temp difference between inlet and outlet ?

Question

WAHED REHAN MOHAMMED ABDUL · Answer

The recommended outlet temperature from outlet of AHU/FCU should be- deg C less than the room temperature. This is common standard value used in calculation. Based on the below temperatures, the recommended supply temperature from AHU/FCU is- deg C.

Basharat Ali · Answer

normally,Chilled water In let temperature should be Fahrenheit,And out let of chilled water should be Fahrenheit.

Sherif Mohammed Ibrahim · Answer

-Temperature difference for chilled water is9.6 to F.

Ismael Hamad · Answer

To define the effectiveness of a heat exchanger we need to find the maximum possible heat transfer that can be hypothetically achieved in a counter-flow heat exchanger of infinite length. Therefore one fluid will experience the maximum possible temperature difference, which is the difference of $\ T_{h,i}- \ T_{c,i}$ (The temperature difference between the inlet temperature of the hot stream and the inlet temperature of the cold stream). The method proceeds by calculating the heat capacity rates (i.e. mass flow rate multiplied by specific heat) $\ C_h$ and $\ C_c$ for the hot and cold fluids respectively, and denoting the smaller one as $\ C_{min}$ .

A quantity:

$q_{max}\ = C_{min} (T_{h,i}-T_{c,i})$

is then found,where $\ q_{max}$ is the maximum heat that could be transferred between the fluids per unit time. $\ C_{min}$ must be used as it is the fluid with the lowest heat capacity rate that would, in this hypothetical infinite length exchanger, actually undergo the maximum possible temperature change. The other fluid would change temperature more slowly along the heat exchanger length. The method, at this point, is concerned only with the fluid undergoing the maximum temperature change.

The effectiveness(E), is the ratio between the actual heat transfer rate and the maximum possible heat transfer rate:

$E \ = \frac{q}{q_{max}}$

where:

$q \ = C_h (T_{h,i} -T_{h,o})\ = C_c (T_{c,o} - T_{c,i})$

Effectiveness is dimensionless quantity between0 and1. If we know E for a particular heat exchanger, and we know the inlet conditions of the two flow streams we can calculate the amount of heat being transferred between the fluids by:

$q \ = E C_{min} (T_{h,i} -T_{c,i})$

For any heat exchanger it can be shown that:

$\ E = f ( NTU,\frac{C_{min}} {C_{max}})$

For a given geometry, $\ E$ can be calculated using correlations in terms of the "heat capacity ratio"

$C_r \ = \frac{C_{min}}{C_{max}}$

and the number of transfer units, $\ NTU$

$NTU \ = \frac{U A}{C_{min}}$ where $\ U$ is the overall heat transfer coefficient and $\ A$ is the heat transfer area.

For example, the effectiveness of a parallel flow heat exchanger is calculated with:

$E \ = \frac {1 - \exp[-NTU(1 + C_{r})]}{1 + C_{r}}$

Or the effectiveness of a counter-current flow heat exchanger is calculated with:

$E \ = \frac {1 - \exp[-NTU(1 - C_{r})]}{1 - C_{r}\exp[-NTU(1 - C_{r})]}$

For $C_r \ =1$

$E\ = \frac{NTU}{1+NTU}$

Similar effectiveness relationships can be derived for concentric tube heat exchangers and shell and tube heat exchangers. These relationships are differentiated from one another depending on the type of the flow (counter-current, concurrent, or cross flow), the number of passes (in shell and tube exchangers) and whether a flow stream is mixed or unmixed.

Note that the $C_r \ =0$ is a special case in which phase change condensation or evaporation is occurring in the heat exchanger. Hence in this special case the heat exchanger behavior is independent of the flow arrangement. Therefore the effectiveness is given by:

$E \ =1 - \exp[-NTU]$

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