Cooling Load - Basics




Cooling load calculations are typically one of the first calculations completed by the HVAC and Refrigeration engineer. These calculations serve as the basis for determining air conditioning equipment sizes. In order to determine the mechanical equipment sizes, the engineer must first determine what heat is being transferred into the building and what heat is being transferred out of the building. The summation of the heat gained and lost by the building will determine the size of the air conditioning equipment.

The various heat gains and losses into a building can be characterized as either external or internal loads. External loads include the conduction and radiation heat loads transferred through roofs, walls, skylights and windows. In addition, outside air can be brought into a building through ventilation requirements or infiltration, which will cause a load upon the system. Internal loads include heat loads from people, both latent and sensible, loads from lighting and miscellaneous equipment like computers, televisions, motors, etc.

Cooling Load Basics for the Mechanical PE Exam

The various heat gains can also be organized into sensible and latent heat gains. Sensible heat gains are those characterized by only a change in temperature and no change in state. Latent heat gains are those characterized by moisture gains. It is important to note that in the table below, that ventilation, infiltration, people and miscellaneous equipment both have sensible and latent heat gains. These individual heat gains are discussed thoroughly in the following sections.

Cooling Load Basics for the Mechanical PE Exam

THERMAL MASS and TIME LAG FACTOR

When completing load calculations it is important to understand the time lag factor. When the sun shines upon a wall face early in the morning, although the wall does experience a heat load, the amount of heat load experienced IN the building at that time is minimal. This is due to the thermal mass of the wall. Thermal mass is also known as heat capacity and is defined as the ability of a material to absorb heat.

The use of thermal mass is shown in buildings that have high thermal mass walls that absorb heat during the day, store the heat during occupied periods and release the heat during the night when it is cool.

UNCERTAINTY

Calculating heat gains and determining cooling loads has very high uncertainty. This is because of the many assumptions that must be made like occupant loads, occupant, schedules, outdoor weather conditions, equipment schedules and heat gains, etc. The engineer should recognize that the following calculations are not the most accurate ways to calculate cooling load and are only shown to highlight concepts that could be tested on the professional engineering exam. There are multiple methods used to calculate cooling load calculations like the Radiant Time Series, Total Equivalent Time Difference and the CLTD/SCL/CLF methods. The CLTD/SCL/CLF method is shown in this section because it is the most practical method that can be tested without a computer and in a relatively short period of time (4-hours, 6 minutes per problem).