Approach Temperature

[jayaeros]

Hi all,

I have 15 kW scroll chiller (R22) with shell and tube configuration on the heat exchangers.

My refrigerant properties at the condenser inlet and outlet as following:-

a) Cond in Temp = 80.6 deg C
Cond in Pressure = 15 barg (T sat = 41.8 C)
b) Cond out Temp = 40.9 deg C
Cond out Pressure = 14.9 barg (T sat = 41.6 C)

While the cooling water inlet and outlet temperature are 29.3 C and 34.7 C respectively with a flow rate of 50.5 Lpm

From this, the approach temperature is 41.6-34.7 = 6.9 deg C.

Why is the approach temperature is too high? Is this acceptable? Please correct if im wrong.

Please comment.

Regards,
jaya

[Gary]

From this, the approach temperature is 41.6-34.7 = 6.9 deg C.

Why is the approach temperature is too high? Is this acceptable? Please correct if im wrong.

The approach temperature should be no more than 11C. Your approach at 6.9C is not too high.

[jayaeros]

Thanks for the reply. I was thinking all this while that approach temperature should be very low; less than 2 degree C.

I have also calculated that the effectiveness of the condenser

Effectiveness = (T Cool water out - T Cool Water in) / (TCond sat-TCool W in)

Effectiveness = (34.7-29.3) / (41.6-29.3) = 0.44

I believe that effectiveness can be increased if the approach temperature is much lower. Thus, indicating the need for the sub-cooling and hence the condenser is actually undersized.

Please correct if my understanding is wrong.

Thanks

[Gary]

There is a difference between questioning what is acceptable and what is optimum.

What is the intended purpose of this system? Optimum cooling in the evaporator? Optimum heating in the condenser? Both?

No doubt lowering the approach will increase the effectiveness, but at what price? Where is the point of diminishing returns?

[Gary]

Looking through your other thread it seems that you are modifying parts of the system without considering the effect the modifications have on other parts of the system. You need to look at the system as a whole.

Start here:

http://www.refrigeration-engineer.co...ad.php?t=19701

[jayaeros]

The system is intended for optimum cooling. Actually, i am trying to find the performance of the system, like internal entropy generation, heat transfer coefficient and etc.

[Gary]

First you need to get the system working right.