CHAPTER 3: THE CONSUMER'S HEAT LOAD
In the design and subsequent operation of a district heating system, the charac-
teristics of the load can be very significant. The load will not only dictate the
combination of supply temperature and mass flow rate necessary for its satisfaction,
but the heat exchanger equipment used at the consumer will also determine the
return temperature of the water. Lowering return temperature is desirable because
it results in larger temperature differences and thus lower mass flow rates, pumping
energy expenditure, and possibly smaller pipes. The importance of this issue is
evidenced by many district heating utilities in Europe having taken significant
actions to achieve large temperature differences. Thus, it is essential that our design
methodology for the distribution piping system account for the characteristics of the
consumer's load and the constraints that result.
The primary type of heat load for most district heating systems is space heating.
In some cases industrial process loads can also be significant. In most cases where
buildings rely on a district heating system for space heat, they also use the system
to heat hot water. Here we will develop simple models for space heating loads only.
SIMPLE MODEL FOR THE CONSUMER'S SPACE HEATING EQUIPMENT
As noted above, in addition to the maximum magnitude of the load placed on the
district heating system by the consumer, several other characteristics of the load are
important. The way in which the load varies is of primary importance. This was
discussed in Chapter 1 and will be addressed in more detail later. The other major
way in which the load affects heat distribution systems is through the response of
the consumer's heat exchanger to changes in supply temperature. To address this
issue, we need a model for the consumer's heat exchanger equipment. We will
develop such a model in this section.
In district heating systems using hot water, the water-to-air heat exchangers of the
consumers can either be directly connected to the network or indirectly coupled by
a heat exchanger. Each type of connection has its advantages and limitations. For the
sake of simplicity, we will assume that the buildings are directly connected in this
work. To address indirect systems, it would be necessary to either develop alternate
models or attempt to modify the model for a direct system developed below.
The normal radiator common on many residential and light commercial hydronic
heating systems can be classified as a cross flow heat exchanger with one of the fluids
mixed (water) and the other fluid (air) unmixed, as described by Kays and London
(1964). Although the term "radiator" is commonly used for these heat exchangers,
they function via both convective and radiative heat transfer within the temperature
ranges normally encountered in practice. A schematic representation of this cross
flow heat exchanger is shown in Figure 3.
Because the water is considered to be ideally mixed, its temperature is assumed
to be uniform in the direction of air flow at any point along the heat exchanger. As
the water moves through the heat exchanger, it varies from the supply temperature
Ts at the water inlet to the return temperature Tr at the outlet. The incoming air
temperature Ta is assumed to be constant along the length of the heat exchanger.
However, the outgoing air temperature Tao will vary along the length of the radiator
owing to the decline in water temperature.
Although it would be possible to describe the performance of a radiator using
traditional approaches, such as those described by Kays and London (1964), simpler
equations have been proposed. These are based on experimental results for such
heat exchangers, an example being the equation given by Bhm (1988)