inside corner causes great pressure loss. The rounded corner on the outside does not help as much as might be anticipated. Item 3. This is a common form of 90° bend that has relatively low resistance.

Item 4. If minimum resistance is desired, this extreme example of a smooth, streamlined fitting can be used, although space requirements will be prohibitive in many installations.

Item 5. Occasionally, a beam or rafter prevents the use of a smooth bend and a right angle bend is necessary. In such cases the use of turning vanes (splitters) will be effective in reducing resistance.

Item 6. Another way to reduce resistance is to change item (3) to a modified version of item (4) by inserting splitters in the sharper bend.

D. Sudden Contraction

When air is suddenly contracted from a large duct to a small duct, a pressure loss occurs See Figure 2.8.

E. Sudden Expansions

The pressure loss resulting from the sudden expansion of air from a small duct to a large duct, are much larger than losses due to sudden contraction. As with sudden contraction, much can be done by making air expansion gradual, rather than abrupt. See Figure 2.9.

To summarize Section 2.1.6, the following items contrib- ute to higher pressure losses in a duct system:

1.Smaller diameter ducts

2.Longer duct lengths

3.Changes in direction of air flow

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