Fluid To Fluid Heat Exchanger

 

HeatExchanger:FluidToFluid

 

Used in:

 

  • Plant and condenser loops, supply and demand side

 

The fluid-to-fluid heat exchanger component (aka heat exchanger or HX) is designed to couple the supply side of one plant or condenser loop to the demand side of another plant or condenser loop. The heat exchanger is a fairly general component and can be configured for use in a situation where any two loops need to be connected together. The only constraints are that one side must be connected to the supply side of one loop and the other side connected to the demand side of a different loop.

 

Plant Fluid to Fluid Heat Exchanger

Note: The adopted convention is that the heat exchanger must always be placed in a supply sub-loop.

Because the heat exchanger is intended to be generic, its two sides are distinguished by the nature of the loop side being connected. One side is called the “Loop Supply Side” to indicate that side of the heat exchanger connected to the supply side of a loop. The other side is called the “Loop Demand Side” to indicate it is connected to the demand side of a loop. The heat exchanger is intended to act as a supply component for the loop connected to it as the “Loop Supply Side” and as a demand component for the loop connected to it as the “Loop Demand Side”. From the point of view of the heat exchanger model itself, the Loop Demand Side fluid serves as the source/sink to supply heating/cooling to the fluid in the Loop Supply Side. The “Loop Supply Side” or “Loop Demand Side” connections of the heat exchanger are automatically enabled and disabled depending on which sub-loop is currently active to facilitate correct connection, i.e. to connect the supply side of the heat exchanger you must first navigate to the supply sub-loop in which the heat exchanger has been located and similarly navigate to the demand sub-loop of the loop to which the demand side of the heat exchanger is to be connected.

 

Various options are available for the heat exchanger component. The heat transfer between the two sides can be modelled using one of seven different models for different types of heat exchangers. Heat transfer is calculated and reported using a sign convention from chilled water applications where positive heat transfer indicates energy was extracted from the Loop Supply Side and added to the Loop Demand Side. A range of options are available for controlling the heat exchanger. One general type of control is “On/off” where the flow through the heat exchanger is either fully on or fully off. Another type of control is “Modulated” where the flow through the Loop Demand Side is controlled to try and meet a target setpoint or load on the Loop Supply Side.

 

General

Name

Enter a unique identifying name for this heat exchanger.

Operation

Availability schedule

Select an availability schedule to be used for supervisory control of the device. The heat exchanger is available for use whenever the schedule value is greater than zero. The heat exchanger is not available whenever the schedule value is zero or less.

Heat Exchanger Settings

Loop demand side design flow rate

This field specifies the design flow rate (in m3/s or gal/min) of the hydronic fluid passing through the heat exchanger on the Loop Demand Side. This field is autosizable. When autosized, this design flow rate is set to equal the design flow rate for the Loop Supply Side.

Loop supply side design flow rate

This field specifies the design flow rate (in m3/s or gal/min) of the fluid passing through the heat exchanger on the Loop Supply Side. This field is autosizable. When autosized, this design flow rate is set equal to the overall design flow rate of the loop that is connected. If a sizing factor is entered, then it is used to multiply this design flow rate.

Heat exchange model type

This alpha field identifies the nature of heat exchanger. Heat exchanger model type is specified by one of the following four key word choices:

 

Heat exchanger U-Factor times area value

This autosizable numerical field is used to specify the overall U-Factor Times Area (UA) (in W/K or Btu/h-°F) for use in the calculation of the heat exchanger effectiveness using the appropriate D-NTU correlation. If 7-Ideal is specified as the heat exchanger type, the effectiveness will be set to 1.0. When set to Autosize Heat Exchanger U-Factor Times Area Value is calculated based on an effectiveness of 1.0 where capacity is such that the temperatures in the plant sizing for the two loops can be maintained.

Control type

This option is used to specify how the heat exchanger is to be controlled during operation. Different applications for connecting two loops will require different control behavior and different control options are needed depending on the desired behaviour. There are a range of options to choose from:

 

Minimum temperature difference to activate heat exchanger

This value specifies the temperature tolerance used in control decisions in (DeltaC or DeltaF). Whenever the control logic needs to compare two temperatures, the value entered in this field is used as a threshold for comparisons.

Heat transfer metering end use type

This option specifies how the metering for heat transfer will be accounted in summary reports with respect to end uses. Although the heat exchanger consumes no energy that needs to be metered, there are also meters for heat transfers that apply to this heat exchanger model. The nature of the end use may vary depending on the application. The available choices are:

 

Temperature Limits

Operation minimum temperature limit

When the Minimum temperature limit checkbox is checked this field is used to provide supervisory control of the heat exchanger. If either of the inlet temperatures are below this limit (°C or °F), the heat exchanger will not operate.

Operation maximum temperature limit

When the Maximum temperature limit checkbox is checked this field is used to provide supervisory control of the heat exchanger. If either of the inlet temperatures are above this limit (°C or °F), the heat exchanger will not operate.

Sizing Factor

Sizing factor

When the Sizing factor checkbox is checked this field can be used to modify the results of autosize calculations. This sizing factor is used for this heat exchanger and modifies sizing results by multiplying them by the factor entered here. This factor is applied to the Loop Supply Side Design Flow rate and in turn affects the heat exchanger UA and the loop demand side flow rate which are derived from that flow rate. This allows fine control over the size of the heat exchanger when using autosize in those fields.