Your little distribution outlet in Seattle is going to insta

Your little distribution outlet in Seattle is going to install a new 3 ton (38 inches x 47 inches x 40 inches) refrigeration unit to cool your walk in freezer room. It will be installed (lifted somehow) on the roof of the building above the freezer unit. You have already braced the roof for the weight. The unit uses 410A refrigerant and has a COP of 3.5. The freezer room itself is fairly well insulated but one wall is located against the side of the building where the building gets morning sunshine. You receive ice cream five days a week, sometimes from one and sometimes from two (semi) trucks a day. Generally you keep the freezer at –10°F though that can vary. You distribute the ice cream to 7-11’s and other small outlets with your own trucks throughout the day (you have 4 small trucks). The freezer door is typically left open during busy times to ease entrance and exits but it plastic strip curtains to try and keep the cold air in.

a) Estimate your daily electrical cost to run the new unit.

b) What \"asumptions\" did you have to make?

Solution

Let us assume the refrigeration unit runs on a reversed Carnot cycle.

COP of the unit = 3.5

COP of the unit = Work done / Heat extraced

So Work done by the unit = Heat Extraced * COP = 3.5*3*3.5 kW= 36.75 kW

Assuming 1 Ton of refrigeration = 3.5 kW

Now the Freezer works 5 days a week.We can assume it runs for 18 hrs a day.

Assuming the freezer works without fail for a 4 week month

Also assuming the electricity cost is $1 per unit

No of hours the freezer unit works= 18*5*4 hours= 360 hr

Eneregy consumed by the freezer in a month = 36.75*360 = 13230 kWh =13230 units

So the cost of running the freezer unit is $13230 / month