Blast Freezer Heat Load calculations

[smpsmp45]

What is the right way of blast freezer heat load calculations.

In one refers to Crack USA manual they say calculate product load & multiply that by 1.5 for System load.
By FAO Bulletins, multiply by 1.3 for system capacity.


I think the issue is working hour calculations.

Typically for 18 Hr working operation the system load is calculated by basic load x 24 / 18. Now if this factor is also applicable for product load.

That factor seems to be applicable for continuous operations. In some cases heat load is with this factor on entire product load & it some cases that factor is not considered on Product load.

Helpman software still gives totally different values much on the lower side.

Can some one throw more light on this.?

[charlie n]

suggest you set working hours and product pull down hours at the same value. (Batch load tome 18 hrs?)This should give the actual product load. In a well built blast freezer room, the wall heat transmission is almost insignificant compared to the product load so if the software calculates transmission load over 24 hrs instead of 18 hours, it will make almost no difference to your final result.

[smpsmp45]

That is true. But why so much difference in calculations by Four esteemed organisations?

[Dacosta]

During the process of blast freezing the product will follows Newton’s Law of Cooling

Tt= Ti + (T0-Ti)e^-rt

Where
Tt is Temperaure at time t
Ti is the ambient temperature
T0 is the initial temperature
R is the cooling rate
t is the time

If we graph this we will notice that the curve is logarithmic. The basic heat load can be represented as a straight line from the initial temperature at t=0 to some point on the graph at time t. (if I could post a picture I would). This picture would show that there is a large area that the straight line does not follow the curve to well. By changing the slope of the line (increasing the blast freezer capacity) we can better follow the curve in the initial cool down.

*Warning calculus next section*

To size a perfect coil to be able to accommodate the initial cooling rate is straight forward. Simply calculate the slope of the line at t=0...

*end calculus section*

Or simply 2.7 times your initial cooling rate. However this is GROSSLY over sized for the cooling rate at the end of the cooling cycle. Therefore we pick some happy medium between these two values, 1.35 or 1.5 and be done with it.


if you want to see a picture of a rough graph let me know and i can e-mail it to you.

[ref717]

You have to consider many factors in calculating the blast freezer heat load. Critical points to consider are:
1.) The product load(kgs.)
2.) Product initial temp. and final temp. required.
3.) Heat transmission load of your aircooler motors.
4.) Air speed and airflow inside the blast freezer.
Other heat loads are negligible such as lights,occupancy,wall transmission.Overloading a blast freezer would mean more time for cooling but still you have to consider a good freezer design.

[josei]

What is the right way of blast freezer heat load calculations.

Can some one throw more light on this.?

A light of Stoecker INDUSTRIAL REFRIGERATION HANDBOOK (Ch18), is this:

"LOAD CALCULATIONS IN PERSPECTIVE

Calculation of refrigeration loads is an inexact process, and because of this
fact, most designers simplify some of the procedures outlined in this chapter.
In making their short cuts, designers usually add generous safety factors
resulting in extra refrigeration capacity for the plant. While this strategy results
in a larger and more expensive refrigeration plant than initially required, this
extra capacity is usually put to good use later as the facility expands or an
unforseen load develops due to new products or processes.


This evaluation of load calculations should not suggest that sloppy
calculations are not without their peril. An observation is that the total plant
capacity provided by the load estimates of most competent designers is adequate,
even generous. Where errors are more likely to occur is in the calculation of an
individual load, resulting in the inability to maintain the desired temperature
in a certain space or achieve the production rate of a certain freezing process.

Experienced designers know that depending on the type of service (long-term
food storage, short-term food storage, freezing operations, etc.), one of the
components of the load will predominate. For example, the product load will be
significant in a freezing operation, infiltration might dominate in short-term
storage, while motors and lights are significant in processing areas.

Good designers intelligently use rules of thumb to check more-detailed
calculations." PS. Rules of thumb derived from experience !!