About Delta T

[tjk2007]

Kindly suggest for my queries

System : Cold Room
Room Temp :0ºC

Total Heat Load : 42 KW
No of Evaporators : 2
Evaporator capacity: 21 KW each
Secondary Coolant :35% Prop.Glycol
Glycol Inlet :-8ºC
Glycol Outlet :-4ºC
Flow rate :9.4 m3 /Hr. for a single unit.

Query:

• Here the ▲T = 4ºC is common. Suppose I make the Inlet /outlet temp.like -10/-6ºC (OR) -1 / -5ºC what will be the result to the room temperature? How can I justify while selecting the equipment?

• Considering 7 cold rooms to the same capacity & same parameters the number of evaporators is 14. I have to store the glycol in a tank and rechilling it thro PHE. How can I calculate the flow rate for PHE?

Waiting for your suggestions.

[Peter_1]

You can say that your capacity will drop +/- linear with the DT.
For me, your DT is 8K (room at 0°C and glycol entering at -8°C)
Now, you will enter at -5°C.
So a drop of +/- 37%
Q= k.A.DT (k and A remains the same)

Flow rate, is it not just 9.4/unit x 14 units?
Evaporating at 5 to 7 K lower then the desired outlet temperature of -8°C (so -13°C to -15°C)
You will need software to calculate this because each manufacturer can tell you what capacity you will have with a specific glycol mixture. Don't forget also that the viscosity will increase with lowering the temperature.

You will need bigger pumps.
Pressure drop will also increase due to the large mass flow of 14 units