Food Engineering

Lesson 20. DESIGN PROBLEMS ON BATCH FREEZERS

Module 2. Food freezing

Lesson 20

DESIGN PROBLEMS ON BATCH FREEZERS

20.1 Introduction

Refrigeration requirements are expressed as the total heat to be removed in order to reduce the ice cream mix from an unfrozen state to some temperature in the freezing range.

Due to the composition of ice cream, the total heat load is divided into three parts:

(a) Sensible heat required to reduce the mix temperature to the initial freezing point

(b) Sensible heat required for unfrozen portion of mix at temperature below F_p

(c) Latent heat requirement

After reaching the initial freezing point, the last two contributions to total heat occurs simultaneously. Since, the composition of normal ice cream mix dictates that all two portions of total heat throughout the range of freezing temperatures encountered. Predictions of refrigeration requirements must include calculation of each phase. The beater assembly of batch ice cream freezer is designed based on dimensional analysis, considering different dimensional numbers like, Reynold’s number, Nussuelt’s number, Prandtl’s numser etc, Reynold’s number helps to design beater assembly and to optimize its speed to control ice crystal formation during freezing. The refrigeration requirement in k cal/kg may be written as:

The total amount of heat which must be extracted from the ice cream freezer are:

1. The total heat extracted from the ice cream to freeze it partially in the freezer
2. The heat equivalent of work done in agitating the mix while being frozen
3. Radiation losses

20.2. Examples

Example 20.2.1

Find out the refrigeration load of an ice cream freezer in tones when capacity of freezer is 700 kg/hr, drawing temperature is -5ºC, initial temperature of mix is 4ºC, initial freezing temperature of mix -2.5ºC, water content of the mix is 63%, specific heat of mix is 0.8 and specific heat of semi solid ice cream is 0.6. The water frozen at -5ºC is 47%. Assume that 85% of the heat equivalent of the work done by the motor appears as heat in the ice cream. The freezer requires 5 kW motor to drive the dasher, and radiation loss is 3% of the total heat.