A Nontraditional HVAC System

by Tariq Amlani

Our group of engineers, architects, builders, bankers and lawyers has been fortunate to have had the opportunity to design, construct and operate two new state-of-the-art hospitals to service the communities of Campbell River and the Comox Valley for Island Health. With a total project budget of $602 million for both hospitals, our proponent team was required to guarantee costs for the design, construction, maintenance, operations, energy consumption and housekeeping for 30 years, based on a schematic level design solution and in a public-private-partnership contract form.

With a total program that includes 248 inpatient beds, 10 operating theatres, 27 isolation rooms, four trauma rooms, two MRI suites, two laboratories and pharmacies, two medical device reprocessing departments, four commercial kitchens, two helipads, lots of supporting spaces and more than 750,000 square feet of building, figuring out the programming and distribution of the services was complex enough.

On top of that, the project agreement mandated that the design solution consume no more than about 300 kWh/m2 of total thermal and electrical energy per year. To put that into perspective, the current average energy consumption for all of the existing acute care hospitals on Vancouver Island is about 495 kWh/m2/yr and the theoretical design for the new Comox Hospital, designed only to meet the current code requirements of ASHRAE 90.1-2010, would consume 394 kWh/m2.

The good news is our integrated design team was able to find a solution and one of the keys, as counter-intuitive as it sounds, was to air-condition the exhaust air before the used air left the building.

In order to ventilate the two buildings, on average we are introducing a total of 200,000 litres of fresh air per second. For each litre of fresh air we bring into the building, we need to exhaust an equal amount, to keep the building balanced. That’s a lot of air. And it’s all nice and warm.

Common traditional solutions for recovering some of this available exhaust air heat include glycol run-around loops, energy recovery wheels and energy recovery plates. These systems work by transferring heat directly from the exhaust air stream to the fresh air stream. The amount of heat they are able to recover fluctuates over the course of the year, as the incoming air temperature changes and as the demand for heating at the central air handlers varies.

For our project, however, we had a VAV reheat system and we found that the heating demand at the central air handlers was relatively low (compared to the hydronic reheat load and the domestic hot water heating load). We were challenged by the fact that in the shoulder seasons, when the incoming air was relatively warm and required little or no heating at the air handlers, the traditional air to air heat recovery systems would not be able to capitalize on the available thermal energy in the exhaust air stream.

Enter the exhaust air heat recovery heat pump system.

By installing cooling coils downstream of the exhaust air fans, we are able to ‘air-condition’ all of that warm exhaust air, from the normal discharge air temperature of 24°C, all the way down to 12°C, before we throw it out to the atmosphere. Unlike traditional air to air heat recovery systems in which the amount of available heat recovery varies depending on the difference in temperature between the exhaust air and the incoming air, this system provides a constant source of heat recovery year-round. In this case, a total of more than 2,500kW of heat.

In order to make it all work, we designed a central heat recovery (screw) chiller plant, which on the evaporator side of the chillers produces a constant supply of chilled water at around 6°C (varies slightly depending on demand and a reset). On the condenser side, we use a pump and three-way mixing valve to recirculate the leaving condenser water back into the inlet condenser connection of the machines. As this condenser water circulates around and around, it continually warms up, until it reaches about 43°C, at which point we crack the three-way valve and discharge the warm condenser water into the hydronic heating loop. Once the recovered heat is into the hydronic loop, we can use it to provide heating at the air handlers, at the reheat coils or to preheat domestic hot water.

For the North Island Hospital project, the use of the exhaust air heat pump heat recovery system is projected to deliver more than $250,000 in energy cost savings annually, helping to earn the project more than $1.7 million in energy incentive funding from BC Hydro. As designed, the heat recovery system will result in an annual greenhouse gas savings of more than 1,500 tons of carbon dioxide per year, versus traditional gas boiler heat generation used in the existing hospitals. PM

Tariq Amlani is a mechanical engineer with Stantec and principal with the firm. Over the past 10 years, he has played key roles on the design, development, construction and commissioning of several new healthcare facilities within B.C., including the Kelowna General Hospital, the Vernon Jubilee Hospital, the Comox Valley Hospital and the Campbell River Hospital.

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