DOAS, Radiant Cooling

How To Model a DOAS With Radiant Cooling

All throughout this blog we have looked at the many aspects of the Dedicated Outdoor Air System (DOAS) coupled with radiant heating and cooling. We have looked at its many benefits, its different components, how to design and optimise such a system. In this post, however, we will look at a theoretical example to demonstrate the multiple steps that need to be taken to model a DOAS with radiant cooling.

Note: we will be using the US imperial measurement system throughout this post as we are following ASHRAE guidelines.

The brief

 

We’re setting out to design a DOAS with parallel sensible cooling. In practice, that results in the decoupling of ventilation and air-conditioning functions or decoupling of sensible and latent load functions. First, we remove the latent loads from the outside air (OA) intake and generated in spaces using a 100% OA ventilation system (i.e. DOAS). Second, we remove the space sensible loads using a parallel mechanical cooling system; such as fan coil units, conventional variable air volume, and ceiling radiant cooling panel (CRCP), independent of the ventilation system.

DOAS with parallel sensible cooling

Step 1, Determine design outdoor air conditions

 

In their ‘Fundamentals’ handbooks, ASHRAE typically gives three design OA data pairs:
Peak dry-bulb with mean coincident wet-bulb temperature: to determine peak sensible loads;
Peak dew point with mean coincident dry-bulb temperature: to calculate the peak latent loads;
Peak wet bulb with mean coincident dry-bulb temperature: to estimate peak total cooling loads.
Among these three, one data set appropriate for designing the DOAS should be chosen by considering the climatic design data set providing the highest design OA enthalpy. For example, the selected cooling coil should meet cooling and dehumidification requirements.

Step 2, Determine target space conditions

 

Typically, aim for ‰79ºF and 50% relative humidity (RH). It corresponds to 73.8 gr/lb humidity ratio (HR) and 58.6ºF dew-point temperature (DPT). This room DPT is lower than the design mean panel surface temperature (62ºF) and as such will not cause condensation on the radiant cooling panel surfaces.

 

Step 3, Determine design cooling load and required ventilation rate for each space

 

With a predetermined OA and our target space conditions, the design sensible and latent cooling load for each space are calculated. Required ventilation for each space is estimated using the minimum ventilation rates recommended by ANSI/ASHRAE Standard 62.1-2004. For example, a classroom requires 10 cfm (cubic feet per minute) of OA per occupant and 0.12 cfm of OA per unit floor area.
In the DOAS/CRCP system, the total supply air is the sum of the required minimum ventilation rates (i.e. 1,649 cfm) and the ventilation air distributed to each conditioned space at constant volume.

 

Step 4, Determine supply air conditions

 

The supply air (SA) must be dehumidified by the DOAS to maintain the target space humidity level in each conditioned space. The required SA humidity ratio for each space can be calculated using the following equation:

required SA humidity ratio for each space

To illustrate the design process, the diagram below shows the design conditions that we have determined so far. The cooling coil (CC) for example cools and dehumidifies the SA preconditioned by the enthalpy wheel to meet SA dryness calculated by the equation above.

dehumidifies the SA preconditioned by the enthalpy wheel to meet SA dryness calculate

 

Step 5, Determine enthalpy wheel effectiveness and design cooling coil load

 

We now need to determine the thermodynamic properties of the enthalpy wheel (see state 2 in the diagram above). The equations below determine, respectively, the dry-bulb temperature (DBT) and the humidity ratio (HR) of the SA leaving the enthalpy wheel.

Determine enthalpy wheel effectiveness
The design enthalpy wheel effectiveness (εs and εL) can be found from the manufacturer and depend on these parameters: desiccant material, wheel entering air conditions, face velocity, and airflow ratio.
Next, the design cooling coil load (Qcc) can be calculated using the following equation:

Qcc

 

Step 6, Determine sensible cooling load for the CRCP system

 

In the DOAS/CRCP system, the ceiling radiant panels should accommodate the remaining sensible load not met by the supply air from the DOAS. In this example, the design sensible load (Qs) was estimated in step 3. The sensible cooling provided by the SA (Qsen,sa) can be calculated using the first equation below. Consequently, the difference between the space sensible load and the SA cooling capacity is the sensible cooling load allocated to the CRCPs installed in each space (second equation).

Determine sensible cooling load for the CRCP system

 

Step 7, Determine design panel cooling capacity

 

In practice, the design cooling capacity per unit panel area (Btu/h·ft2) is determined by the panel manufacturer (ANSI/ASHRAE Standard 138-2013), based on the difference between the room temperature and the mean panel surface temperature (or mean fluid temperature).

 

Step 8, Determine required CRCP area

 

The required CRCP area (Ap) for each space is easily calculated by dividing the panel sensible cooling load (Qsen,p) estimated in step 6 by the unit design panel capacity (qp) determined in step 7.

Determine required CRCP area

In this post, a general DOAS/CRCP system design procedure was presented using a simple design example in 8 steps. System design tools and verified simulation models would significantly improve the uptake of such systems, but for now, these steps may provide an accurate estimation of how a DOAS/CRCP system would operate.

 

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