HVAC system selection 2

[scythe13]

Hi,
I am working on a project to evaluate the HVAC system for a 21 storey residential building.
I have attached a typical floor plan which involves Fan Coils layout. As you can see there are no mechanical ventilation system to furnish fresh air is considered.
Do you think according to ASHRAE Standard 62.1, this is an acceptable design?

Also, I am considering an alternative option:

-VRF (variable refrigeration flow) system for the apartments units
plus, air cooled DX units to provide fresh in the corridors.

Do you think it's a good viable option? (the total cooling load of the apartments is 185 tons)

Many thanks

 

[cdxx139]

As a mechanical engineer, you are responsible in determining if the system meets ASHRAE 62.1.

But to get you started, which ASHRAE 62.1 version? 2007 allows a natural ventilation option. Around here, the operable window area must meet 4% of the floor area served. I believe 2010 does not allow natural ventilation any more.

62.1 has many requirements and calculations. No one can determine if it meets it by simply looking at the plans. (Although you may be able to determine that it doesnt meet it by looking, but I am not at that level...YET)

knowledge is power

 

[Drazen]

Do you know that VRF system has mandatory limitation as to system refrigerant charge to volume of smallest sleeping room?

In Europe that is covered by EN 378, but it is actually only transmission from Japanese standard established earlier.

That forces you to separate systems, which can be troublesome for large residential buildings for matters of space - where to position many outdoor units.

There is also issue of metering - some VRF manufacturers offer metering system as an inherent part of VRF, but there is no actual means of calibration and verifying, whihc can be troublesoms when disputes occur.

In system you presented heat meter can be installed it position of connection to risers.

I am curious about overall hydraulic concept of this system. Are all fan coils connected to network via three-way valves? That was standard solution for long time, but is very energy inefficient for large systems, where you can have large mass of water flowing around for nothing.

 

[scythe13]

Drazen,
yes, I'm aware of the concerns related to VRF. It's also covered in ASHRAE 15.

As for energy submetering, I didn't find any info regarding how it's calculated, except for this (which does not explain how it divides the energy consumed by the outdoor unit between tenants):
"The modularity of the VRF also enables relatively simple submetering of electricity (i.e., placing an electric meter on one or a few condensing units is relatively simple, accurate and inexpensive"- (Goetzler, 2007)

Do you know how the work of outdoor unit is sub-metered?

About incorporating 3-way control valves, it's what I want to change and instead use 2-way valves with variable flow system. Thanks for noticing that!

 

[317069]

- waht is RT in your drwing?
- is there any duct?
- what about washroom and kitchen exhaust?
- exhaust fan in washroom considered as a mechanical ventilation
- is it a rental units or condominum?
- 62.1 standard allows natural ventilation but there are conditions have to be met.

 

[Drazen]

yes, vfd pumps and two-way valves should be right choice for such networks.

as regards to vrv metering, i remember there are some daikin models that offer metering based on flow delivered through expansion valves, that way be near real consumption, enabling that one system can be used for multiple apartments.

of course, there is an issue of fixed cost part of charge, but that issue exists anyhow, with any kind of system.

though i consider 62.1 best standard, it is not mandatory within my scope of activities, so my knowledge of it is limited. as colleagues mentioned, it can be determined whether natural ventilation is allowable or not.

that additionally opens an issue of how to revise your project - you need to analyze calculations in current design and see how ventilation heat losses/gains are covered, which directly affects your fc units sizing.

 

[scythe13]

317069,
-RT: Room Thermostat
-There are ducts for exhaust fans, water pipes, ... but I see no space left for AHU ducts
- Exhaust fans are considered for kitchens and bath rooms
-Rental

Drazen,
If all the valves in the system are two-way type, the flow will vary with the load; thereby I can use flow meter instead of heat meter to submeter energy usage. Am I right?

- I guess for tightly sealed homes, using just exhaust ventilation system along with natural ventilation is not enough.
One option is supplying fresh air thru fan coils, what's your idea?

 

[Drazen]

Heat meter is actually flow meter with two temperature sensors added which measure real-time temperature difference, and calculator added in control unit which integrate energy consumed, so mechanically you actually have flow meter; most of those devices are designed to cope with varying flow all the time.

As regards to decision on ventilation, some reference to standard has to be established. I think exhaust-only limited to sanitaries can be tolerated in tight homes, but that is not what will bring you whole-apartment ventilation.

If you install exhaust for whole apartment, you will need to add at least make-up air grills, than the main issue is whether heat recovery is mandatory or not.

Using FC's to condition fresh air reverts back to mentioned issues - heat recovery and calculation of ventilation loss for FC sizing, they cannnot be avoided.

 

[317069]

- your system is hydronic system for rental appartment. in rnetal apartments usualy heat is included in the rent, in you case cooling also is the same, why do you want to go with sub metering and make it complex.
- as for ventilation, some provide fresh air through corridor, and other use ventilation through windows. someone will say windows are not alwayes open, we can say still better than mechanical ventilation which has more building sik risk as 62.1 says. exhaut fans can support ventilation.

 

[scythe13]

317069,
There are some reasons behind submetering:
-Submetering provides financial incentives to save energy
-It's fair for the tenants to be billed based on the energy they consumed. Using a fixed charge is not fair and may cause tension between tenants and landlords

 

[317069]

- "Sub metering provides financial incentives to save energy" could you please explain it with an example.
- I didn't hear that tenant argued for heat rate included in the rent; this is the rent including heating and cooling, takes it or leave it, easy.

 

[scythe13]

Find the attachment!

 

[317069]

Attachment didn't say anithink just like a commercial for a product. it didn't show how can we save energy by using submetering,
let us go with your idea, but, for your 21 stories building what about submetering also for the elevators, tenant in 5th floor consupts less elevator energy than tenant at 15th floor.
if you want you can go with submetering nothing wrong with it but is not nessessary.
your main question was about ventilation and 62.1 standard

 

[trashcanman]

Run, do not walk away from Variable refrigerant systems.
1. Very proprietary. You will be stuck for parts from the original manufacturer.
2. If one unit fails, you just might have to shut down all the units on the same circuit for repair.
3. If you spring a refrigerant leak, it will be hard to fix without shutting down the whole system, plus you have the cost of the refrigerant to consider.
If you want a central system, go hydronic. Easy to design, install, repair. A leak is just water - cheap. Each unit can be easily metered for chilled or hot water.

If you want unitary, go with PTAC units.

 

[Falah7582]

for such buildings, the main concern of tenants is bills and specially AC bills, from my experience I would say DX units for each apartment is better than hydronic central system due to accumulative costs for the system requirements to cool an area yes in general the COP of a hydronic system is better that DX systems but DX is efficient and can be monitored to reduce operating costs.