By Pieter de Bod, Pr. Eng. LEED BD+C
WSP prides itself to have been involved with the successful opening of a new healthcare facility, a medical office building (MOB) in Mansfield, Texas, US.
The facility is conveniently located for patients in a prime location with easy access from main local routes. Mansfield is a suburb approximately 25 miles southwest of Dallas.
The owner of the building is a healthcare group and is one of the largest faith-based, nonprofit health systems in the United States (US), and the largest in North Texas in terms of inpatients and outpatients served. The healthcare group is also affiliated with several organisations — from all aspects of the health care industry — to better serve the more than 7 million residents of North Texas.
The 7 440m2 (80 000ft2) greenfields project features four floors of outpatients’ clinics and offices specifically constructed and designed for use by physicians and other health care personnel to provide healthcare services to their patients. Medical office buildings or day clinics have elements or design principles intended to improve patient outcomes and enhance the patient experience, whilst patients do not sleep over at the facility. Facilities where patients sleep over are referred to as inpatient facilities.
As the life expectancy of people increases and their chronic conditions naturally escalate, it places a higher demand on comprehensive care for the ageing patients, hence the need for more in and outpatient facilities like hospitals and day clinics respectively.
Modern healthcare is endlessly evolving, so practitioners are best suited to facilities that allow those healthcare changes to occur seamlessly. With the Covid-19 pandemic continuing to cause disorder, healthcare facilities globally are scrambling to use all available technology to increase patient and healthcare safety. Therefore, medical office buildings should have the infrastructure in place to meet these evolving needs.
Physicians, their staff, and patients require a pleasant working environment for optimal comfort and job functionality. It is therefore crucial that this new facility had a fully functional HVAC system.
Not only do clients have a high expectation of a well-functioning HVAC system installation, but they are also involved from the start of the project, providing their inputs, and making key decisions. Working with educated clients (like this client), is always appreciated.
WSP was appointed as the MEPF (Mechanical, Electrical, Plumbing and Fire) consultants on this project. Construction started in summer of 2019 and the building was occupied in winter of 2020.
The HVAC system
The new air-conditioning, heating and ventilation system comprises of four large Daikin Variable Air Volume (VAV) Direct Expansion (DX) Rooftop Package Units (RTPUs) located on the roof of the building. The building is divided into four different thermal zones: – two perimeter zones and two internal zones. Each zone will be served by a separate RTPU because the temperature profile for perimeter and internal zones are different. For example, the south perimeter zone may require more cooling during the afternoon when solar influx typically peaks, while the north perimeter zone may require less cooling.
Each RTPU serves all four levels and assigned to serve the following zones:
|RTPU-1||103 kW cooling (354 MBH)||Serving the north perimeter zone.|
|RTPU-2||214 kW cooling (731 MBH)||Serving internal zone 1.|
|RTPU-3||198 kW cooling (677 MBH)||Serving internal zone 2.|
|RTPU-4||145 kW cooling (497 MBH)||Serving the south perimeter zone.|
Each rooftop unit comprises of supply and return air fans with volumetric fan tracking, Merv 8 and Merv 14 filters, multiple scroll compressors, dual R410 refrigerant circuits, condenser fans and coils, and the capability to operate at -17.8°C (0°F). The return fan speed tracks up and down with the supply fan speed.
Each RTPU is also equipped with a Microtech Unit Controller and a BACnet communication interface. The rooftop units are mounted on matching 914mm (36 inch) high flat roof curbs.
The RTPUs do not have humidification devices.
Medium pressure supply and return ductwork is routed from the RTPU on the roof to a riser shaft where the ducting will drop down vertically in shafts and then branches off to main duct runs on every floor to serve single duct Variable Air Volume air terminals (VAV ‘boxes’). The VAV boxes are designed to have minimal pressure drop and low noise levels. The onboard controller of each VAV box controls the airflow rate of conditioned air into an occupied space in response to a thermostat control signal.
The incorporation of a multipoint crossflow airflow sensor allows the VAV box to compensate instantly for any change in supply air pressure that might alter the supply volume resulting in a pressure independent variable air volume system. Electrical reheat coils in the VAV box are factory mounted and ETL listed to meet electrical safety standards including complying with CSA 236/UL.
The electric heaters are a side-mounted and slide-in style, specifically designed to mount in the extended terminal casing allowing for easy field maintenance. To ensure safe operation, these electric heaters use thermal safety switches and low watt density heating elements to minimise hot spots. The control damper features a double gasket.
Galvanised steel medium pressure ductwork is constructed to a static pressure class of 1000Pa (4” water gauge). The ductwork from the VAV ‘boxes’ are low pressure galvanised steel ductwork constructed to a static pressure class of 250Pa (1” water gauge). All supply and return ductwork are externally insulated and constructed in accordance with Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) Standards for appropriate pressure class.
