Ventilation to prevent high temperature in chiller plant in basement

[EloyRD]

Good evening, community:

I'm reviewing the HVAC design of a chiller plant in the basement of a 30-floor building.

The main equipment present in this room are:
- Chiller / 360 ton / 351 kW (x3 units)
- Primary loop pump / 1512 GPM @ 60 ft w.c. / 30HP (x4 units)
- Secondary loop pump / 2268 GPM @ 130 ft w.c. / 100 HP (x3 units)
- Chiller / 50 ton / 38 kW (x1 unit)
- Primary loop pump / 120 GPM @ 90 ft w.c. / 7.5 HP (x2 units)

To ventilate and prevent a temperature rise greater than 10°C (18 °F) -as is requested in ASHRAE 15-, the design proposes two AHU to provide a total 240000 BTU/H cooling capacity.

If I take for the pumps a mechanical inefficiency that turns into heat of 20%, I obtain 65 kW = 221 000 BTU/H. That number is almost the full required cooling capacity, and it seems OK to neglect any heat load from the chiller carcass or motors.

None of the engineers in the office remember seeing a design of a basement chiller plant with AHU; we have only seen designs with forced ventilation. But it seems that the heat load is high enough that cooling is required.

I have two strong doubts and would love any comments or a reference to some guide or bibliography about what is standard practice:
- Is it OK to neglect the heat load from the chiller accessories?
- Do AHU are usually required or proposed in design for chiller plants in a basement?

Thanks for any help.
Regards.
Eloy Ruiz

 

[ZDR1985]

Hi,

Why mechanical efficiency is used to estimate the heat generated by motors?

Based on your data, assuming all pumps and chillers are running at the same time and motor heat dissipation is 5% of motor rating (95% efficiency), I get 72 kW of heat, that is approximately 6 m3/s (12,713 cfm) of ambient air.

What is the size of room? Maybe you don't need cooling, ventilation is enough and will save you a lot of money.

 

[pocketengineer]

Mechanical ventilation is perfectly ok for chiller plants in the basement, even in hot climate countries. While it is perfectly ok to air-conditioned the chiller plants, most owners will go after the energy (operational) cost. After all, business men look at the bottom line.

 

[EloyRD]

Hi ZDR1985,

The room is around 150 meter2 (around 1600 sq. ft.)

Thanks for pointing out to the 95% electrical efficiencies from the motor, I didn't take that into account. For the pump, I was referring to a hydraulic-and-mechanical efficiency of 80% and that what is left turns into heat to the surroundings. Maybe I'm having some conceptual error about the issue, does that heat goes to the surroundings or only rises the temperature of the pumped-chilled-water slightly?

For the chiller, I am thinking that the heat produced in the compression process is transferred to the condensate circuit or the chilled water and that the heat to the environment is negligible.

Do you see any fault in my understanding?

Thanks

 

[EloyRD]

Hi pocketengineer,

Thanks for your comment about what is usual practice for HVAC in chiller plant. I'll see into replacing the AHU for fans if I can justify that the only items that release heat are the motors.

Regards.

 

[lukaiENG]

A couple of questions:
1. where did the 240000 BTU/hr AHU cooling capacity come from - it seems arbitrary.
2. why are you neglecting the chiller heat gains? You will have heat gains off the motors just like for the pumps and these are unlikely to be insignificant. You should be able to get this data from the equipment data sheets or your sales rep.

You are right in your revised opinion that the hydraulic inefficiency of the pump increases the temperature of the water (through friction) rather than increasing the room temperature. It is the motor efficiency that you should account for (per previous poster's advice)

I agree that you can probably ventilate the space - with the caveat that if you are in a very humid location you need to consider the dew point temperature of the OA vs. the temperature of the pipework. Mechanical plantrooms rarely have good insulation (at least after a couple of years) and you risk having condensation forming on your pipework in humid environments.

 

[Compositepro]

Mechanical energy losses from pumps goes into the pumped fluid. Ultimately, all energy will end-up as heat. The only question is where does it dissipate to the environment.

 

[EnergyProfessional]

doesn't the chiller and the chilled pipe cool enough (even when insulated)? the chiller mot doesn't heat that much, some are even hermetically sealed and cooled by refrigerant (smaller scroll chiller).
If the chiller runs chances are it is hot outside, so ventilating won't help.

Same with all the boiler rooms where they feel the need to add unit heaters... like all the boilers, pipes and un-insulated strainers etc. don't heat enough....

 

[ZDR1985]

EloyRD,

150 m2? That is a small room and ventilating it will result to over 40 air changes per hour, imagine that draft inside the room.

Probably you have to double check on the chiller motors heat (open type or hermetic type), which maybe not that much as I assumed to be the same as pump motors that are air cooled.