Mechanical Load Calculation

Mechanical systems have a very important role in buildings, keeping indoor conditions that are healthy, comfortable and safe for humans. Some examples of mechanical equipment are boilers, furnaces, air conditioners, chillers, air handlers, cooling towers, fan coils, water heaters, booster pumps, fire pumps and automatic sprinklers.

Building mechanical systems use large amounts of energy and water, especially if they are oversized or lacking smart controls. Using larger equipment than necessary also increases project costs, without offering any benefits in return. However, mechanical engineers do not recommend undersized equipment either, since this can create unsuitable or dangerous conditions. More is not better in mechanical design, and the best performance is achieved when equipment capacity is optimized for the building.

Mechanical load calculation is a very important step of the building design process. When the capacity of all mechanical installations is optimized for the needs of your building, you can achieve high performance at an optimal cost.

• Consider that many mechanical components are heavy and bulky, and modifications can be expensive and difficult.
• By working with qualified mechanical engineers during the design phase, you can avoid expensive change orders during construction.

The entire mechanical design process depends on load calculations. Even if you use the highest quality equipment, performance will be poor if mechanical load calculations were inaccurate.

Pumping systems have many important roles in buildings, and the following are some examples:

• Water booster pumps ensure an adequate water supply pressure, when the pressure provided by the local water utility is not enough.
• Hydronic pumps circulate hot water or chilled water, when used as a heat transfer medium in HVAC systems.
• Fire pumps ensure an adequate water supply for fire protection systems during emergencies.

Regardless of the application, a pumping system must meet an important condition: providing the right pressure and flow. The suction pressure must also be high enough to prevent cavitation; microbubbles form and collapse with low pressure, wearing down the pump impeller and other components. The mechanical load calculation for building projects also includes the workload of pumps.

Even when pumps are sized properly, their workload varies depending on building conditions. For this reason, the full capacity of a pumping system is not used all the time.

• Energy consultants recommend using variable frequency drives (VFD), which can adjust pump speed according to workload. This is much more efficient than restricting flow with a valve at the pump outlet.
• A valve wastes pumping power in the form of pressure drop, while a VFD ensures that the pump gets the right power input in the first place.

While an accurate load calculation ensures the right pump capacity, a VFD uses that capacity as efficiently as possible.

Fans are the cornerstone of ventilation systems, establishing the airflow needed in the building. The fans in ventilation systems can be classified into two types, based on how they move air:

• Supply fans move air into the building, causing a positive pressurization.
• Exhaust fans remove air from the building, causing a negative pressurization.

The two fan types are used individually or in combination, depending on how the ventilation system is designed. Building codes require exhaust fans in rooms that produce plenty of air pollution, such as kitchens and restrooms.

The capacity of a ventilation system is specified in cubic feet per minute (CFM) or air changes per hour (ACH). When airflow is specified in CFM, the number does not depend on the space being ventilated, since cubic feet and minutes are both fixed units. However, the definition of ACH changes depending on indoor space dimensions. For example, 5 ACH in a large conference hall is a much higher airflow than 5 ACH in a small office.

ASHRAE provides two methods for calculating the mechanical ventilation load, the Ventilation Rate Procedure (VRP) and the Indoor Air Quality Procedure (IAQP).

The VRP uses airflow tables that have been determined experimentally by ASHRAE, finding the optimal ventilation rate for each type of indoor space. The VRP rates consider three main factors:

• Purpose of the ventilated space: Classroom, office, restaurant, warehouse, etc.
• Floor area: Assuming the same occupancy classification, a larger space needs more ventilation to preserve air quality.
• Number of occupants: A full office needs more ventilation than an equally-sized office that is only 25% full, for example.

The VRP does not require air pollutants to be monitored directly. ASHRAE has already taken care of that task, when finding the optimal airflow rates for each type of indoor space. However, ventilation systems designed with the VRP have a key limitation - they cannot respond to a sudden increase in air pollution.

The IAQP is based on controlling air pollutants directly, without using the prescriptive airflow values in the VRP. This provides more flexibility when designing the ventilation system, and the process can be summarized in three main steps:

• Identify all the air pollutants that can be expected in the building.
• Determine the maximum concentration of each pollutant, based on reliable sources like the WHO or US EPA.
• Specify a ventilation system that provides enough airflow to keep all pollutants below their respective thresholds.

Many building codes make the VRP mandatory, since it uses prescriptive airflow rates from ASHRAE, instead of the open design approach of the IAQP. However, both methods can be combined to design a ventilation system that meets building codes, while enhancing indoor air quality.

Fire protection systems are treated separately by building codes, and their design requirements are covered extensively by NFPA standards. However, many fire protection systems can be considered mechanical installations from an engineering standpoint. The following are some examples:

• Fire pumps
• Standpipes
• Automatic sprinklers
• Sprinkler booster pumps
• Smoke control systems
• Post-fire smoke purge systems

When other building systems malfunction, they normally cause inconvenience or disruption. On the other hand, if fire protection systems do not respond correctly, the consequences can include lost lives and massive property damage.

Mechanical load calculation is also important when designing fire protection systems. For example, fire pumps and sprinkler booster pumps must deliver enough pressure and flow during a fire emergency. At the same time, smoke control systems must have fans of enough capacity to remove smoke, especially in key areas like staircases and building exits.