Jan Stoeken reveals why it’s worth giving proper consideration to HVAC
If not thought about, HVAC can be a highly disruptive issue on a project. HVAC for offshore and industry comprises more than just air conditioning. Other aspects such as dilution of explosive and toxic gases, smoke removal in case of fire, ventilation of escape routes, heat removal of E&I rooms under emergency conditions and heat recovery is also part of HVAC engineering.
Developing HVAC begins with inventory of the applicable regulations, client standards, room requirements and internal and external heat loads. Although HVAC is based on physics and human metabolism, the different regulations and standards result in various designs. This already starts with the minimum fresh air ventilation rates per room function, which determines the basis for most designs. The minimum ventilation rates are increased if required for dilution of gases or odours.
The external heat loads due to local climate or the client’s imposed outdoor design conditions combined with the air flow define the capacity of the air handling units and connected utilities. Keep in mind that capacity and equipment size are directly related and that the possibility for reducing the size of HVAC equipment is therefore directly related to the applied regulations and standards.
High internal heat emissions are best removed by local cooling equipment, but whether this is allowed depends on the project specifications. If it is not allowed, increased supply airflows and consequently large ducting will be necessary. Especially for engine rooms, it’s usually a lively discussion.
It’s the task of HVAC engineers to define a solid design basis that takes into account all the abovementioned factors.
Heating or cooling of the fresh airflow to the desired supply air temperature is an energy-consuming process. In modern designs the use of waste heat is common practise. Energy for heating is obtained from main power generation (gas turbines or diesel generators) with electrical back-up in case the main power generation fails or operates on partial load. The feasibility of using waste heat is depending on the operating philosophy of the platform and the required energy for heating. Especially in harsh climates, additional heat recovery systems could be necessary.
There are more (efficient) heat recovery systems available, also for cooling, which is common practise in onshore commercial buildings, but the reliability in offshore applications is not yet demonstrated. Safety, reliability and proven design are keywords in equipment selection, resulting in a conservative approach that is not directly in favour of relatively new and more energy efficient solutions.
To avoid excessive overcapacities in HVAC equipment design, a thermal building simulation program is in place, using design climate files based on real climate data in combination with an accurate 3D model (the disadvantage of oversized HVAC equipment is unnecessary space consumption, a not optimal capacity control and unnecessary investment costs).
The program is used to establish equipment capacities and to investigate the thermal behaviour of rooms under specific circumstances. For example, what is the temperature curve in UPS and battery rooms during failed or stopped ventilation/cooling and what is the time period left for intervention?
A new development is integrating HVAC calculation software with standard 3D model software. Present focus is on integrating the thermal simulation software and duct pressure drop and noise calculation with Revit MEP.
For more information visit www.engineerlive.com/iog
Jan Stoeken is with PDE in the Netherlands
