Hi, please help I'm lost and confused on how to even begin to answer this question

[EmersonArguijo]

Can you calculate the refrigeration capacity in (kw) of a system that is required to freeze 3.5 tonnes of fresh lean beef. The beef is to be frozen within 8 hours. The beef enters the freezer pre-chilled at 4°c and the required storage temperature is -25°c.

The following information may help assist in your calculation.
(1) freezing point of beef is -1.7°c
(2) 'c' = specific heat capacity for beef (above freezing) is 3.22kJ/Kg K
(3) 'c' = specific heat capacity for beef (below freezing) is 1.67 kJ/kg K
(4) LH = Latent heat of fusion of beef is 233 kJ/kg

 

[Deleted member 4993]

Can you calculate the refrigeration capacity in (kw) of a system that is required to freeze 3.5 tonnes of fresh lean beef. The beef is to be frozen within 8 hours. The beef enters the freezer pre-chilled at 4°c and the required storage temperature is -25°c.

The following information may help assist in your calculation.
(1) freezing point of beef is -1.7°c
(2) 'c' = specific heat capacity for beef (above freezing) is 3.22kJ/Kg K
(3) 'c' = specific heat capacity for beef (below freezing) is 1.67 kJ/kg K
(4) LH = Latent heat of fusion of beef is 233 kJ/kg

How much "energy" ( E₁)will be needed to bring the beef to -1.7^oC?

Use E₁ = m * c * [4 - (-1.7)]

Where m = mass and c = corresponding specific heat
..... and continue.....

 

[EmersonArguijo]

Okay so this is what i have managed to answer, please tell me if I'm on the right track and thank you for helping me begin.

1) Q = M x C x ΔT
= 3.5t x 1.67kJ/kg K x 4°c - ( -1.7°c)
= 25.08 (not sure what symbol to use here)

2) Q = M x LH
= 3.5t x 233kJ/kg
= 815kJ/kg

3) Q = Q1 + Q2 + Q3
= 25.08 + 815 + ??

I don't follow what Q3 is to complete the task.

 

[Deleted member 4993]

Okay so this is what i have managed to answer, please tell me if I'm on the right track and thank you for helping me begin.

1) Q = M x C x ΔT
= 3.5t x 1.67kJ/kg K x 4°c - ( -1.7°c)
= 25.08 (not sure what symbol to use here)

2) Q = M x LH
= 3.5t x 233kJ/kg
= 815kJ/kg

3) Q = Q1 + Q2 + Q3
= 25.08 + 815 + ??

I don't follow what Q3 is to complete the task.

Mass of beef = 3.5 Ton = 7000 kg
ΔT = 4°c - ( -1.7°c) = 5.7°C = 5.7 K
C = 3.22 kJ/(Kg K )

Q1 = 3500 * 5.7 * 3.22 kg * K * kJ/(kg K ) =64239 kJ ..................................... edited

Q3 calculation would follow the exact same equation as Q1 - but the numbers will be different.

Correct your calculation of Q2 including correct mass.

Continue....

 

[EmersonArguijo]

Can you explain how 3.5t = 7000kg

I've by some miracle understood everything else besides that. Thank you again.

 

[ksdhart2]

Can you explain how 3.5t = 7000kg

I've by some miracle understood everything else besides that. Thank you again.

I suspect that's an error on Subhotosh Khan's part, confusing the definition of ton. If we assume the "ton" in question is a short ton, then we have:

\(\displaystyle 1 \: \text{ton} = 2000 \: \text{lbs} \approx 907.185 \: \text{kg}\)

On the other hand, if we assume it's a long ton (sometimes also called a metric ton or tonne), then we have:

\(\displaystyle 1 \: \text{ton} = 1000 \: \text{kg} \approx 2204.62 \: \text{lbs}\)

However, in neither case is it true that a ton is 2000 kilograms.

 

[JeffM]

I suspect that's an error on Subhotosh Khan's part, confusing the definition of ton. If we assume the "ton" in question is a short ton, then we have:

\(\displaystyle 1 \: \text{ton} = 2000 \: \text{lbs} \approx 907.185 \: \text{kg}\)

On the other hand, if we assume it's a long ton (sometimes also called a metric ton or tonne), then we have:

\(\displaystyle 1 \: \text{ton} = 1000 \: \text{kg} \approx 2204.62 \: \text{lbs}\)

However, in neither case is it true that a ton is 2000 kilograms.

Except a long ton is not the metric tonne. A long ton is 2240 pounds, which is 20 times 112 pounds, which was called a "hundred weight" and equaled 8 stones. The history of weights and measures and their names is comprehensible only if you realize that standardization of weights and measures is a relatively recent development.

The

 

[Deleted member 4993]

Except a long ton is not the metric tonne. A long ton is 2240 pounds, which is 20 times 112 pounds, which was called a "hundred weight" and equaled 8 stones. The history of weights and measures and their names is comprehensible only if you realize that standardization of weights and measures is a relatively recent development.

The

You are correct - I was wrong.

1 Ton = 2204 lbs ~ 1000 kg

That is what I meant to say.....

 

[EmersonArguijo]

So far I have:

Q = M × C x ΔT

= 3500 x 3.22 x 5.7
Q1 = 64,239

= m x LH
= 3500 x 233
Q2 = 815,500

= 3500 x 1.67 x 23.3
Q3 = 136,188.5

Q1 + Q2 + Q3
= 64, 239 + 815,500 + 136,188.5
= 1,015,927.5


What have i miss is it time? And how do I do that

 

[Deleted member 4993]

So far I have:

Q = M × C x ΔT

= 3500 x 3.22 x 5.7
Q1 = 64,239

= m x LH
= 3500 x 233
Q2 = 815,500

= 3500 x 1.67 x 23.3
Q3 = 136,188.5

Q1 + Q2 + Q3
= 64, 239 + 815,500 + 136,188.5
= 1,015,927.5


What have i miss is it time? And how do I do that

What "physical quantity" you did calculate when you calculated Q1, Q2 and Q3?

What is the unit of Q1, Q2 and Q3?

What is the unit of Q1 + Q2 + Q3?