Impact of the Multipass V single pass in freezing of product
The freezing time of a given product is a function of the impinging air temperature on the product and the air velocity over the product.
Due to the reduced evaporator face area available in the Multipass system the evaporator is deeper in the direction of the air flow hence the air leaving the evaporator is usually colder.
The smallest temperature difference between the impinging air and the product occurs when the air leaving the evaporator is some -36C and the product mass average temperature is say -18C
This represents a temperature differential of 18K
At this product leaving temperature every 1K colder air represents approximately 5% increase in the temperature differential between the cooling medium (air) and the product.
It should be noted that at this stage of the cooling process, the bulk of the latent heat of fusion has been removed. Due to the greater thermal conductivity of the frozen material, the product temperature reduction is more responsive to changes (reduction) in the temperature of the cooling medium.
At the other end of the air path where the air passes over the entering product, the air temperature could be in the order of -30C. Due to the longer air path and the reduction of air circulating volume in the Multipass.
Considering the temperature differential in (product to air temperature 7 –(-30)= 37C the impinging air temperature to product difference is still 37 C so eve on entry impinging air temperature has a beneficial impact on the heat transfer between patented the product and the impinging air temperature.
The above illustrates the real benefits of the Multipass and the true counter flow arrangement between the freezing medium (air) and the product flow and the associated very substantial total energy cost savings originating from the air circulating fan energy reduction and the engine room.
Refer to energy saving calculations of Multipass