Heat Exchangers¶

This section contains supplementary information for the section on heat exchangers in the component guide, see Exchanger.

Calculation Method¶

In TAS Systems, there are three options to calculate the maximum rate of heat transfer for the exchanger:

Efficiency

NTU Method

Duty

Efficiency¶

Upon choosing this option the user is asked to enter the sensible efficiency of the exchanger. The sensible efficiency of the exchanger is calculated by the following formula when the supply air’s flow rate is equal to the exhaust air’s flow rate:

\[\text{Sensible Efficiency} = \frac{T_{\text{supply}} - T_{\text{fresh}}}{T_{\text{return}} - T_{\text{fresh}}}\]

Where \(T_{\text{supply}}\) is the temperature of the supply air immediately after going through the exchanger, \(T_{\text{return}}\) is the temperature of the return air before entering the exchanger and \(T_{\text{fresh}}\) is the temperature of the supply air before entering the exchanger.

In the rare cases where the exchanger is used with the air entering it at the top and bottom at different flow rates, the efficiency is worked out with the following equation:

\[\text{Efficiency} = \frac{(T_{\text{aft. f1}} - T_{\text{bef. f1}})Q_{\text{f1}}}{(T_{\text{bef. f2}}-T_{\text{bef. f1}})\times{\min{(Q_{\text{f1}},Q_{\text{f2}})}}}\]

Where \(T_{\text{aft.}}\) is the temperature of the air exiting the exchanger on the selected air flow, \(T_{\text{bef.}}\) is the temperature of the air entering the exchanger on the selected air flow, \(Q\) is the flow rate of the selected air flow, \(\min{(x,y)}\) means the smaller of the two values and the subscript \(\text{f1}\) and \(\text{f2}\) relate to whether it is the first or second air stream this value should be taken from.

This equation will sometimes give you a negative efficiency, which would be caused in this case by the air stream \(f1\) transferring heat to the air stream \(f2\). The efficiency entered into the efficiency field should always be the absolute value.

In the Sensible Efficiency Tab, you can add a modifier to the Sensible Efficiency.

NTU Method¶

Upon choosing this method, the user will need to input:

The heat transfer surface area

The heat transfer coefficient

The Exchanger type

After entering these details, TAS will then work out the rate of heat transfer of the exchanger.

Duty¶

This option models a heat exchanger that has a built-in heat pump that is used to transfer heat from one air stream to the other. As there is a built-in heat pump, the user will need to provide the Duty and the Bypass Factor.

Duty¶

The duty of a component is the upper limit on the amount of power a component can provide. If, in a certain hour, the power demand on the component is greater than the duty of the component, the component will not be able to meet this demand.

For the heat pump in the heat exchanger this would mean it would no longer exchange heat between the two air streams. In TAS Systems, the demand (or load) met by a component is reported for each hour in the results section.

In the Duty tab, you will be able to choose these 3 options as well, but with the sized and value options you will be able to add a modifier.

Bypass Factor¶

The Bypass Factor field determines the amount of air that will bypass the heat pump.

The value is entered as a factor between 0 and 1 and this factor is then multiplied against the flow rate of the air going through the exchanger.

Modifiers can be added to the bypass factor using the Bypass Factor tab.

Latent Method¶

With the Latent method field, you are provided with two options to calculate the latent energy transfer of the exchanger; HumRat and Enthalpy.

HumRat¶

Upon choosing this option, the user is asked to enter a latent efficiency. The latent efficiency is calculated by the following formula when the supply air’s flow rate is equal to the exhaust air’s flow rate:

\[\text{Latent Efficiency} = \frac{\text{Supply HR} - \text{Fresh Air HR}}{\text{Return HR} - \text{Fresh Air HR}}\]

Where \(\text{Supply HR}\) is the humidity ratio of the supply air immediately after going through the exchanger, \(\text{Return HR}\) is the return humidity ratio of the return air before entering the exchanger and the \(\text{Fresh Air HR}\) is the humidity ratio of the supply air before entering the exchanger.

In the rare cases where the exchanger is used with the air entering it at the top and bottom at different flow rates, the latent efficiency is worked out with the following equation:

\[\text{Efficiency} = \frac{(\text{HR}_{\text{aft. f1}} - \text{HR}_{\text{bef. f1}})Q_{\text{f1}}}{(\text{HR}_{\text{bef. f2}}-\text{HR}_{\text{bef. f1}})\times{\min{(Q_{\text{f1}},Q_{\text{f2}})}}}\]

Where \(\text{HR}_{\text{aft}}\) is the humidity ratio of the air exiting the exchanger on the selected air flow, \(\text{HR}_{\text{bef}}\) is the humidity ratio of the fluid entering the exchanger on the selected air flow, \(Q\) is the flow rate of the selected air flow, Min(x ,y) means to take the minimum value of the two entries, and the subscript \(f1\) and \(f2\) relates as to whether it is the first or second air stream this value should be taken from.

Please note that this equation will sometimes give you a negative efficiency, which would be caused in this case by the air stream \(f1\) transferring heat to the air stream \(f2\).

The efficiency entered into the efficiency field should always be the absolute value.

In the Latent Efficiency tab you can add a modifier to the Latent Efficiency.

Enthalpy¶

Upon choosing this option, the user is asked to enter enthalpy efficiency. The enthalpy efficiency is calculated using the following formula when the supply air’s flow rate is equal to the exhaust air’s flow rate:

\[\text{Enthalpy Efficiency} = \frac{H_{\text{supply}} - H_{\text{fresh}}}{H_{\text{return}} - H_{\text{fresh}}}\]

Where \(H_{\text{supply}}\) is the enthalpy of the supply air immediately after going through the exchanger, \(H_{\text{return}}\) is the enthalpy of the return air before entering the exchanger and \(H_{\text{fresh}}\) is the enthalpy of the supply air before entering the exchanger.

In the rare cases where the exchanger is used with the air entering it at the top and bottom at different flow rates, the latent efficiency is worked out with the following equation:

\[\text{Efficiency} = \frac{(H_{\text{aft. f1}} - H_{\text{bef. f1}})Q_{\text{f1}}}{(H_{\text{bef. f2}}-H_{\text{bef. f1}})\times{\min{(Q_{\text{f1}},Q_{\text{f2}})}}}\]

Where \(H_{\text{aft}}\) is the humidity ratio of the air exiting the exchanger on the selected air flow, \(H_{\text{bef}}\) is the humidity ratio of the fluid entering the exchanger on the selected air flow, \(Q\) is the flow rate of the selected air flow, Min(x ,y) means to take the minimum value of the two entries, and the subscript \(f1\) and \(f2\) relates as to whether it is the first or second air stream this value should be taken from.

Please note that this equation will sometimes give you a negative efficiency, which would be caused in this case by the air stream \(f1\) transferring heat to the air stream \(f2\).

The efficiency entered into the efficiency field should always be the absolute value.

In the Enthalpy Efficiency tab you can add a modifier to the Latent Efficiency.