Middleby Marshall PS200 Blowers, belts and pulleys, Types of Heat Transfer, PS555/570-series

SECTION 2 - AIR FLOW AND HEAT TRANSFER

SECTION 2

AIR FLOW AND HEAT TRANSFER

I.TYPES OF HEAT TRANSFER

Heat constantly moves from a warm object to a cold ob- ject. Heat moves in three different ways: thorough con- duction, through radiation, and through convection. Middleby Marshall conveyor ovens use all three of these paths to cook the product.

•Conduction - Heat is transferred through surface con- tact (such as pizza dough in contact with a pan).

•Radiation - Heat is transferred from light-colored ob- jects to dark-colored objects. The darker the color of an object, the greater the amount of heat that it ab- sorbs. The lighter the color of an object, the greater the amount of heat that it reflects. Middleby ovens use light-colored interiors (aluminized or stainless steel, depending on the oven model) to reflect as much heat as possible to the product.

•Convection - Heated air will transfer heat to objects with which it comes into contact. Unless the air flow is controlled, hot air will rise and be replaced by cooler air. Middleby Marshall ovens use blowers (one or two large fans, depending on the oven model) to force air through adjustable air fingers onto the product.

The intensity of the heat at the point at which it is sensed is called temperature. The difference in temperature between the food product and the heat source - that is, the pan, oven interior, or heated air, as described above - determines the speed at which the heat flows. The greater the differ- ence in temperature, the faster the heat flows to the prod- uct.

In Middleby Marshall conveyor ovens, the heat transfer from conduction is greatly dependent on the customer’s product and cooking surface (pans, etc.). Likewise, the heat trans- fer from radiation is effectively constant, because the color of the oven interior cannot be changed. For these reasons, the best way to optimize heat transfer is by regulating the convection air. This can be done in two ways:

•Adjusting the blowers and blower speed to change the quantity and velocity of air delivered onto the product.

•Adjusting the air finger configuration to change the pat- tern in which the vertical columns of air are delivered to the product.

II. BLOWERS AND BLOWER SPEED

A.PS200-series ovens

PS200-series ovens have a single blower motor mounted inside the machinery compartment, with a pulley on the end of the motor shaft. A belt connects this pulley to another on the shaft of the blower fan. The air veloc- ity can be changed by using a larger or smaller pulley on the motor shaft, and changing the length of the belt to match the new pulley. See Figure 1.

However, note that there are only two approved belt/ pulley combinations for PS200-series ovens; one for 60Hz ovens, and one for 50Hz ovens. Field modifica- tions to these approved combinations are not permit- ted unless under the specific directions of the Middleby Technical Service Department. Any changes to con- vection air delivery MUST be made using the air fin- gers.

Figure 1

Blowers, belts and pulleys

PS200-series

PS360-series

Blower

Wheel

Orientation

7

PS555/570-series

Early

(PS570)

Current

(PS570S,

PS570G, all

PS555)

Blower Wheel

Orientation