Various exhaust fans are installed on the roof. One exhaust fan is installed for the air extraction of the core toilets and janitor rooms, while four exhaust fans are provided for the extraction of the tenant areas. Each fan is connected to a variable frequency drive/variable speed drive. The type of exhaust fans used are centrifugal vent set fans with bolted and continuously welded steel construction, electrostatic baked polyester powder coating, fan housing with access door and drain, flat blade fan wheel, OSHA belt guard and weather cover, extended lube lines, inlet flex duct connection, steel isolation rails and spring isolators, non-fused disconnect switch.
During summer months when ambient temperature may reach more than 40°C (104°F) it is vitally important to ensure optimal air circulation over the air-cooled condenser coils in the rooftop units. The perforated screen wall and opening below of screen wall improves airflow.
Heating cables can be overlapped without overheating or burnouts. The heat tracing system is controlled by standalone controller and connected to an ambient-sensing thermostat housed in a NEMA 4X polyurethane-coated cast aluminium housing with stainless steel hardware. Warning labels are installed for safety reasons.
It is fairly standard to install a unitary heating unit in each fire escape staircase to maintain a temperate space during winter months to prevent sprinklers from freezing.
Aluminium fixed pattern 3 cone stamped quare ceiling diffusers are used distribute the supply airflow in the corridors, toilets, receptions, and clinic spaces.
Slot diffusers are used around the perimeter zones in some rooms like the exercise area. As with any other project numerous balancing devices were installed to balance and commissioning the air system allowing the HVAC system to run efficiently and effectively.
A new stand-alone DDC building automation system is installed to monitor and control all HVAC systems.
The air-conditioning, heating, and ventilation services have been designed to comply to several codes as follow:
- 2015 International Building Code (with local amendments)
- 2015 International Energy Conservation Code (with local amendments)
- 2017 National Electric Code (with local amendments)
- 2015 International Mechanical Code (with local amendments)
- 2015 International Plumbing Code (with local amendments)
- 2012 International Fire Code
- 2012 NFPA 101 Life Safety Code
- 2012 NFPA 99 Standard for Health Care Facilities
- ASHRAE Standard 90.1 2013 Energy Standard for Buildings Except Low-Rise Residential
- ASHRAE Standard 62-2013 Ventilation for Acceptable Indoor Air Quality.
Climate in Mansfield, Texas
Mansfield is part of the Dallas-Fort Worth metroplex and is located in North Texas. It has a humid subtropical climate that is characteristic of the Southern Plains of the United States. Mansfield experiences mild winters and hot summers and categorised by the Köppen climate classification as Cfa. The Köppen climate classification is one of the most widely used climate classification systems (See Figure 1).
The typical summers in North Texas are very hot and humid, with temperatures ranging between 32°C and 42°C. Heat waves occur occasionally and can be severe. During the summer, the region receives warm and dry winds from the north and west, as well as hot, humid air from the Gulf of Mexico.
During the winter season temperatures range between 3°C and 14°C. Due to the city’s location inland from the Gulf Coast, the city’s climate is characterised by a relatively wide annual temperature range, as well as significant weather variations in a given month. The average snowfall per year is approximately 66mm. Since Mansfield lies at the lower end of the ‘Tornado Alley’, and tornadoes can be a threat to the city.
Some of the major challenges on this project included the space constraints with limited ceiling where the contractor had to fit VAV boxes and ducting. However, all challenges were easily overcome through the regular multi-disciplinary coordination sessions that stimulated open communication between all the professionals involved.
Another challenge, especially for the contractor, was to test and balance and commission the HVAC system to ensure that design parameters were achieved.
WSP modelled the MEPF systems in Revit, which is a modern building information modelling software for architects and engineers, designers and contractors and developed by Autodesk.
Revit allows designing and coordinating the MEPF system components in 3D, annotates the model with 2D drafting elements, and accesses building information from the building model’s database.
About the author
Pieter de Bod (Pr. Eng.) is an accredited LEED BD+C and Green Star Professional and is currently working at WSP in Dallas, Texas, US.
List of professionals and suppliers
|Architect and Prime Consultant||The Beck Group|
|Consulting MEPF Engineers||WSP USA|
|General Contractor||The Beck Group|
|Mechanical Contractor||Don Burden and Associates Inc.|
|VAV Terminals||Enviro-Tec; Vendor is Texas Air Systems Inc||72|
|Fans||Loren Cook; Vendor is Texas Air Systems Inc||9|
|Diffusers, Grilles||Naylor Industries||Various|